It's fun to watch chemistry labs explode on video, but you know what's even more fun? Watching a chemistry experiment in action, with a good explanation of what's going on. That's science at its most awesome. Here are ten of our favorite chemical reactions caught on video — which also include excellent commentary on what you're seeing.
10) Hydrogen combustion
When hydrogen and oxygen are bound together in a ratio of 2:1, you get one of the most important molecules on Earth: water. So how do you get hydrogen to pair up with oxygen? One way is demonstrated here. H2 gas in the balloon reacts with oxygen in the air to give H2O and a spare oxygen atom. That oxygen atom can then reach with hydrogen to make more water.
This reaction generates heat, which can, in turn, provide enough energy to spark a chain of H2 + O2 reactions. But you need an energy source to start the chain reaction in the first place — that's what the electric match in this video is for. It's an incredibly fast reaction (the hydrogen on the Hindenberg, for instance, is said to have burned out in just 90 seconds), one that's responsible for some of the most notable explosions of the 20th century — Chernobyl and the Challenger disaster, for example.
9) The barking dog reaction
The barking dog experiment involves the reaction of either nitrogen monoxide — NO — or nitrous oxide — N2O) (aka laughing gas) — with carbon disulfide. Why is it called the barking dog? Watch the video. In this clip, the reaction is performed with N2O, which is mixed with the vapors from added carbon disulfide. On a molecular level, this reaction is similar to the hydrogen-oxygen reaction; a chain-reaction-igniting "activation energy" is provided in the form of a match, and as the combustion wave travels down the tube, it compresses the gas ahead of it, giving rise to the characteristic "barking" noise, as well as a bright blue light. This blast of blue is one of the few examples of chemical luminescence in the gas phase, and is so bright that the reaction was once used as a flash in low-light photography.
8) Potassium in water
Potassium is a highly reactive metal. Expose it to air and it will quickly interact with oxygen and water vapor (which helps explain why you don't find it in nature). Expose it to liquid water, however, and things get especially explosive, rapidly generating potassium hydroxide and hydrogen gas, the latter of which quickly ignites. It reacts so violently with water, in fact, that it is actually stored in anhydrous oil so that when you remove it from a storage container it doesn't interact with water vapor in the air.
7) NaK alloy and water
If you continued watching the video on potassium above, you may have heard chemist Martyn Poliakoff and his beautifully coiffed hair start talking about NaK, a sodium-potassium alloy with borderline mythically reactive potential. Sodium, potassium, lithium, and all the other elements in the first column of the periodic table (excluding hydrogen) are what are known as alkali metals, and all of them are extremely reactive. Mix sodium and potassium together and — according to Poliakoff — you get an especially reactive alloy. I couldn't find any videos of NaK reacting mid-air, but I did find the clip you see here; and the violent reaction of the NaK amalgam with water in a bowl placed over snow-covered ground is definitely a sight to behold (though we can't say we're particularly fond of this pair's lack of attention to safety).
6) Hydrogen peroxide and sodium iodide
This one's an oldie but a goodie, and it's one you may remember from your high school chemistry class. When you combine hydrogen peroxide with potassium iodide, the result is a prodigious volume of oxygen gas. Perform this reaction in a standard beaker, and this gas — which is colorless — will just dissipate without much incident. But throw some dish soap into the mix and it becomes possible to visualize just how much gas is being produced by the reaction. The process is also pretty exothermic (i.e. the reaction produces a fair bit of heat), which is why these kids burned themselves doing the same experiment.
5) The Old Nassau Reaction
The Old Nassau demonstration (also known as "the Halloween reaction" for reasons that should be readily apparent) is what is known as a "clock" reaction, on account of the delayed chemical interactions that give rise to the conspicuous changes in the mixture's color.
Here, three clear solutions of sodium bisulfate, mercury(II)chloride, and potassium iodate are combined. The first two solutions are mixed together, then added simultaneously to the potassium iodate. After a few seconds, the mixtures turn orange-red as mercury iodide precipitates out of solution (the mercury iodide is said to "precipitate out" because it is not soluble in the mixture). A few seconds later, the mixture rapidly becomes black in color as a starch-iodine complex is formed. Notice how the reaction kinetics differ between the beakers? That's because the mixtures in this demonstration have been diluted from left to right with increasing amounts of water, which slows the rate of the reactions.
4) The Briggs–Rauscher reaction
This experiment belongs to a small number of descriptively named oscillating chemical reactions. Oscillation reactions are a little like clock reactions that repeat themselves.
According to Wired, this experiment was first demonstrated over 30 years ago, but researchers are still trying to understand how this reaction — which involves an impressive cocktail requiring various quantities of water, hydrogen peroxide, soluble starch, potassium iodate, sulfuric acid, malonic acid and manganese sulfate — works. Aaron Rowe explains what we do know:
Several reactions take place at once. One of them produces iodine, which gives the amber color. Hydrogen peroxide reduces other chemicals into iodide ions. Along with normal iodine, the charged particles interact with starch to create it a blue-black color. The speeds of those transformations are constantly changing. As one overtakes the other, the color suddenly changes.
Researchers have observed the stirred mixture go through upwards of 15 cycles before finally ending as a blue-black mixture. It's worth pointing out that the explanation provided by Rowe is a vastly simplified account of what's going on in the video seen here. A much more in-depth explanation (which, believe it or not, has still been simplified) can be found here.
3) The Tollens reaction — aka the "silver mirror" test
Chemists often need to know whether a certain compound contains an aldehyde or a ketone — two structurally similar chemical groups commonly found in organic compounds. One way to do this is with Tollens' reagent, a clear mixture of sodium hydroxide, silver nitrate, and ammonia that has the chemical formula Ag(NH3)2NO3.
There's some sodium hydroxide in the mixture as well, but the first component is the important bit, because when Tollens' reagent is mixed with an organic compound that contains an aldehyde, the Ag(NH3)2NO3 will react, causing elemental silver to fall out of the solution and onto the inner surface of whatever container the reaction is taking place in. If that container is glass, it will take on the appearance of a mirror.
In the demonstration you see here (which was performed at the 2011 Ig Nobel awards ceremony), a glucose solution (glucose contains an aldehyde group) is added to an unusually high volume of Tollens' reagent and swirled around the inside of the round bottom flask in which the reaction is taking place. Just wait for it — the result is pretty impressive. (It bears mentioning that a half-naked man in silver body paint is completely optional for this reaction to take place.)