Safe endothermic reaction that gets below freezing?

Question

I have a project/idea I am working on but I am only 14 years old and I need some help.

I'm looking for an endothermic reaction that gets below freezing, but the chemicals before the reaction and after the reaction is over are safe to touch and safe for the environment. The reaction can be harmful to plant life, but only for a short time before it decays or whatever needs to happen for it to be safe. I do not want it to have long term effects.

Additionally, two important parts are that these substances need to be easy to obtain and need to not to be a fertilizer.

Answer 1

The reaction of baking soda and citric acid (both are easy to come by and safe) in water does cool the water noticeably. I have tried it multiple times, and while it does not reach freezing, it will reliably reach $6\ ^\circ \mathrm{C}\ \left(43\ ^\circ \mathrm{F}\right)$, which is cold to the touch.

Add 4 teaspoons each of sodium bicaronate (baking soda) and citric acid to a cup. Then add the smallest amount of water you can and start stirring!

You can tell a chemical reaction is occurring because a gas is produced:

$$\ce{3NaHCO3 + H3C6H5O7 -> Na3C6H5O7 + 3CO2(g) + 3H2O}$$

But, wait! We want a temperature below freezing! It might seem a little like cheating, but the reaction still works. Carbon dioxide bubbles are still produced, and the temprature gets down to around $-6\ ^\circ \mathrm{C}\ \left(21\ ^\circ \mathrm{F}\right)$.

If you do not have a thermometer, you can prove the second version is colder than the freezing point of water by doing the following:

1. Get a small, flat piece of wood and dampen the top surface of it.
2. Do the reaction in a glass, using 4 teaspoons each baking soda and citric acid and replacing the water with crushed ice.
3. As the temperature drops, the water between the wood and the glass will freeze, freezing the glass to the wood. If you lift the glass, the wood will stick to it.

Answer 2

The evaporation of hexane is endothermic. n-Hexane has a low enthalpy of vaporization at $31.56\ \mathrm{kJ\ mol^{-1}}$ at $25^\circ \mathrm{C}^{[1]}$, compared to water, which is $43.99\ \mathrm{kJ\ mol^{-1}}$ at $25^\circ \mathrm{C}^{[2]}$. This means that hexane quickly and easily absorbs heat from its surroundings, and can get well bellow $0^\circ \mathrm{C}$. Indeed, I recall performing an experiment in high school whereby paper towels dipped in different solvents were wrapped around the end of thermometers, and the lowest temperatures were recorded. By far hexane was the lowest at something below $-10^\circ \mathrm{C}$.

n-Hexane is definitely not a fertilizer, and doesn't do any serious damage to the environment like chlorofluorocarbons do. Your main concern would be storing it in a cool well ventilated space and keeping it away from open flames, as it is very volatile and flammable. Additionally I would recommend you work with nitrile gloves, as they are insoluble in hexane. Hexane is a very non-polar molecule and can dissolve your epithelial cell membranes. It's not immediately dangerous, however, so just take caution not to get too much on your skin.

If you find hexane too hard to obtain, acetone would have similar qualities, and is easily obtained at the grocery store as nail polish remover.

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[1] Majer, V.; Svoboda, V., Enthalpies of Vaporization of Organic Compounds: A Critical Review and Data Compilation, Blackwell Scientific Publications, Oxford, 1985, 300.

[2] Marsh, K. N., Ed., Recommended Reference Materials for the Realization of Physicochemical Properties, Blackwell, Oxford, 1987.