CBSE Class 7 Science Physical and Chemical Changes Worksheet Set 11

Answer: Fast: explosion of a cracker, burning of cooking gas. Slow: tooth decay, growth of a plant.

Teacher's Note:
1. Fast changes finish in a very short time.
2. Slow changes take hours, days or years.

Answer: Periodic: beating of the heart, change of seasons. Non-periodic: flash flood, train accident.

Teacher's Note:
1. Periodic changes repeat after a fixed interval of time.
2. Non-periodic changes occur at irregular times.

Answer: Irreversible. The ash and gases formed cannot be changed back into paper.

Teacher's Note:
1. A new substance is formed on burning.
2. Such changes cannot be reversed by simple methods.

Answer: Undesirable, because it spoils the food and makes it unfit to eat.

Teacher's Note:
1. Desirable changes are useful to us.
2. Undesirable changes cause loss or harm.

Answer: Changes that are useful to us are desirable changes. Formation of curd from milk is an example.

Teacher's Note:
1. Another example is ripening of fruits.
2. Whether a change is desirable depends on its effect on us.

Answer: 1. No new substance is formed. 2. It is usually reversible and the change in shape, size or state is only temporary.

Teacher's Note:
1. The chemical composition stays the same.
2. Melting of ice is a good example.

Answer: 1. A new substance with different properties is formed. 2. It is generally irreversible and often involves heat, light, gas or colour change.

Teacher's Note:
1. The original substance cannot be easily recovered.
2. Rusting and burning are examples.

Answer: Burning forms new substances such as ash, carbon dioxide and water vapour. These cannot be turned back into paper.

Teacher's Note:
1. Formation of new substances is the sign of a chemical change.
2. The change is also accompanied by heat and light.

Answer: Only the size and shape of the paper change. No new substance is formed and the pieces are still paper.

Teacher's Note:
1. The composition of the paper is unchanged.
2. In a physical change only physical properties change.

Answer: Presence of oxygen (air) and moisture (water).

Teacher's Note:
1. Iron must be in contact with both together.
2. Painting or oiling iron keeps these away and prevents rusting.

Answer: (a) boiling of water. Water only changes into steam and no new substance is formed.

Teacher's Note:
1. Boiling of an egg, burning of wood and making curd form new substances.
2. They are therefore chemical changes.

Answer: (a) periodic change. Eclipses recur at regular intervals that can be predicted.

Teacher's Note:
1. The motion of the Sun, Earth and Moon repeats.
2. Periodic changes can be predicted.

Answer: periodic; fast (and chemical).

Teacher's Note:
1. Day and night repeat every 24 hours.
2. A cracker burst takes only a moment.

Answer: chemical; chemical.

Teacher's Note:
1. Rusting makes a new reddish-brown substance.
2. It is also slow and irreversible.

Answer: False. Cooking rice changes it permanently and forms new substances, so it is a chemical change.

Teacher's Note:
1. The cooked rice cannot become raw rice again.
2. Heat causes the permanent change.

Answer: True. Rust is a new substance and cannot be turned back into iron by simple methods.

Teacher's Note:
1. It is also a slow and chemical change.
2. Moisture and air are necessary.

Answer: False. Breaking changes only the shape and size, so it is a physical change.

Teacher's Note:
1. No new substance is formed.
2. Pieces are still the same material.

Answer: Reversible: melting of wax (can solidify again), evaporation of water (vapour can condense), stretching a rubber string (it returns when released). Irreversible: tearing of paper.

Teacher's Note:
1. Reversible changes can be undone.
2. Torn paper cannot be restored to its original state easily.

Answer: Melting of wax near the flame is physical because wax solidifies again. Burning of the wax vapour forms carbon dioxide and water, which is a chemical change.

Teacher's Note:
1. Both kinds of change occur at the same time.
2. The chemical change is the one that cannot be reversed.

Answer: Burning of fuel gives out heat and light. A burning cracker gives out heat, light and sound.

Teacher's Note:
1. Changes in matter are accompanied by energy changes.
2. Some changes absorb energy instead, such as melting of ice.

Answer: A lot of heat is released and slaked lime (calcium hydroxide) forms with a hissing sound. It is a chemical change.

Teacher's Note:
1. A new substance is formed with release of heat.
2. Slaked lime is used in whitewashing.

Answer: 1. Physical: no new substance is formed; chemical: a new substance is formed. 2. Physical: mostly reversible; chemical: mostly irreversible. 3. Physical: little or no energy change; chemical: heat, light or other energy is usually given out or absorbed. 4. Physical: only physical properties change; chemical: chemical properties change as well.

Teacher's Note:
1. Examples of physical changes: melting, freezing.
2. Examples of chemical changes: burning, rusting, curd formation.

Answer: Heating sugar strongly turns it black (caramel and carbon). It is an irreversible chemical change.

Teacher's Note:
1. Gentle heating only melts it, which is physical.
2. Strong heating decomposes it.