Question

Which of the following is not a chemical change
A. Burning of fuels
B. Rusting of Iron
C. Drying of clothes
D. Formation of curd from milk

Answer 1

Hint: A chemical change is a process that involves the chemical transformation of a chemical substance to another. They are most irreversible reactions but however, there are some changes that are said to be reversible in nature.

Complete step by step answer:
A chemical change is a change in which one chemical substance is said to have transformed into one or more other chemical substances. These changes occur through chemical reactions, in which the molecules and the ions rearrange themselves to form different structures and thus have different properties.
Now, by keeping in mind the definition of chemical change, let us look at the options provided to us.
a) Burning of fuel: It is known as combustion. Combustion is a chemical change since it involves a process in which a substance, in this case a fuel, reacts with oxygen in the air to give off heat. Therefore, burning fuel gives us a chemical change.
b) Rusting of Iron: It refers to the formation of rust on to the surface of the iron. Rust is a mixture of iron oxides on the surface of iron objects. This is also a chemical change since in this the iron from the iron surface reacts with the oxygen present in the air and this reaction takes place in the environment containing water (air containing high levels of moisture in it). Rusting of iron results in the formation of a red flaky substance that is easily crushed into powder.
c) Drying of clothes: It occurs through a process called evaporation. It is not a chemical change since the drying of clothes does not lead to a change in the chemical composition of the clothes and hence does not change its chemical properties. Therefore, it is a physical change.
d) Formation of curd from milk: It is known as fermentation. In this process the lactic acid bacteria reacts with the casein. The bacteria from the lactose uses enzymes to produce energy and the byproduct of this energy is called lactic acid which acts on the proteins present in the milk and denatures them. Therefore, it is also a chemical change.

Hence, the correct option is option (c).

Note: For a chemical change there are few that need to be kept in mind.
1. Color change
2. Formation of a precipitate
3. Formation of a gas or gas bubbles
4. Odour or taste change
5. Volume change
6. Production of light and many such changes.