Materials Their Properties And Separation Methods

The reason why the surface of a lake freezes in severe winter, but the water at its bottom is still liquid is due to the unique property of water. Water attains its maximum density at 4C. When a lake cools, the surface water loses heat, becomes denser and sinks. This process continues until all the water in the lake is at 4C. With further cooling, a stable, lighter layer of water forms at the surface. When this surface water freezes, it floats and stays at the surface of the lake until a solid sheet of ice is formed. The water at the bottom remains liquid because it is denser and hence, does not rise to the surface to freeze.

**Crystallization of sodium chloride:** This is a physical change. The sodium chloride (table salt) is changing its form from a dissolved state in water to a solid crystal structure, but the chemical composition remains NaCl. **Melting of ice:** Similar to crystallization, melting of ice is a physical change. Water (H2O) molecules are rearranging from a rigid solid structure to a more fluid liquid state, but the chemical makeup stays the same. **Souring of milk:** This is a chemical change. Milk sugar (lactose) is broken down by bacteria into lactic acid. This process involves the formation of new substances and alters the chemical composition of the milk. Therefore, only statement 3 is an example of a chemical change.

Water molecules are polar, meaning they have a positive charge on one end (the hydrogen atoms) and a negative charge on the other end (the oxygen atom). This polarity allows water molecules to attract and interact with other polar or charged molecules, facilitating the dissolution of a wide range of substances in water. Therefore, option (a) is correct.