Corrosion and rancidity

Corrosion: It is the process in which metals are slowly eaten up by the action of air moisture or chemicals. For example rusting is a form of corrosion in which iron is eaten up by the action of air and moisture and a reddish brown coating of iron oxide is formed as shown in the following chemical reaction.

Rancidity: When the substance containing oils and fats are exposed to air they get oxidised and become rancid due to which their smell, taste and colour change. This process is known as rancidity. For example when a when butter is kept open for a long time then its smell and taste gets changed.

Chemical Equations

Chemical Equations:  It is the symbolic representation of a chemical reaction where the reactant entities are given on the left hand side and the product entities on the right hand side of the equation. In complete chemical equation all reactants and products are written along with their physical state. A chemical equation must be balanced so that it can follow the law of conservation of mass. For example

Identify the substances that are oxidised and the substances that are reduced in the following reactions.

3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.

Answer:
(i)The substance oxidised is sodium and the substance reduced is oxygen gas.
(ii) The substance oxidised is Hydrogen gas and the substance reduced is copper oxide.

Give an example of a double displacement reaction other than the one given in Activity 1.10.

2. Give an example of a double displacement reaction other than the one given in Activity 1.10.

Answer:

Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer:When an iron nail dipped in the copper sulphate solution than iron displaces copper from the copper sulphate because iron is more reactive than copper. Therefore the colour of the copper sulphate solution changes.
The reaction involved is

Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

Answer:
The chemical reaction involved in the electrolysis of water is

From the above reaction it is clear that the amount of hydrogen gas and the oxygen gas formed are always in the ratio of 2:1. Therefore the volume of gas collected in the first test tube is double the volume of the gas collected in second test tube.

A solution of a substance ‘X’ is used for white washing. (i) Name the substance ‘X’ and write its formula. (ii) Write the reaction of the substance ‘X’ named in (i) above with water.

1. A solution of a substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Answer:

(i)The substance ‘X’ which is used in white washing is calcium oxide or quick lime and its formula is CaO.
(i)The reaction involved is

Write the balanced equation for the following chemical reactions. (i) Hydrogen + Chlorine Hydrogen chloride (ii) Barium chloride + Aluminium sulphate Barium sulphate + Aluminium chloride

2. Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine  Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate   Barium sulphate + Aluminium chloride
(iii) Sodium + Water   Sodium hydroxide + Hydrogen
Answers:

Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

3. Write a balanced chemical equation with state symbols for the following reactions.
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Answers:

Why should a magnesium ribbon be cleaned before burning in air?

1. Why should a magnesium ribbon be cleaned before burning in air?

Magnesium ribbon must be cleaned before burning in air so that the the layer of magnesium oxide (which is formed due to reaction of magnesium with air) can be removed in order to get the desired chemical reaction.