There are three substances X, Y and Z. The substance X does not have a fixed melting point or boiling point and it still shows the individual properties of its constituents. The substance Y is a pure substance which occurs in nature as such. The substance Y has a fixed melting point and boiling point but it cannot be broken down into simpler substances by any chemical means. The substance Z is also a pure substance whose properties are entirely different from those of its constituents. The substance Z can, however, be divided by electrolysis into two substances which belong to the same class of substances as Y.
(a) What type of substance could X be? Name one substance like X.

(b) What type of substance could Y be? Name one substance like Y.

(c) What type of substance could Z be? Name one substance like Z.

(d) Which process involves absorption or release of an appreciable amount of energy: formation of substance X or formation of substance Z?

(e) Name the three groups into which all the substances like Y are divided on the basis of their properties.

Separate the following into physical and chemical changes:

Sublimation of a solid, Decomposition of water into hydrogen and oxygen by passing electric current,

Formation of clouds, Making a fruit salad from raw fruits, Dissolving carbon dioxide in water.

(a) What is a physical change? Give two examples of physical changes.

You are given two samples of water labelled as ‘An’ and ‘B’. Sample ‘A’ boils at 100°C and sample ‘B’ boils at 102°C. Which sample of water will not freeze at 0°C? Comment.

Rusting of an article made-up of iron is called:

Classify the following as physical or chemical changes:

(i) Cooking of food

(ii) Boiling of water

(iii) Cutting of trees

(iv) Dissolving salt in water

(v) Digestion of food

(vi) Melting of ice

Which of the following are physical changes and which are chemical changes?

(a) Burning of a magnesium wire

(b) Freezing of water

(c) Rusting of iron

(d) Glowing of an electric bulb

Classify the following as physical or chemical changes:

(i) Formation of curd from milk

(ii) Condensation of steam

(iii) Growth of a plant

(iv) Breaking of a glass tumbler

The rusting of an iron object is called:

Which of the following are chemical changes?

(i) decaying of wood

(ii) burning of wood

(iii) sawing of wood

(iv) hammering of nail into wood

Which of the following are physical changes?

(i) melting of iron metal

(ii) rusting of iron metal

(iii) bending of an iron rod

(iv) drawing wire of iron metal