Science [4 Mark Questions]

The effects of chemical reactions are:

1. Evolution of gas:
Gas evolution may take place as a result of chemical reactions. Example: When dilute hydrochloric acid is added to a solution of sodium carbonate or sodium bicarbonate carbon dioxide gas is evolved.

2. Change in colour:
Certain chemical changes produce change in colour. Example : When you place a iron nail in a solution of copper sulphate, the blue colour of copper sulphate slowly changes into green due to chemical reaction between iron copper sulphate solution.

3. Change in state:
Certain chemical reaction change of state is observed. Example : When you bum a piece of camphor, smoke comes out as result of chemical reaction between solid camphor and oxygen. Here, there is a change of state from solid to gas.

• Spoilage of food and vegetables:
  Food spoilage may be defined as any change that causes food unfit for human consumption. The chemical reactions catalyzed by the enzymes result in the degradation of food quality such as the development of bad taste and odour, deterioration, and loss of nutrients.
  Example 1: Rotten eggs develop a bad smell due to the formation of hydrogen sulphide gas.
  Example 2: Decaying of vegetables and fruits due to microbes.
• Rancidity of fishes and meat:
  Fishes and meat containing high levels of polyunsaturated fatty acids that undergo oxidation cause a bad odour when exposed to air or light. This process is called rancidity.
• Apples and fruits turn brown when cut:

1. Apples and some fruits turn brown due to a chemical reaction with oxygen in the air. This chemical reaction is called browning.
2. The cells of apples, fruits and other vegetables contain an enzyme called polyphenol oxidase or tyrosinase, when coming in contact with oxygen catalyses a biochemical reaction of plants, phenolic compounds to brown pigments known as melanins.

Generally, there are three types of pollutions viz air, water, and land pollution. Due to increasing human activities lot of chemical substances are produced artificially which harms all the living and non – living things.
Types of chemical substances and their effects:

S. No	Type of pollution	Chemical substances responsible for the pollution	Effects
1.	Air pollution	Carbon dioxide, Carbon monoxide, oxides of sulphur, oxides of nitrogen, Chlorofluorocarbons, methane, etc.	Acid rain, Global warming, respiratory problems, etc.
2.	Water pollution	Waste water containing chemical substances (e.g dyeing industries), detergents, oil spillage, etc.	Decrease in quality of water, skin diseases, etc.
3.	Land pollution	Fertilizers like urea, various pesticides, herbicides, etc.	Spoilage of land, cancer, respiratory diseases, etc.

The conditions required for a chemical reaction to take place:

1. Physical contact
2. Solution of reactants
3. Electricity
4. Heat
5. Light
6. Catalyst