State two ways to prevent the rusting of iron.

The two ways to prevent rusting of iron are as follows- (i) Galvanization: Articles that are exposed to extreme moisture such as roof sheets and pipes are protected from rusting by galvanizing. Galvanization is a method of protecting steel and iron from rusting by coating them with a thin layer of zinc. The iron coated with zinc is called galvanized iron or simply G l . As zinc is more reactive than iron, it reacts with oxygen in the presence of moisture to form an invisible layer of basic zinc carbonate Z n C O 3 . Z n O H 2 which protects it from further rusting. The galvanized iron articles remain protected from rusting even if zinc coating is broken. This is because zinc is more reactive than iron. (ii) Painting: In materials like railings, window, grills, lamp posts, iron gates, iron bridge, railway coaches, bodies of cars, buses and trucks, etc., painting the metal surface does not allow them to come in contact with the moist air and thus, prevents rusting.

Name two metals which are found in nature in the free state.

The two metals which are found in nature in the free state are gold and silver. Apart from this platinum and copper also exist in the free state. They are non-reactive in nature. They are placed at the bottom of metal reactivity series.

What happens when manganese dioxide is heated with aluminium powder. Write only chemical equation.

It is a displacement reaction in which heat is evolved. Aluminium is more reactive than manganese. Therefore, the following chemical reaction will take place when manganese dioxide is treated with aluminium powder to form aluminium oxide and manganese. 3 MnO 2 s + 4 Al s → 3 Mn l + 2 Al 2 O 3 s + Heat

Why do ionic compounds have high melting points?

There are strong electrostatic forces of attraction between oppositely charged ions in ionic compounds. Considerable amount of energy is required to break strong inter-ionic force of attraction. Therefore, they have high melting points. On dissolving in water ionic compounds form a solution in which ions can move. This means that these solutions can conduct electricity.

Food cans are coated with tin and not with zinc because

zinc has a higher melting point than tin

You must have seen tarnished copper vessels being cleaned with lemon or tamarind juice. Explain why these sour substances are effective in cleaning the vessels.

Copper reacts with moist carbon dioxide in the air to form a green layer called patina. As a result, the copper vessel loses its shine and is said to be tarnished. This is due to the formation of copper carbonate which is basic in nature. The citric acid present in the lemon or tamarind helps to neutralise the basic copper carbonate thereby dissolving the layer by neutralisation reaction. This is the reason why tarnished copper vessels are cleaned with lemon or tamarind juice to give the surface of the copper vessel its characteristic lustre.

What are amphoteric oxides? Give two examples of amphoteric oxides.

The oxides which show basic, as well as acidic behaviour are known as amphoteric oxides. They can act as either an acid or base in a reaction to produce a salt and water. Examples of amphoteric oxide are zinc oxide and aluminium oxide