Give scientific reasons :

*1. Sodium and potassium is stored under kerosene.

Ans. 1. Sodium (Na) and Potassium (K) belongs to group IA, so they are alkali metals. 2. They are highly reactive metals. 3. Sodium and potassium react with oxygen in air at room temperature to form metallic oxide. They catch fire and start burning when kept open in the air. Hence, it is stored under kerosene oil to prevent its reaction with oxygen and moisture.

2. Kitchen articles like pressure cooker, pan etc. are anodized.

Ans. 1. Anodising is a technique used to make aluminium surface resistant to corrosion and abrasion. 2. In this process, the anode reaction results in formation of black coloured thin film of aluminium oxide on the surface of article. 3. Kitchen articles like pressure cooker, pan etc are constantly subjected to heat, pressure and water causing corrosion and abrasion. 4. To avoid this, kitchen articles are anodized.

3. Aluminium oxide is an amphoteric oxide.

Ans. 1. Metal oxides that show both acidic as well as basic properties are known as amphoteric oxides. 2. They react with acid as well as base to form salt and water. 3. Aluminium oxide behaves as an acid while reacting with a base and behaves as a base while reacting with an acid to form salt and water.
Hence, aluminium oxide is an amphoteric oxide.


4. Sodium and chlorine are poisonous substances but sodium chloride is edible.

Ans. 1. Sodium chloride is a compound of sodium and chlorine. 2. Physical and chemical properties of constituent elements are not retained in a compound (i.e. properties of sodium and chlorine are different as compared to sodium chloride.) 3. When chemically bonded, these two poisonous substances form a compound sodium chloride so safe that we eat it every day.

5. Some elements occur in free state while others occur in the combined state.

Ans. 1. Some elements like noble gases do not have the tendency to combine with others or even amongst themselves because they have a stable electronic configuration of duplet or octet. Therefore, they do not have the tendency to lose or gain or share electrons. Therefore, they occur in free state. 2. While most of the elements like hydrogen, oxygen, sodium, chlorine etc. are active elements. These elements have incompletely filled outermost orbits and hence show the tendency to lose or gain or share electrons in a chemical change. Therefore, they occur in combined state.

6. Ionic compounds are formed due to the combination of metals and non- metals.

Ans. 1. A metallic atom loses electrons and gets converted into a positively charged ion (cation).
2. A non-metallic atom gains electrons and gets converted into a negatively charged ion(anion). 3. These cations and anions have opposite charge and experience a strong attraction for each other, so they remain tightly held together forming ionic or electrovalent bond. Hence ionic compounds are formed due to the combination of metals and non-metals.


7. An aqueous solution of sodium chloride conducts electricity.

Ans. 1. A substance shows conduction of electricity due to the movement of electrons or ions. 2. In aqueous solution, sodium chloride splits into sodium ions (Na+) and chloride (Cl–). These ions are free to move in the solution. Hence, the aqueous solution of sodium chloride conducts electricity.

8. Pure water is a poor conductor of electricity but aqueous solution of NaCl conducts electricity.


Ans. 1. Pure water does not contain any free ions 2. However, when the ionic compound NaCl is dissolved in water, positively charged Na+ ion and negatively charged Cl– ion get separated from each other. 3. Therefore, Na+ and Cl– ions are present in water i.e. aqueous solution of sodium chloride. These ions are free to move in the solution and hence aqueous solution of NaCl conducts electricity.

*9. Elements like gold, silver are used to make jewellery.

Ans. 1. Gold is a yellow shining metal. 2. Gold metal does not corrode because it is a highly unreactive metal that remains unaffected by air, water vapour and other gases in the atmosphere. 3. Gold does not tarnish and retains its luster for years. 4. Since gold does not corrode, gold ornaments look new after years. So, it is used to make jewellery because of its bright shiny surface and high resistance to corrosion.

10. Elements like gold, silver are found in free or native state.

Ans. 1. Metals like gold, silver are placed at the bottom in the reactivity series. 2. They are least reactive metals. 3. So, elements like gold, silver are found in free or native state.

11. Electrolysis of active metals like Na, K, Al is not carried out by using aqueous solution of their chloride.

Ans. 1. Decomposition of an electrolyte by passage of electric current through it is called as electrolysis. 2. The metals extracted by electrolysis by passing electric current are very pure, they do not contain impurities. 3. If we carry out electrolysis of aqueous solution of sodium chloride, as soon as sodium metal is produced, it will react with water present in aqueous solution to form sodium hydroxide. Thus we get sodium hydroxide instead of sodium metal. So, sodium, potassium, calcium metals are obtained by electrolysis of molten chloride.