[♪ INTRO] As far as chemical reactions go, none are quite as altogether beautiful, useful,  and dangerous as fire.

Fires require three things to come  together: a fuel source, heat, and oxygen. While you’re probably familiar with most  things that can catch fire, like wood or gas, there are some everyday materials  with unexpectedly fiery properties.

And since a lot of them are found in our homes, knowing about them might even keep you safe! So here’s 5 weirdly flammable things. To create fire, you need a lot of heat to  gain enough kinetic energy for molecules from the fuel source to break their  interactions and transition to the gas phase.

Those molecules can then react with oxygen in the surrounding air in a process called oxidation. The oxidation process then releases  even more heat, which helps free more molecules from the fuel to react with oxygen in a self-sustaining chain reaction we call fire. The heat and light released from  that process make up the familiar, visible flames we see in burning objects.

The minimum temperature at which all this  can happen is called the flash point. And different materials  have different flash points. Anything that has a flash point of 37.8°C degrees Celcius and upwards is considered  “combustible,” such as wood and paper.

That’s because they take  some work to light on fire since their flash point is  way above room temperature. On the other hand, things with flash  points closer to room temperature, like alcohols, are called “flammable.” Because they can evaporate quickly  and their fumes can catch fire. So, in general, the lower the flash point, the more easily something can be set on fire.

And something that can catch fire  easily, although it has a pretty high flash point, is plain ol’ flour  like the kind used to make bread. While it seems simple and harmless,  as far as cooking ingredients go, in the right circumstances,  flour is pretty explosive. The main component of flour is  starch, which is a carbohydrate.

The molecules in a carbohydrate  contain lots of energetic bonds between atoms of carbon and hydrogen. Oxidation can release the energy in  those bonds, making them easy to burn. When it’s all lumped together in a pile,  flour isn’t especially likely to catch fire, but when it’s dispersed in the air, like a  cloud of dust, that’s a whole different story.

That’s because the individual  grains of flour are small, giving them a high surface  area relative to their size. So when they’re suspended in the air,  they have access to lots of oxygen surrounding them, making it even easier to burn. What’s more, because those grains are so  small, they burn up really fast and light up nearby grains in the  process, spreading the fire way quicker than it would in a  single, lumpy fuel source.

So if a spark or nearby flame  catches part of a floury dust cloud, it can set the whole thing alight! In 2015, that exact scenario happened  at a water park in Northern Taiwan. A celebration involving bursts of  colored corn flour turned into a fireball that injured hundreds of people.

Thankfully, most cooking recipes that include  flour involve getting it wet with water, making it much less dangerous in the kitchen. But even so, it pays to be  careful when throwing bags of flour around any flames, like a stove! The same goes for other powdery foods  like powdered coffee creamers too.

And fire risks aren’t limited to  the stuff in the kitchen, either. Many nail polish removers, and  other household stain removers, contain a key ingredient: acetone. Acetone is great at dissolving things  like nail polish and can be easily washed away with water.

That combination naturally makes  it great at removing stains, but unfortunately, acetone, as you’ve  probably guessed, has another property: it’s really flammable! Its flash point is at -18°C,  making it easy to set alight. And like flour, its molecules  have similar carbon-hydrogen bonds that release lots of  energy during oxidation.

Finally, acetone molecules don’t  interact as strongly with each other in liquid form, so they have a tendency  to evaporate from the surface. In other words, it's pretty “volatile.” That’s why nail polish remover has  such a strong and distinct smell, since the acetone floats  easily into the air as a vapor. And as we saw for flour, when small,  tiny bits of material float around, it makes them much easier to set on fire.

In fact, people have gotten seriously hurt  applying nail polish remover near open sources of flames like candles, since  acetone vapor can set alight very quickly. So if you’re going to use nail  polish remover on a quiet night in, safety officials recommend you  don’t have any candles nearby. In other cases, certain  materials themselves might not be combustible or flammable,  but can still cause a fire.

Glass doesn’t burn very well at all. But it can still turn into a secret  arsonist, under the right conditions. This time the culprit is the physics  of glass, rather than its chemistry.

When light interacts with a  dense but transparent material, like glass, it effectively gets slowed down. And because of that, if a ray of light  approaches that material at an angle to its surface, the path of that  ray is also bent away from the surface as it travels through it. Physicists describe this as Snell’s law.

So, if you have a curved  piece of glass, like a lens, it can focus lots of incoming rays of  light to a small spot called a focal point. That’s how magnifying glasses  work, using Snell’s law. The light comes from a wide field  of view, but narrows down to a spot where we can see it all  if we align it with our eyes.

Magnifying glasses can also focus a distant,  bright light source like the sun to a spot where a lot of the heat and light  gets concentrated to a single point. And much like a lens, curved pieces of  glass-like snow globes, flower vases, mirrors, and even some windows can  produce a similar kind of effect. It can also just so happen that a combustible  or flammable material might happen to align with the effective focal point  of that glass, and be set on fire.

Fires have been caused by  objects like mirrors sitting near window sills in people’s homes. On a large scale, the effect  can be even more dramatic. In 2013, a curvy building in  London made of glass melted part of a car on a nearby street  as it reflected the sun’s rays!

Which happened for around two hours a day, so the building had to be retrofitted with  sunshades so it could stop melting things. So it might be a good idea  to keep crystal flower vases, bottles of water, and mirrors  away from the windowsill. So far, we’ve talked about  things catching fire when they receive heat from some other source.

But there are strange instances  where the fuel itself can generate enough heat to  basically set itself on fire. Linseed oil is made from flax  seeds, and it’s often used on wooden furniture to protect it and make  grain patterns in the wood more visible. At first, it might not be all  that surprising that linseed oil can catch fire, since, well,  lots of oils can be burned.

But in the right conditions,  linseed oil can basically set itself on fire because of the way it dries. Most liquids dry by evaporating  off water or some other solvent in the mixture, and  leaving the rest behind. But linseed oil dries through  oxidation, basically the same process through which you create fire.

That doesn’t mean that it  bursts into flames immediately since there’s normally not enough heat  to make the reaction self-sustaining. But, as linseed oil dries through  oxidation, it generates heat, and if it gets hot enough,  it can set itself on fire. That’s especially problematic since people apply linseed oil to furniture by  rubbing it in with pieces of cloth.

And then when they’re done with the  cloths, they often throw them into a pile. As the oil in the pile of cloth  dries, the cloths heat up, and their collective heat can increase  until the oil and cloths catch fire. In 2014, a pile of linseed-soaked cloths was identified as the source of  a fire in a home in Maryland.

In fact, in 1991, a whole  high-rise tower in Philadelphia caught on fire for the same reason. So if you do use linseed oil  to spruce up the furniture, safety officials recommend  you wash the rags you use and hang them out where there’s  lots of airflow before storing. Finally, there’s an object that  really doesn't seem like it should be flammable but burns  incredibly easily: ping pong balls!

Specifically, older ping pong  balls are surprisingly flammable. A few people have noticed this and  speculated that it’s because there was a gas put into the balls during the manufacturing  process that burns really well, but, to be clear, it’s just ordinary air in there. It’s the actual material of the balls, which  is called celluloid, that burns really well.

Until about 2020, ping pong  balls were still being made of that stuff since it’s a  light and durable plastic that gave the balls the right  properties for being whacked about. Although it has a high flash point of over 175°C, once celluloid hits that point,  it can start burning without needing a spark or flame to get  started, and it burns really easily. Celluloid gets its ignitable properties  from a substance called nitrocellulose, which was used as a replacement for  gunpowder in weapons during the 1800s.

By the middle of the century,  American entrepreneurs discovered that dipping nitrocellulose in wax turned  it into a hard, enduring substance, and patented it as celluloid. Which was  one of the first manufactured plastics. Celluloid enjoyed a long  stint in lots of applications, famously including rolls of film  like the kind used in projectors.

Unfortunately, when the lamp  in the projectors got hot, they had the tendency to make the celluloid  film dramatically burst into flames. Because of its flammable tendencies,  celluloid fell out of use in most cases, though ping pong balls were the exception  until 2014, when they started to be phased out because of the fire risks  of transporting lots of balls around. But older ping pong balls  are still made of celluloid, so you probably want to keep them  away from any sources of flame.

And celluloid hasn’t totally disappeared either. It’s still used to make, amongst  other things, guitar picks! But thankfully, as far as we  know, there haven’t been any unintentional fires caused by guitar picks,  so for now, there’s nothing to fret about.

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