Physical and Chemical Changes

Landslides

Reason — Changes that are not repeated at regular intervals are called non-periodic changes. They may occur randomly at any time, like landslides.
Change of day and night is a periodic change as this change occurs regularly every 24 hours due to Earth’s rotation. Seasons repeat in a fixed cycle every year due to Earth’s revolution around the Sun. Full moon and new moon nights occur in a regular cycle known as the lunar cycle.

permanent change

Reason — Burning of a piece of paper into ash is a permanent change because paper cannot be obtained back from ash. A new substance, ash, is formed and the change is irreversible.
Periodic changes occur at regular intervals (like day and night). Natural changes occur without human involvement (like rain, seasons). Reversible changes can be undone.

Rusting of iron

Reason — The changes that take longer time to complete are called slow changes. They take hours, days, months or even years to occur. Examples: Rusting of iron.
Wood burns quickly once ignited. Crackers burn and explode in seconds. The fan starts rotating almost immediately.

A glass cut into pieces

Reason — A physical change is a change in which no new substance is formed and the chemical composition of the original substance remains the same, even though some of its physical properties like colour, state, shape, size, etc. may change. A glass cut into pieces will have different sizes and shapes, but the chemical composition remains the same.
Cooking of food is a chemical change, new substances are formed and it is irreversible. Milk changes into curd due to bacterial action. A new substance is formed, so it is chemical. The egg undergoes a permanent change due to heat. It cannot return to its original form.

all of the above

Reason — A chemical change is a permanent and irreversible change in which new substances are formed whose chemical composition and physical and chemical properties are different from those of the original substance. During a chemical change, exchange of energy takes place, depending on the nature of the reaction.

Both A and R are true but R is not the correct explanation of A.

Explanation — A is true as changes which result in the formation of new substances with entirely different composition and properties are called chemical changes.
R is also true because chemical changes are generally permanent and irreversible. However, R states another characteristic of chemical changes; it does not explain the definition given in A.

Therefore, Both A and R are true but R is not the correct explanation of A is the correct option.

A is true but R is false

Explanation — A is true as a reversible and temporary change is one that can be undone by changing conditions (like melting and freezing). R is false as burning a piece of paper is an irreversible change as a new substance ash is formed.

Therefore, A is true but R is false is the correct option.

Both A and R are true and R is the correct explanation of A.

Explanation — A is true as sublimation of camphor is a physical change as no new substance is formed. When the camphor vapours cool down, they change back to solid camphor as a sublimate.
R is true as no new substance is formed in a physical change.

Therefore, Both A and R are true and R is the correct explanation of A is the correct option.

Both A and R are true but R is not the correct explanation of A.

Explanation — Assertion is correct as iron rusts only when both oxygen (air) and water are present for some time. Reason is also true as rust is a reddish-brown substance (hydrated iron(III) oxide). However, R just describes rust; it does not explain why air and water are needed for rusting.

Therefore, Both A and R are true but R is not the correct explanation of A is the correct option.

A is false but R is true.

Explanation — A is false because when a candle is lit, the wax melts (a physical change), then burns to produce carbon dioxide, water vapour, light, and heat (a chemical change).
R is true as molten wax solidifying on the floor is also a physical change. Thus, melting is a physical change, while burning is a chemical change.

Therefore, A is false but R is true is the correct option.

(a) Freezing

(b) chemical

(c) composition

(d) exothermic reaction

(e) gaseous state

(a) Sublimate

(b) Magnesium oxide (MgO)

(a) False
Correct Statement — Heating of an iron rod is a physical change.

(b) True

(c) False
Correct Statement — Rusting of iron is a slow change.

(d) True

(e) False
Correct Statement — There is no change in the mass of a substance in a physical change.

(a) Drying of wet clothes — Physical change

(b) Curdling of milk — Chemical change

(c) Butter getting rancid — Chemical change

(d) Boiling of water — Physical change

(e) Burning of paper — Chemical change

(f) Melting of wax — Physical change

(g) Burning of coal — Chemical change

(h) Formation of clouds — Physical change

(i) Rusting of iron — Chemical change

(j) Roasting of potatoes — Chemical change

(k) Formation of alloys — Physical change

(l) Making of a sugar solution — Physical change

(m) Glowing of an electric bulb — Physical change

(n) Manufacture of salt from sea water — Physical change

(a) Earthquakes

(b) Change of day and night

(c) Chopping vegetables

(d) Photosynthesis

(e) Switching on a fan

(f) Burning of candle

(a) Physical change — A physical change is a change in which no new substance is formed and the chemical composition of the original substance remains the same, even though some of its physical properties like state, colour, shape, size, appearance etc. may change.

(b) Chemical change — A chemical change is a permanent change in which new substances are formed whose chemical composition and physical and chemical properties are different from those of the original substance.

Freezing of water is a physical change because the change is reversible and the chemical composition of water remains the same. No new substance is formed. Only change of state occurs i.e. water (liquid) is converted to ice (solid) in freezing.

Evaporation of water is a physical change because the change is reversible and the chemical composition of water remains the same. No new substance is formed. Only change of state occurs i.e. water (liquid) is changed to vapour (gaseous) state.

When a piece of paper is burnt a new substance ash is produced which cannot be changed back into paper. This change is permanent and irreversible, so it is a chemical change.

Cutting a cloth piece does not produce any new substances. There is no change in the chemical composition of the cloth piece after cutting. Hence, even though it is irreversible, still it is a physical change.

The inflated balloon bursts when brought near a lighted bulb. This is because, the air inside the balloon becomes hot, the kinetic energy of its particles increases the pressure on the inner walls of the balloon and it bursts. It is a physical change.

Sublimation of camphor is a physical change that is reversible. When the evaporated vapours of camphor cool down, they change back to solid camphor as a sublimate. This shows that no new substance is formed, and the change is temporary and reversible.

The characteristics of a physical change can be summarised as follows :

1. No new substance is formed.
2. There is no change in the chemical composition of the original substance.
3. The change is temporary and it can be reversed by reversing the conditions.
4. The change is only in the state, size, shape, colour, texture or the smell of some or all of the substances that undergo physical change.
5. There is no change in the mass of the substances involved in a physical change.
6. There is usually no loss or gain of energy as a result of physical change.

The characteristics of a chemical change can be summarised as follows :

1. New substance(s) is/are formed.
2. The composition of the original substance changes completely.
3. The change is permanent and irreversible.
4. There is a change in the mass of the original substance.
5. There is an exchange of energy during a chemical change which means that energy in the form of heat and light may be released or absorbed.

When a candle is lighted, wax near the wick burns to produce new substances such as carbon dioxide and water vapour, along with the evolution of heat and light. This shows a chemical change. At the same time, some wax melts and the molten wax solidifies again on cooling. This is a physical change. Hence, burning of a candle is an example of both a physical and a chemical change.

A → Sublimation
B → Deposition
C → Solid camphor / sublimate
D → Oxygen / air
E → Heat

The change from solid camphor to white vapours and back to solid camphor is a physical change because no new substance is formed.

The change in which magnesium combines with oxygen to form magnesium oxide is a chemical change because a new substance is formed.

Gain in mass takes place in the second case, where magnesium combines with oxygen from air to form magnesium oxide.

(a) The changes below are :

(i) Solid wax melting and resolidifying is a physical change because no new substance is formed. Only the state of wax changes, and the change is reversible.

(ii) Molten wax at the wick burning to produce light, heat and smoke is a chemical change because new substances such as carbon dioxide and water vapour are formed, and energy is released in the form of heat and light.

(b) The gases produced when molten wax is burnt are carbon dioxide and water vapour.

(c) Respiration is an exothermic reaction occurring in living beings. During respiration, glucose reacts with oxygen to form carbon dioxide and water, releasing energy in the form of heat.