A glass of water turns cold when glucose is added to it and is stirred. When acid is added to a small quantity of basic solution, heat is released. In both the cases conventional heating agents like fire or conventional cooling mechanism like refrigeration is not employed. But how does simple addition of one type of compound to the other results in heating or cooling? What makes a system appear hot or cold? Read further for the answers.
Molecules are bunch of atoms bound together by chemical bonds. When strong bonds are formed high amount of energy is released the reverse of which, breaking bonds, require large amounts of energy. The opposite is true with the weak bonds. Chemical bonds are of different types like covalent bonds, ionic bonds, co-ordinate covalent bonds, etc. Apart from the so-called chemical bonds other inter-molecular forces that bind different molecules are also disrupted during the process of mixing of two substances. The weak-bond strong-bond concept described earlier applies to these inter-molecular forces as well.
An object appears hot when it is releasing energy and appears cold when it drains out energy from your body. When you are suffering with fever, your body is hot and is releasing energy. Since the surroundings are draining heat energy out of your body you feel cold. Now equipped with this knowledge, let’s discuss the questions posed above. When acid and base are mixed, weak bonds are broken and strong bonds are formed. When a weak bond is broken, small amount of energy is absorbed and during the formation of a strong bond, large amount of energy is released and hence there is a net release of energy. In a way, this is similar to the process of freezing or condensation taught during our Physics tuition classes, where specific latent heat of fusion or vaporization comes in.
This energy released enters the body when the container is held and hence you observe that the container is hot. On the other hand when glucose is added to water, strong bonds are broken and weak bonds are formed and hence energy is absorbed. So when you hold the glass of water, to which glucose is added, the glass absorbs heat energy from your body and hence you feel that the glass is cold.
Technically the reactions in which heat energy is released are called as exothermic reactions. Mixing of detergent in water, acid-base reactions, sodium in water are some examples of exothermic reactions. Reaction of sodium with water is highly exothermic and results in explosion. The reactions in which energy is absorbed are called as endothermic reactions. Glucose in water is one such endothermic reaction.