MAK LABS — Std 7 General Science Chapter 13 Changes: Physical and Chemical

1Distinguish between the following.

(a) Physical change and chemical change

Physical change	Chemical change
No new substance is formed.	A new substance is formed.
The composition does not change.	The composition changes.
It is usually temporary (reversible).	It is usually permanent (irreversible).
Example: melting of ice.	Example: burning of wood.

(b) Periodic change and non-periodic change

Periodic change	Non-periodic change
It occurs again and again after a fixed time.	It does not occur after a fixed time.
We can tell when it will happen.	We cannot tell when it will happen.
Example: day and night.	Example: earthquake.

(c) Natural change and man-made change

Natural change	Man-made change
It occurs on its own.	It is brought about by human beings.
We cannot control it.	We can control it.
Example: ripening of fruit.	Example: cooking food.

2Under which different types can the following changes be classified?

(a) Conversion of milk into yoghurt

Chemical, slow, useful, irreversible, man-made change.

(b) Bursting of a cracker

Chemical, fast, irreversible, man-made, harmful change.

(c) Occurrence of an earthquake

Natural, fast, harmful, non-periodic change.

(d) Revolution of the earth around the sun

Natural, periodic, slow, useful change.

(e) Stretching of a spring

Physical, fast, reversible, man-made change.

3Give reasons.

(a) While purchasing canned food, its expiry date should be checked.

Canned food remains good only for a certain period. After the expiry date, the food gets spoiled. Its taste and nutrients change. It may cause food poisoning. So we should check the expiry date.

(b) An iron article should be given a coat of paint.

Iron rusts when it comes in contact with air and moisture. Rusting makes the article weak. A coat of paint stops air and moisture from reaching the iron. So the article does not rust.

(c) A wooden article should be polished.

Wood absorbs moisture from the air. It may swell, crack or rot. Termites may also attack it. Polish stops moisture from entering the wood. It also makes the article shiny.

(d) Copper and brass utensils should be tinned.

Copper and brass react with sour food items like curd and lemon. Harmful substances are formed. They are bad for health. A coat of tin (kalhai) stops this reaction. So these utensils are tinned.

(e) A dry handkerchief gets wet at once on dipping in water, but it takes long for a wet handkerchief to dry.

A dry handkerchief soaks water at once. So wetting is a fast change. A wet handkerchief dries by evaporation. Evaporation takes place slowly. So drying is a slow change.

4What will you take into account while identifying the following?

(a) A physical change in a substance.

No new substance is formed.
Only the shape, size or state changes.
The change is usually temporary.

Example: melting of wax.

(b) A chemical change in a substance.

A new substance is formed.
Colour, smell or taste may change. Gas, heat or light may be given out.
The change is usually permanent.

Example: rusting of iron.

5Read the paragraph and note down the various types of changes it mentions.

It was nearing six o’clock in the evening. The sun was setting. A breeze was blowing. Leaves on the tree were shaking. Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys. Then he felt hungry and went into the house. Mother made a dough from wheat flour and fried purees. While eating hot purees, his attention was drawn outside the window. It had started raining. There was lightning, too. Sahil was enjoying his dinner in the dim light.

Setting of the sun – natural, periodic, slow change
Blowing of breeze – natural, non-periodic change
Shaking of leaves – natural, physical, fast change
Making toys from wet soil – man-made, physical change
Feeling hungry – natural, periodic change
Making dough from flour – man-made, physical change
Frying puris – man-made, chemical, irreversible change
Rain – natural, physical, useful change
Lightning – natural, fast, harmful change

★Project: Visit a place where work like powder coating, spray painting is done. Make a note of the information you obtain.

Powder coating and spray painting protect metal articles from rusting. They also make them look attractive.
The article is first cleaned of rust, oil and dust.
Spray painting: Liquid paint is sprayed with a spray gun. It dries in air.
Powder coating: Dry colour powder is sprayed. The article is heated in an oven. The powder melts and forms a hard coat.
Workers wear masks, gloves and goggles for safety.

13. Changes: Physical and Chemical

Exercises