There are so many diabolical possibilities, but which chemical is the most dangerous?

It comes down to a combination of effect and exposure—how much makes a deadly dose and what exactly will it do to you? Nerve agents are widely considered the most toxic chemical weapons owing to their tiny toxic limits and devastatingly rapid impacts on the human body: Just 10 milligrams (that’s ten thousandths of a gram) of VX is enough to cause death within minutes. Yet just one person has been killed by the nerve agent over the last decade.

Meanwhile, more than 100,000 people are accidentally poisoned in the U.S. every year by common household chemicals such as bleach and disinfectant, even though these substances are slower-acting and far less toxic than VX. And some common chemicals can be fatal when combined. For instance, combining drain cleaner and bleach will release poisonous chlorine gas. 

Those two examples highlight a key problem in ranking chemicals in order of danger: To evaluate danger, you need to know how likely you are to encounter a chemical.

Safety professionals define danger using a combination of two factors: hazard and risk. 

"A hazard is something with the potential to cause harm. Risk is the likelihood that harm will arise and the severity of that harm," said Richard Webb, the health, safety, environment and well-being officer at the University of Cardiff's School of Chemistry. The hazard is therefore a fixed property of a tool or chemical, while the risk varies depending on how that object is used.

We automatically consider this balance of factors every day. Take the example of a kitchen knife: We know the blade is sharp and will cut things, including us, in the right circumstances. But it's how we use and store the knife that determines whether it poses a danger to us, Webb told Live Science. 

Related: Can foxgloves really give you a heart attack?

This same logic applies to chemicals. "Even a very hazardous chemical does not pose any risk if there is no exposure," a spokesperson for the Finland-based European Chemicals Agency told Live Science. Botulinum toxin, VX and chlorine trifluoride are therefore extremely hazardous but very, very low risk to the average person.

"Some hazardous chemicals are also essential for our health in small doses," added the spokesperson, "whereas in higher exposures they may be lethal."

Ordinary table salt is an excellent example. The small amount in our diets is vital to maintain the correct ion balance within our bodies, but too much can cause severe health problems, like high blood pressure and heart failure. Outside the body, large quantities of that same salt act as a weedkiller by overwhelming plants' ion balance to the point of death.

Even determining which chemicals are the most hazardous is fraught with difficulty, as there are so many ways they could cause harm. In the European Union, classification, labeling and packaging regulations define nine hazardous characteristics, including toxic, explosive and corrosive. But again, Webb emphasized that which of these is most dangerous depends on the context.

For example, although chlorine is a common disinfectant in pools today, the concentrated gas was used as a chemical weapon in World War I and caused both chemical burns and respiratory irritation. The key difference though, is that pools include only a small amount of chlorine, and that small amount is dissolved into the water. "The thing that makes it high risk is the fact it's a gas," Webb said. 

On paper, sodium cyanide looks much worse. "It's famously poisonous. It binds to your hemoglobin permanently, which stops it from carrying oxygen so you can't respire," Webb said. However, as a solid, it's much easier to handle, meaning scientists using this toxic compound can more readily avoid the nasty effects of exposure. 

"If you work with it safely — you wear your PPE [personal protective equipment], work in a fume hood and wash your hands when you finish — the likelihood of contaminating yourself is pretty low," Webb explained.

This means our safety is often within our own control. Anything can become dangerous if it's not handled properly, but there are steps we can take to reduce the likelihood of harm. 

"The most important thing is knowing exactly what the hazards are and what you can do to minimize the risk," Webb said.

The wine, which the scientists described as "reddish liquid" in appearance, was found in a roughly 2,000-year-old tomb during a house construction project in Carmona, a town in Seville, in 2019.

The use of wine in Roman-era burial rituals is well documented, but discovering a wine sample this old, in its liquid state, was "rather exceptional and unexpected," the scientists wrote in their paper, published June 16 in the Journal of Archaeological Science: Reports. 

"It's a sunken tomb that was excavated from the rock, which allowed it to remain standing for 2,000 years," José Rafael Ruiz Arrebola, an organic chemist at the University of Córdoba and a senior author of the study, told The Guardian. 

Wine contains distinct chemical compounds that reflect not only its flavor and appearance but also its origins. But after many years, these chemicals often undergo substantial decay that makes them difficult to characterize, the scientists wrote in the paper. 

During the funerary ritual, cremated ashes were mixed with the liquid, making it murky, the scientists told The Guardian. 

Using analytical techniques including high-performance liquid chromatography and mass spectrometry, the scientists sifted through element-by-element to find components that belonged to the liquid. 

Wine grapes contain distinct plant compounds known as polyphenols that serve as a "barcode," marking their varietal and the conditions in which they were grown and harvested. However, "few studies have been conducted on polyphenols in archaeological wine remains," the scientists wrote in the study. 

When the scientists found polyphenols in the liquid sample, their suspicions were confirmed — the ancient liquid was indeed wine.

From looking at historical texts, they suspected that the wine would have been similar to modern fino wines produced from regions in southern Spain. The scientists compared the polyphenol content of the ancient wine to today's wines to determine that the wine was likely from Doña Mencía, a city in southern Cordoba.

While the liquid is reddish, it lacked syringic acid, a compound that is produced by red wine when it decomposes, which confirmed that the original wine was actually white. 

Despite having mostly decayed, the ancient wine isn't, "the least bit toxic," according to microbiological analyses, Arrebola told The Guardian. Nevertheless, the scientists did not taste it.

Other ancient wine analyses have focused on dried remnants, such as the 8,000-year-old fingerprints of tartaric acid, a compound of grapes and wine, found on a clay jug in the Republic of Georgia. Because the new study analyzed liquid wine, this discovery is one of a kind.

"We have been lucky to find it and analyze it — it's something you only see once in your life," Arrebola told CNN.

Cotton is susceptible to this kind of laundry blunder in a way that synthetic fibers, like polyester, are not. A large part of this vulnerability comes down to the individual fibers of the cotton clothes, Jillian Goldfarb, an associate professor of chemical and biomolecular engineering at Cornell University, told Live Science in an email. 

"Cotton fabric is made by weaving together fibers from a cotton plant, which themselves are made mostly of cellulose, a natural biopolymer," she said. "Cotton … is prone to shrinking because its fibers swell when they get wet and then contract as they dry."

If you've ever sweat in cotton clothes, you know firsthand how well they can absorb moisture. On the other hand, synthetic fabrics — like polyester, nylon and spandex — are more resistant to sweat and shrinkage because their tightly woven fibers don't swell in water.

On a chemical level, weaving cotton fiber for clothing introduces tension that creates a hydrogen bond network, Erika Milczek, a chemist and CEO of biotechnology company CurieCo, told Live Science. 

Related: When did humans start wearing clothes?

When variables like heat and water are introduced, this hydrogen bond network can transform, causing fabric to either relax or contract. This is also the science responsible for wrinkles in your clothes, Milczek said.

The science of shrinking

When it comes to accidentally shrinking your cotton clothes, not all items are made equally, Goldfarb said. 

"Even when they're made of the same material, some cotton fabrics are more prone to shrinking than others depending on how the fibers are assembled into a fabric," she said. "Woven cottons, while they will certainly shrink, see considerably less shrinkage than knit cottons."

Imagine the intersection of woven cotton fibers like a hashtag, where some fibers are woven under others, Goldfarb said. Yarn woven horizontally is called the "warp," and yarn woven vertically is called the "weft."

"As the yarns swell when they're wet, they push the wefts closer together, shrinking in one direction," Goldfarb explained. "When the moisture is taken out of the fabric, the fibers contract." This means that shrinking actually begins before the clothes ever hit the dryer. Shrinking is the dual consequence of water-logged fibers and high heat.

Exactly how much your clothes shrink is determined by a number of factors, Milczek said. For example, it depends on whether you wash your clothes in water alone or add detergent — detergent further disrupts hydrogen bonds — and whether you dry your clothes at high heat or low heat or hang them to dry. 

"The temperature [when line drying] is considerably lower, so evaporation occurs much more slowly, and the fibers are not 'stressed' by the heat in shrinking," Goldfarb explained. A line-dried shirt also experiences more consistent humidity between the outdoors and your closet, which can result in less shrinkage, she said.

Saving a shrunk shirt 

For some, this knowledge may come a little too late. But don't fret; there may still be hope for your shrunken garments. 

One obvious answer, Milczek said, is to look for clothes that are shrink resistant to begin with. These include cotton clothes with synthetic blends or cotton clothes that have been preshrunk. 

If that won't do the trick, there is a science-backed way to attempt to "unshrink" your clothes. 

"Depending on the quality of the yarn and the weave … if we swell the fibers and allow them to dry under tension, it is possible to "unshrink" some cotton fabrics, at least temporarily," Goldfarb said. 

One way to do this at home is to use a steam iron, she said. This reintroduces moisture into the garment to expand the fibers while applying mechanical force to stretch them back out. But tread lightly — this method can also easily swing too far in the opposite direction.

"Of course, it's easy to 'overstretch' your cotton this way, and if it's done unevenly, you can be left with a rather warped item of clothing." Goldfarb said. 

In essence, rocks are aggregates of two or more minerals. Minerals, meanwhile, are solids that, with a few exceptions such as diamonds, lack carbon and are arranged in an orderly, repeating "crystal structure." 

"Minerals are basically the building blocks of rocks," Erika Anderson, an honorary curator of mineralogy and petrology at the University of Glasgow's Hunterian Museum in Scotland, told Live Science. "It's kind of like atoms in a molecule, so minerals are the atoms." 

Each type of mineral has a unique crystal structure, which results from its chemical composition and dictates a set of physical properties, such as hardness, color or magnetism, according to the United States Geological Survey (USGS). For instance, halite — the natural form of sodium chloride (NaCl), from which table salt is made — is a soft mineral that forms clear, cube-shaped crystal fragments. Different minerals, like aragonite (CaCO3) and calcite (CaCO3), can have the same chemical makeup, but their crystal structure and physical properties differ because of how they each formed.

"For each mineral, they will have a set way that those atoms bond together," Anderson said. "Some minerals have the exact same elements in them, but they're bonded differently, so it makes them different minerals."

Related: Fountains of diamonds that erupt from Earth's center are revealing the lost history of supercontinents

A good example of a mineral is quartz, which is found across the world and in different rocks, such as granite and quartzite, Anderson said. Quartz is made of the chemical elements silicon and oxygen and has the chemical formula (SiO2). The mineral is colorless in its pure form, but impurities can either make quartz crystals appear opaque or stain them pink, purple, yellow or brown.

As of May 2024, the International Mineralogical Association — the scientific body responsible for identifying, approving and naming minerals — listed 6,050 mineral species. Experts distinguish minerals based on their crystal structure, which is the specific way in which their atoms or elements are arranged.

While some minerals like halite have relatively simple crystal structures, others can contain 10 or more elements, such as khomyakovite and georgbarsanovite.

"We're constantly finding new minerals, because we're exploring areas that might have conditions we didn't know about," Anderson said.

The rock cycle

There are three main types of rock — igneous, sedimentary and metamorphic — with varying mineral mixes depending on where and how the rock came to be. 

Igneous rocks — which form as magma solidifies either deep within Earth or at the surface after a volcanic eruption, for example — contain a limited number of minerals that crystallize, Richard Bevins, an honorary professor of Earth sciences at Aberystwyth University in the U.K., told Live Science in an email. "These are termed the common rock-forming minerals and include feldspar, olivine, pyroxene, mica, quartz and amphibole."

Igneous rocks may be subjected to high heat and pressure, or exposed to fluids that alter their mineral composition. Once their mineral composition changes, the rocks are considered metamorphic, with examples including phyllite, schist, quartzite and marble. Igneous and metamorphic rocks on Earth's surface inevitably erode and break up as wind and water go to work on them. The fragments are transported and form deposits that solidify into new rocks called sedimentary rocks. "Sedimentary rocks are principally composed of minerals present in the rocks that were eroded to form the sediment," Bevins said. 

In general, the process by which rocks are continually recycled and transformed by geological processes is known as the rock cycle.

Some rocks are mono-mineralic, meaning they only contain one mineral. Limestone, for example, is a sedimentary rock made exclusively of the mineral calcite (CaCO3). Glacier ice too, is a type of rock composed of crystals of water.

In 2014, scientists proposed naming a new type of rock derived from plastic pollution: plastiglomerate. A team found that plastic littering a beach in Hawaii had melted and glued natural sediments together, forming rock-like lumps. And like rocks, researchers said those plastiglomerates may forever remain in the geological record and mark the fragment of Earth's history that we inhabit. 

Editor's Note: This story was updated at 10:30 EDT to note that most, but not all, minerals lack carbon (diamond being a notable exception). It was also updated to note that the crystal structure dictates a mineral's physical properties. 

Promethium is one of the 15 lanthanide elements at the bottom of the periodic table. Also known as the rare earths, these metals exhibit a number of useful properties, including strong magnetism and unusual optical characteristics, making them particularly important in modern electronic devices. 

"They are used in lasers; they are part of the screens of your smartphone. They are also used in very strong magnets in wind turbines and electric vehicles," Ilja Popovs, a research and development staff member at Oak Ridge National Laboratory (ORNL) and co-author of a new study published in the journal Nature, told Live Science. 

'Scarce and difficult to study'

Promethium itself, which was discovered by ORNL scientists in 1945, has a few minor applications in atomic batteries and cancer diagnostics. But scientists have a very limited understanding of the element's chemistry, precluding more widespread uses. 

Studying the radioactive element has posed a decades-long challenge, partly due to the difficulty of securing a suitable sample, team member Alexander Ivanov, also a research and development scientist at ORNL, told Live Science.

"Promethium doesn't have a stable isotope — they're all radioactive, meaning that they are decaying [into other elements] with time," Ivanov said. "You get this element through a fission process, so it's scarce and difficult to study."

ORNL is the U.S.' only producer of promethium-147, an isotope of the element with a radioactive half-life of 2.6 years. Using a method developed last year, the researchers separated this isotope from nuclear reactor waste streams, creating the purest possible sample for study.

Related: Atoms squished closer together than ever before, revealing seemingly impossible quantum effects

Then, the team combined this sample with a ligand — a molecule specially designed to trap metal atoms — to form a stable complex in water. The coordinating molecule, known as PyDGA, formed nine promethium-oxygen bonds, giving researchers the first-ever opportunity to analyze the bonding properties of a promethium complex.

However, the analysis itself was no trivial matter. 

"Because promethium is radioactive, once it's decaying, it's getting transmuted into the adjacent element, which is samarium," Ivanov said. "So you will have a tiny amount of contamination in the form of samarium." 

'The last puzle piece'

The team therefore used an extremely specialized, element-specific technique called synchrotron-based X-ray absorption spectroscopy. High-energy photons generated by a particle accelerator bombarded the promethium complex to build a picture of the positions of atoms and the lengths of bonds. Subtle differences in the metal-oxygen bond lengths then allowed the team to focus on the key promethium-oxygen bond, discounting any contaminating samarium.

Crucially, this information enabled a comparison of promethium's properties with other rare earth complexes for the first time. 

"Promethium was the last puzzle piece among those elements," Popovs said. The ligand provided a way to have a stable complex for all of the lanthanides — the same element ratios and the same kind of geometry. That allowed the team to "study the fundamental physical chemical properties of these complexes across the whole series," Popovs explained.

Lanthanides are naturally found as mixtures of elements, so understanding periodic trends such as bond lengths and complex-forming behaviors helps scientists develop new and more efficient methods to separate these valuable metals. 

Now, the ORNL team is studying promethium in water to build a clearer picture of the coordination environment and chemical behavior of this unusual element. 

"Hopefully, the fundamental insights that we're providing will inform other scientists how to design better separation technologies and can perhaps spur more interest in studying it for other applications," Popovs said.

Natural diamonds form in Earth's mantle, the molten zone buried hundreds of miles beneath the planet's surface. The process takes place under tremendous pressures of several gigapascals and scorching temperatures exceeding 2,700 degrees Fahrenheit (1,500 degrees Celsius).

Similar conditions are employed in the method currently used to synthesize 99% of all artificially created diamonds. Called high-pressure and high-temperature (HPHT) growth, this method uses these extreme settings to coax carbon dissolved in liquid metals, like iron, to convert it to diamond around a small seed, or starter diamond. 

However, the high pressures and temperatures are difficult to produce and maintain. Plus, the components involved affect the diamonds' size, with the largest being about a cubic centimeter, or about as big as a blueberry. Besides, HPHT takes a fairly long time — a week or two — to produce even these tiny gems. Another method, called chemical vapor deposition, eliminates some requirements of HPHT, like high pressures. But others persist, like the need for seeds.

The new technique eliminates some drawbacks of both synthesis processes. A team led by Rodney Ruoff, a physical chemist at the Institute for Basic Science in South Korea, published their findings April 24 in the journal Nature. 

Related: Scientists may have pinpointed the true origin of the Hope Diamond and other pristine gemstones

The diamond crucible

The novel method was a long time in the making. "For over a decade I have been thinking about new ways to grow diamonds, as I thought it might be possible to achieve this in what might be unexpected (per 'conventional' thinking) ways," Ruoff told Live Science by email.

To start out, the researchers used electrically heated gallium with a bit of silicon in a graphite crucible. Gallium may seem like an esoteric element, but it was selected because a previous, unrelated study showed that it could catalyze the formation of graphene from methane. Graphene, like diamond, is pure carbon, but it contains the atoms in one layer rather than in the gemstone's tetrahedral orientation. 

The researchers housed the crucible in a home-built chamber maintained at sea-level atmospheric pressure, through which superhot, carbon-rich methane gas could be flushed. Designed by co-author Won Kyung Seong, also of the Institute for Basic Science, this 2.4-gallon (9 liters) chamber could be readied for experimentation in just 15 minutes, allowing the team to rapidly undertake runs with different concentrations of metals and gases. 

Through such tweaking, the researchers figured that a gallium-nickel-iron mixture — coupled with a pinch of silicon — was optimal for catalyzing the growth of diamonds. Indeed, with this blend, the team obtained diamonds from the crucible's base after just 15 minutes. Within two and a half hours, a more complete diamond film formed. Spectroscopic analyses showed that this film was largely pure but contained a few silicon atoms.

The minutiae of the mechanism that formed the diamonds are still largely murky, but the researchers think a temperature drop drives carbon from the methane toward the crucible's center, where it coalesces into diamond. Plus, without silicon, no diamonds form, so the researchers think it may act as a seed for the carbon to crystallize around. 

However, the new method has its own challenges. One problem is that the diamonds grown with this technique are tiny; the largest ones are hundreds of thousands of times smaller than the ones grown with HPHT.  That makes them too small to be used as jewels.

Other potential uses — for example, in more technological applications like polishing and drilling — for the diamonds synthesized with the new technique are unclear. However, because the process involves low pressure, Ruoff said, it might significantly scale up diamond synthesis. 

"In about a year or two, the world might have a clearer picture of things like possible commercial impact," he added.  

The water-battery has a lifetime of over 1,000 charge-discharge cycles, the team reported April 23 in the journal Nature Energy.

One of the most important properties of any battery is the energy density — how much energy the battery contains relative to its size or weight. Lithium-ion batteries have a particularly high energy density and are widely used in electric cars and portable devices. However, the liquid component, known as the electrolyte, typically contains organic chemicals which can catch fire or explode if the system overheats. 

Related: How do electric batteries work, and what affects their properties?

In contrast, water-based batteries are much safer but generally have a lower energy density thanks to the narrow voltage window in which they operate. However, by hacking the chemistry taking place inside the water electrolyte, Li’s team have dramatically boosted both the energy density and the overall performance of aqueous batteries.

Electrolyte solutions are actually a mixture of many different chemicals, each controlling a different aspect of the battery's performance. Additives called mediators help move electrons across the solution by undergoing a series of supporting oxidation and reduction (redox) reactions.

For aqueous batteries, the most common mediator is iodine: through a sequence of individual redox reactions, this halogen element can transfer up to six electrons per cycle, converting iodide (I–) to iodate (IO3–). However, slow reaction rates and unwanted byproducts mean that this additive usually results in a low-energy-density battery.

To improve the efficiency of this mediating redox sequence (and therefore the overall energy density), Xianfeng Li from the Chinese Academy of Sciences, and colleagues developed a mixed halogen electrolyte, containing both I– and bromide (Br–) ions in an acidic solution. Introducing bromine, another halogen element capable of transferring electrons, provided a stepping stone for this difficult chemistry, increasing the reaction rate and suppressing the formation of nuisance byproducts. 

Related: Future electric cars could go more than 600 miles on a single charge thanks to battery-boosting gel

Through detailed electrochemical and spectroscopic analyses, the team demonstrated that the bromide ions participated in the redox reactions alongside the iodide, forming a vital intermediate and boosting the speed and efficiency of the electron transfer sequence.

The researchers then began a series of experiments to evaluate the impact of this “hetero-halogen” electrolyte on the overall performance of several common battery types using different materials as the negative terminals (anodes).

The new electrolyte nearly doubled the energy density compared with standard lithium-ion batteries when used with cadmium anodes, which are typically found in high-energy portable devices such as power tools. . Meanwhile, vanadium systems, which are often attached to power plants and renewable energy generators for grid energy storage, demonstrated particularly long lifetimes, maintaining peak performance over more than 1,000 charge-discharge cycles.

In both cases, the team reported improved energy efficiencies and calculated that the aqueous hetero-halogen system would be cost-competitive compared with current lithium-ion technologies.

The team hopes that this substantial performance enhancement will lead to wider use of water-based batteries as a safer, high-energy-density alternative to existing systems.

We often think of space as a yawning chasm of nothingness between stars, but this apparent emptiness is alive with chemistry as atoms come together and break apart to create stars and planets over millions of years. Understanding how simple organic molecules such as methane, ethanol and formaldehyde form helps scientists build a picture of not only how stars and galaxies are born but also how life began.

However, detecting these basic building blocks of life is no mean feat. Every molecule possesses a unique energy "barcode" — a collection of specific wavelengths of light that the molecule can absorb. At a quantum level, each absorbed wavelength corresponds to a transition between one rotational energy level and another, and every molecule has a different-but-well-defined set of energy levels where these transitions may occur. This barcode of energy transitions is easily measured for samples in the lab, but astrochemists must then hunt out this same energy signature in space.

"When we observe interstellar sources with radio telescopes, we can collect the rotational signal from the gaseous molecules in these regions of space," first study author Zachary Fried, an astrochemist at MIT, told Live Science in an email "Because the molecules in space obey the same quantum mechanical laws as those on Earth, the rotational transitions observed in the telescope data should line up with those measured in the lab."

Related: Scientists made the coldest large molecule on record — and it has a super strange chemical bond

This approach is exactly how Fried and colleagues — part of a research team led by Brett McGuire, an assistant professor of chemistry at MIT — detected 2-methoxyethanol, a 13-atom molecule in which one of the hydrogen atoms of ethanol is replaced with a more complex methoxy (O–CH3) group. This level of complexity is particularly unusual outside the solar system, with only six "species" larger than 13 atoms ever detected. 

"These molecules are typically much less abundant than smaller hydrocarbons that have simpler formation routes," Fried said. "Additionally, the spectral signals of these molecules are distributed over a greater number of transitions, thus making the individual spectral peaks weaker and more difficult to observe."

But it wasn't simply luck that led the team to this discovery; they also used artificial intelligence. The team had previously developed a machine-learning method to model the abundance of different molecular species in different regions of space. "Using these trained models, we can predict which undetected molecules may be highly abundant, and thus strong detection candidates," Fried said.

Methoxy-containing species had previously been detected in a part of the Cat's Paw Nebula, also called NGC 63341, and in IRAS 16293, a binary system in the Rho Ophiuchi cloud complex, located 457 light-years from Earth. As such, the team had a good idea of where to look for the new molecule.

Fried began by measuring the rotational spectrum of 2-methoxyethanol samples in the lab; he recorded a total of 2,172 possible energy signals for the molecule. Then, using the Atacama Large Millimeter/Submillimeter Array (ALMA), a set of 66 radio telescopes in Chile, the team collected readings from both the Cat's Paw Nebula and IRAS 16293 and analyzed the signals for the distinct energy signature of 2-methoxyethanol.

While no corresponding energy traces were detected in IRAS 16293, the team ultimately identified 25 matching signals from the Cat's Paw Nebula and confirmed the presence of 2-methoxyethanol in this star-forming region. 

"This enabled us to investigate how the differing physical conditions of these sources may be affecting the chemistry that can occur," Fried said. "We hypothesized several causes of this chemical differentiation, including variations in the radiation field strength, along with different dust temperatures in these two sources [at different stages] of star formation."

The team hopes the findings may inform future studies to identify other as-yet-undetected molecules in space. 

"The feasibility and efficiency of these pathways can be closely tied to the physical conditions of the interstellar source," Fried said. "By investigating which other species are involved in the formation and destruction of the detected molecules, we can determine other species that may be candidates for detection."

But why does it work? Why does striking a flint rock against a piece of steel start a fire, whereas rubbing two random rocks together doesn't?

All fire-starting methods have a similar goal: generating enough heat to ignite a fuel source.

When scraped together, flint and steel can generate this heat quickly because of the way the iron in the steel reacts with the surrounding air when it's shaved off by the flint, said Peter Sunderland, a fire scientist at the University of Maryland.

This is how a classic pocket lighter works, according to Sunderland. Each flick of the wheel rubs flint against steel, igniting the butane fuel inside and producing a flame.

But understanding exactly why this combination is so effective requires digging into the chemistry of oxidation. Oxidation is when a chemical element or compound combines with oxygen, changing its properties. When this process happens to iron, it's known as rusting. Using flint and steel to start a fire harnesses a side effect of oxidation: heat.

Related: What's the longest-burning fire in the world?

Early humans made tools out of flint because the rock can be shaped into arrowheads and sharp blades. Flint is much harder than steel, so striking the two together shaves off tiny bits of iron from the steel.

Iron oxidizes very easily when it's exposed to the air, but the process usually happens very slowly. A neglected car or a piece of abandoned farm equipment will take many years to become covered in rust, for example.

However, these tiny iron particles from the steel oxidize within fractions of a second, though they wouldn't look rusty to the naked eye. This creates very hot sparks. The process happens so quickly because the bits of iron have much more surface area than a bulk piece of iron.

"What's important is the surface-to-volume ratio," Sunderland told Live Science. With a small iron shaving, "the volume is basically zero, but there's lots of surface area."

So, when a tiny piece of iron is shaved off, many iron atoms are suddenly exposed to the air and can oxidize all at once. The chemical reaction rapidly generates a tremendous amount of energy as heat. And if enough of these burning-hot iron shavings fall into a pile of dry leaves or twigs, they can ignite the kindling and get a fire going.

It can be challenging to get the sparks to turn into a flame, so it's helpful to have something that the sparks can more easily ignite to accelerate the process. Shavings of steel work well, Sunderland said — they'll flame up when sparks land on them. Historically, people used a "char cloth" — a burnt piece of fabric that ignites easily and then slowly smolders, giving the kindling around it time to light.

Before steel was widely produced, humans might have generated sparks by scraping flint against other iron-rich rocks, such as pyrite, better known as fool's gold.

Other fire-starting technologies use similar principles. Magnesium fire starters, a popular off-the-shelf option, take advantage of the fact that magnesium burns very hot. So scraping shavings of magnesium into a pile of tinder and then generating sparks by scraping an iron-containing rod above them can quickly get a crackling fire going.

Matches use a completely different set of chemical reactions, but they have a similar goal: generating a lot of heat quickly to ignite a fuel source.

Sometimes, this process happens accidentally, said Sara McAllister, a research mechanical engineer with the U.S. Forest Service in the Missoula Fire Sciences Lab. For instance, wildland fires can start when someone tows a trailer with a chain dragging behind on the pavement, creating sparks. Or clashing power lines generate sparks that set dry grass ablaze.

"They're all kind of in the same realm: tiny, hot particles that land in dry kindling," McAllister told Live Science.

The new material, dubbed "goldene," could have important applications in carbon dioxide conversion and hydrogen generation, the researchers said.

To make goldene, the team employed a 100-year-old technique used by Japanese iron smiths to isolate single layers of the precious metal. They reported their work in the journal Nature Synthesis on April 16. 

Researchers are particularly interested in two-dimensional materials because of their unusual optical, electronic and catalytic properties. The extremely high surface area of these substances relative to their volume means they behave very differently than chemically-identical bulk solids, and numerous examples of 2D materials have been reported since the discovery of graphene in 2004.

Related: World's thinnest electronic device is 2 atoms thick

However, most of these materials are prepared from nonmetals or mixed compounds, and creating single-atom sheets of pure metals is much more challenging.

"Metals do not like to be lonely," Michael Yeung, a solid-state chemist at the University at Albany, told Live Science in an email. "Because the bonding in metals is delocalized, they readily will bond into themselves and agglomerate. Preparing a single layer is quite a feat because you are fighting against the metal's desire to bond with not only itself but with other sheets."

Previous attempts have run into this problem. Several teams have created a single layer of gold atoms embedded within a supporting solid, such as graphene-coated silicon carbide — "like a sort of 'sandwich' structure, using graphene as a pseudo-bread and the gold as the meat," Yeung said. But extracting the goldene from these complex layered solids proved problematic, with the gold atoms coagulating into nanoparticles as soon as the support was removed.

Shun Kashiwaya, an assistant professor in the Department of Physics, Chemistry and Biology at Linkӧping University in Sweden, and colleagues turned this approach on its head to successfully isolate goldene sheets for the first time.  

They began by creating a layered structure of titanium, silicon and carbon, which they then covered with a surface layer of gold. Over 12 hours, gold particles diffused into the material, replacing the silicon layer with gold and creating a goldene sheet embedded within the solid. However, rather than trying to remove the gold layer, the team carefully etched away all of the surrounding solid, leaving the gold sheet untouched.

They figured out the technique when study coauthor Lars Hultman, a professor in the Department of Physics, Chemistry and Biology at Linkӧping University, was researching chemical etchants. Hultman found a 100-year-old method used by Japanese smiths to etch away carbide residues in steel, Kashiwaya told Live Science. Called Murakami's reagent or alkaline potassium ferricyanide, the solution etched away the surrounding titanium carbide support, without affecting the goldene sheet.

To perfect the method, the team experimented with different reaction conditions and concentrations of the etching solution. Crucially, they found that adding a cysteine as a surfactant, or a chemical which decreases the surface tension of a liquid, stabilized the isolated sheets and prevented the gold atoms from clustering and combining into nanoparticles. 

The freestanding goldene sheets were up to 100 nanometers long and are hundreds of times thinner than ordinary gold leaf.

Kashiwaya and Hultman believe that, due to goldene's enhanced chemical reactivity, it could have important applications in reactions to convert carbon dioxide into fuels such as ethanol and methane and water into hydrogen. They are currently working on improving the synthetic method. 

"We aim to explore goldene's fundamental physical and chemical properties and further develop the synthetic process to increase both the goldene sheet area and yield," Kashiwaya said. "We also envision applying this approach to produce other elemental 2D materials (metallenes) beyond goldene."

Yeung is particularly interested in the preparation of new 2D materials made possible by this method. "The ability to selectively etch what is normally stable means that a bunch of new materials can be made," he said. 

The next step could be creating a single layer of silver using aluminas as the base, Yeung said.