Take a small amount of calcium oxide or quick lime in a beaker.
Slowly add water to this.
Touch the beaker as shown in Fig. 1.3.
Do you feel any change in temperature?
A solution of slaked lime produced by the reaction 1.13 is used for white washing walls. Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also CaCO3.
Let us discuss some more examples of combination reactions.
In simple language we can say that when two or more substances (elements or compounds) combine to form a single product, the reactions are called combination reactions.
(ii) Do you know that respiration is an exothermic process?
We all know that we need energy to stay alive. We get this energy from the food we eat. During digestion, food is broken down into simpler substances. For example, rice, potatoes and bread contain carbohydrates. These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. The special name of this reaction is respiration, the process of which you will study in Chapter 6.
C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + energy (Glucose).
(iii) The decomposition of vegetable matter into compost is also an example of an exothermic reaction.
Identify the type of the reaction taking place in Activity 1.1, where heat is given out along with the formation of a single product.