Class 10 Chemistry — Important Questions with Answers

What it is

A more reactive metal displaces a less reactive one from its salt solution.

Answer

Iron is more reactive than copper, so it displaces copper from copper sulphate: Fe + CuSO4 -> FeSO4 + Cu. The blue colour of CuSO4 fades as it is replaced by the pale green FeSO4, and a brown copper coating forms on the nail. This is a displacement reaction.

What it is

In a redox reaction one substance gains oxygen (oxidised) while another loses it (reduced), together.

Answer

Hydrogen gains oxygen to form water, so H2 is oxidised. Copper oxide loses oxygen to form copper, so CuO is reduced. Since oxidation and reduction happen together, this is a redox reaction; here H2 acts as the reducing agent and CuO as the oxidising agent.

What it is

pH measures how acidic or basic a solution is, from 0 (acid) to 14 (base), with 7 neutral.

Answer

The pH scale runs from 0 to 14 and measures the concentration of hydrogen ions (H+) in a solution. A pH of 7 is neutral, below 7 is acidic and above 7 is basic. As a solution becomes more acidic, the H+ concentration increases and the pH value decreases.

What it is

A neutralisation reaction is an acid reacting with a base to give a salt and water.

Answer

A neutralisation reaction is the reaction between an acid and a base to form a salt and water. The general equation is: Acid + Base -> Salt + Water (for example, HCl + NaOH -> NaCl + H2O). An everyday use is taking an antacid (a mild base) to neutralise excess acid in the stomach and relieve acidity.

What it is

Metals higher in the reactivity series displace those lower down, never the reverse.

Answer

In the reactivity series zinc lies above copper, so zinc is more reactive and loses electrons more readily. Therefore zinc displaces the less reactive copper: Zn + CuSO4 -> ZnSO4 + Cu. Copper, being less reactive than zinc, cannot displace zinc, so Cu + ZnSO4 does not react.

What it is

Carbon forms millions of compounds because it makes four strong covalent bonds and chains (catenation).

Answer

Carbon has four valence electrons, so it shares electrons to form four strong covalent bonds rather than gaining or losing electrons. Two special properties help: catenation (carbon atoms link with other carbon atoms to form long chains, branches and rings) and tetravalency (it can bond with four other atoms such as hydrogen, oxygen, nitrogen). Together these give a huge number of stable compounds.

What it is

A homologous series is a family of carbon compounds with the same general formula, differing by CH2.

Answer

A homologous series is a group of organic compounds having the same general formula and the same functional group, in which each successive member differs from the previous one by a -CH2- unit. Two characteristics are: (1) all members have similar chemical properties because of the same functional group, and (2) their physical properties such as boiling point change gradually as the molecule gets bigger. The alkanes (CH4, C2H6, C3H8, ...) are an example.