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Practice each question, then open the answer or worked solution.
What type of oxides are formed when non-metals combine with oxygen?
Solution: When non-metals combine with oxygen they form either acidic or neutral oxides. Ex: N2O5 or N2O3 is an acidic oxide; CO is a neutral oxide.
Give reasons
(a) Platinum, gold and silver are used to make jewellery.
(b) Sodium, potassium and lithium are stored under oil.
(c) Aluminium is a highly reactive metal, yet it is used to make utensils for cooking.
(d) Carbonate and sulphide ores are usually converted into oxides during the process of extraction
(a) Platinum, gold and silver are used to make jewellery as these metals are very less reactive. Hence, they are not affected by air, water or most chemicals. These metals have a lot of luster and they are malleable and ductile in nature, and also high corrosion resistance in nature.
(b) Sodium, potassium and lithium readily react with water to produce a lot of heat. As a result, hydrogen evolved in the reaction results in a fire. On exposure to water they react with moisture (water droplets) present in the atmosphere, in order to prevent contact with water. Hence, these metals are stored under oil.
(c) Aluminium forms on its surface a nonreactive surface of aluminium oxide. Such coating prevents other compounds from reacting to aluminium. So aluminium is used to produce utensils for cooking.
(d) Reducing metal oxide into free metal is easy. Additionally, because it is easier to obtain metals directly from their oxides than from their carbonates or sulphides, the carbonate and sulphide ores are first transformed to oxides to obtain the metals.
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.
Solution: Tarnished copper vessels are cleaned with lemon or tamarind because this sour substance contains acids which dissolve the coating of copper oxide or basic copper carbonate present on the surface or tarnished copper vessels. This makes them shining red-brown again. Hence, they are very effective in cleaning tarnished copper vessels.
Differentiate between metal and non-metal on the basis of their chemical properties.
| Metals | Non-metals |
| When metals are heated with oxygen, they form ionic oxides which are basic in nature and form bases on dissolving with water. This turn red litmus paper to blue. | When non-metals are heated with oxygen, they form covalent oxides which are acidic in nature, which form acid on dissolving with water. This turn blue litmus paper to red. |
| They are electro positive, lose electrons readily and become a positive ion. | They are electro negative, gain electrons and become negative ions. |
| Metals are lustrous. | Non-metals are non-lustrous; graphite is the exception |
| Reducing agents. | Good oxidising agents. |
| Metals are the good conductors of electricity and heat. | Non-metals are non-conductors of electricity and heat; graphite is the exception |
| All metals are solids except mercury. | Non-metals are in solid-liquid and gaseous states. |
A man went door to door posing as a goldsmith. He promised to bring back the glitter of old and dull gold ornaments. An unsuspecting lady gave a set of gold bangles to him which he dipped in a particular solution. The bangles sparkled like new but their weight was reduced drastically. The lady was upset but after a futile argument the man beat a hasty retreat. Can you play the detective to find out the nature of the solution he had used?
Solution: The goldsmith used a solution called Aqua regia which is referred to as royal water in Latin. It is the mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio of 3:1. Aqua regia is capable of dissolving noble metals like gold and platinum. When upper-layer of dull gold ornament is dissolved they lose their weight.
Give reasons why copper is used to make hot water tanks and not steel (an alloy of iron).
Solution: Copper is used to make hot water tanks and not steel (an alloy of iron) because copper does not react with either water or steam, whereas iron reacts with steams to corrode the tank.