Questions and Answers

Does baking soda lower water temperature? If so, why?

Baking soda added to water raises the temperature slightly. Chemical reactions are either endothermic or exothermic. Endothermic means you have to put energy (heat) in to make the reaction go while exothermic means there's energy (heat) left over. Left over heat will raise the temperature. Baking soda and water is exothermic and so the water gets a little warmer. This is because the binding energy of the chemical bonds of the products has an excess over the binding energy of the components. Therefore, energy is released and the water warms up.

Endothermic reactions do not usually go off at room temperature without heating. Baking soda is NaHCO₃ and water is H₂O. Added together you get NaOH and H₂CO₃ which are sodium hydroxide and carbonic acid. Carbonic acid is the fizz in soda. Eventually the mixture releases CO₂ gas when the H₂CO₃ breaks down into H₂O and CO₂. The solution of H₂O and CO₂, if left alone uncorked, will lose its fizz. The water CO₂ mixture will cool off slightly when the fizz (CO₂) is released because the CO₂ takes some energy with it. As the water evaporates it cools off because the warm molecules of water evaporate first! That's right! The warm ones!

When we say temperature we really mean the average temperature of the molecules in a glass of water. Some are colder than average, some are warmer and many are close to the average. Evaporation requires energy, called the latent heat of vaporization. This heat comes from the warm water molecules. When the water evaporates it cools the glass off. That's why sweating makes you cool off too.

A reaction that is endothermic will take place at a much slower rate because there are always some warmer molecules in the mixture. When we warm up the chemicals we raise the average temperature and a lot of the molecules can then react. When you mix bread dough the mix usually gets noticeably warmer after you put in all the ingredients. This is probably safe to try out at home with some adult help. Just ask your folks to help you whip up a batch of "soda bread." You can eat the result!

Author:

Paul Brindza, Experimental Hall A Design Leader (Other answers by Paul Brindza)