Physical and Chemical Changes Test - 1

Change brought about by a person or the nature, which is useful is called desirable change. On the other hand, change brought about by a person or the nature, which is harmful is called desirable change.

Formation of curd is a desirable change as it is more easily digestible than milk. On the other hand, rusting of iron is undesirable change as rusting makes an article of iron weak.

When a substance is heated, its constituent particles move more vigorously. As a result, the average distance between the particles increases and the substance expands. Thus, during summers, the joints of a bridge expand and become loose.

The change from a dough to an unbaked roti represents a reversible change. A dough ball can be rolled out into a roti. Similarly, an unbaked roti can be rolled back into dough.

This question requires the student to know various chemical and physical changes. Corrosion of iron, in the presence of oxygen and moisture, is known as rusting. Rusting is a chemical change, as an oxide of the metal is formed on its reaction with oxygen. Pure iron cannot be obtained after it has been corroded.

The correct answer is D.

Why alternative A is wrong:

Freezing is a process wherein a liquid changes into solid. This is a physical process as it does not involve any chemical change. Hence, this alternative is incorrect.

Why alternative B is wrong:

Cutting of an object splits the object into multiple parts without changing its constitution. Hence, cutting is a physical (and not chemical) change. Hence, this alternative is incorrect.

Why alternative C is wrong:

Melting is the process wherein a substance changes from solid to liquid. This process cannot be classified as a chemical change. Hence, this alternative is incorrect.

During a physical change, the chemical composition of the substance undergoing the change remains the same. No chemical reactions take place during a physical change. A physical change involves a change in physical state and can usually be reversed. No new substance is formed during the change.

Melting is the process of phase change in which a substance changes from its solid state to its liquid state by absorbing heat energy.

Evaporation is the process of phase change in which a substance changes from its liquid state to its gaseous state by absorbing heat energy.

The correct answer is B.

Why alternatives A, C and D are wrong:

Freezing is the process of phase change in which a substance changes from its liquid state to its solid state by releasing heat energy.

Condensation is the process of phase change in which a substance changes from its gaseous state to its liquid state by releasing heat energy.

Changes that are not accompanied by the formation of a new substance are called physical changes whereas changes that are accompanied by the formation of a new substance are called chemical changes. In the process of cutting paper, no new substance is formed. Hence, it is a physical change. On the other hand, when paper is burnt, ashes and smoke are produced. Moreover, smoke and ashes cannot be converted to paper again. Therefore, burning of paper is a chemical change.

A process where a change can be reversed is called a reversible process. A process where a change cannot be reversed is called an irreversible process.

When a matchstick is burnt, ash and gas are produced. Thus, the formation of a new substance takes place. These new substances cannot be converted back into the original substance. Hence, the burning of a matchstick is an irreversible process.

If the new substance formed cannot be converted back into the original substance, then the change is said to irreversible

Earthquake does not occur repeatedly or at regular intervals of time. So, it is a non-periodic change.