Question

Answer the following questions:
(A) Write any two chemical changes that take place due to physical contact between the reactants. [1 Mark]

(B) Sunil studied the characteristics of two metal samples X and Y which were exposed to the atmosphere for a long time. He observed a black coating on X and a hard brown layer on Y. Based on his observations, can you identify X and Y? [1 Mark]

(C) Explain the term rancidity. Mention any three methods for preventing rancidity of food.​ [3 Marks]

Answer 1

(A) (i) When quick lime (calcium oxide) comes in contact with water, it forms slaked lime (calcium hydroxide). [0.5 Marks]
(ii) When a matchstick is rubbed on the sides of a matchbox, a chemical reaction takes place to form heat, light and smoke. [0.5 Marks]

(B) $\bullet$When silver is exposed to an atmosphere containing hydrogen sulphide ${\text{H}}_2\text{S}$) gas, it forms a black layer of silver sulphide (${\text{Ag}}_2\text{S}$). It is called tarnishing of silver. Therefore, sample X is silver [0.5 Marks]
$\bullet$Iron forms a reddish brown layer of rust, (${\text{Fe}}_2{\text{O}}_3.{\text{nH}}_2\text{O}$), when it comes in contact with moisture and oxygen. This is called rusting of iron. Thus, sample Y is iron [0.5 Marks]

(C) The phenomenon which leads to the spoilage of food due to oxidation of oils and fats is called rancidity. It makes food undesirable and unsafe for consumption. [1 Mark]
Methods for preventing or retarding rancidity of foods are:​
•by adding anti-oxidants to foods containing fats and oils
•by keeping food in a refrigerator
•by storing food in air-tight containers [2 Marks]