a) Production of heat:
Chemical reactions produce heat energy. These reactions are called exothermic reactions. (ex) When water is added to quick lime, a lot of heat is produced.
b) Production light:
i. some chemical reactions produce light. (ex) when a piece of magnesium ribbon is burnt in a flame, bright light is produced with heat.
ii. Fireworks produce different colored lights due to a chemical reaction.
c) Production of sound:
i. some chemical reactions produce sound. (ex) Crackers produce sound by undergoing some chemical reactions.
ii. Hydrogen gas continues burning a match stick with a pop sound due to chemical reaction.
d) Production of pressure:
i. Some chemical reactions generate pressure.
ii. Some chemical reactions produce gases which build up pressure in a closed container.
iii. If the pressure level goes beyond the limit, we get the explosion.·
iv. (ex.) When explosives, fireworks are ignited, they explode due to the pressure generated by gases from the chemical reactions.
e) Evolution of gas:
i. In some chemical reactions. gases are evolved.
ii. (ex.) When dilute hydrochloric acid is added to a solution of sodium carbonate, carbon dioxide gas is evolved.
f) Change in color:
i. Some chemical reactions produce a change in color.
ii. (ex.) When an iron nail is placed in copper sulphate solution, the colour changes into green due to chemical reaction between iron and copper sulphate solution.

a) Environment is the place around us that comprises both living and non-living things.
i) Our environment provides air to breathe, water to drink and the land to produce food.
ii) Due to human activities like increasing industries, increasing number of automobiles, etc., a lot of chemical substances are produced artificially.
iii) So there is an unwanted change in physical, chemical and biological properties of the environment.
iv) This is termed as pollution.
v) This harms all the living and non-living things.
Types of pollution:

b) Rusting: when iron materials are exposed to air and water, they undergo a chemical reaction called rusting. This makes iron soft and weak.
c) Tarnishing: When silver objects are exposed to air, they undergo a chemical reaction and the colour changes to black. This is called tarnishing. When copper objects are exposed to air, they undergo a chemical reaction and the colour changes to green.

Biological effects:
a) Spoilage of food and Vegetables:
i. Food spoilage may be defined as any change that causes food unfit for human consumption.
ii. The chemical reactions catalysed by the enzymes result in the degradation of food quality.
iii. It develops bad tastes, odour, deterioration and loss of nutrients.
iv. (eg.) 1. Rotten eggs develop a bad smell due to the formation of hydrogen sulphide gas.
v. (eg.) 2. Decaying of vegetables and fruits due to microbes.
b) Rancidity of fishes and meat:
i. Fishes and meat contain high levels of polyunsaturated fatty acids.
ii. When exposed to air or light these polyunsaturated fatty acids undergo oxidation causing bad odour.
iii. This process is known as rancidity.
c) Apples and fruits turn brown when cut:
i. The cells of apples, fruits and other vegetables contain an enzyme called poly phenol oxidase or tyrosinase.
ii. This enzyme when comes in contact oxygen catalyses a biochemical reaction of plants phenolic compounds to brown pigments known as melanins.
iii. This chemical reaction is browning.

(i) Chemical reactions based on contact:
Certain chemical reactions take place only when the reactants are brought in contact with each other in their physical states.
Eg: When quicklime comes in contact with water, it forms slaked lime.
(ii) Chemical reactions based of solution of reactants:
When we mix two substances in solution form, the chemical reaction takes place to form new substances.
Eg: Barium chloride solution reacts with sulphuric acid solution to form barium sulphate as a white precipitate and hydrochloric acid.
(iii) Chemical reactions based on electricity:
Some chemical reactions proceed only by the passage of electricity. Such reactions are called electrochemical reaction or electrolysis.
Eg: When Brine is electrolysed it produces chlorine and hydrogen gases along with sodium hydroxide.

(i) 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 development of bad tastes and odour, detorioration and loss of nutrients.
Eg: Rotten eggs develop a bad smell due to the formation of hydrogen sulphide gas.
Decaying of vegetables and fruits due to microbes.
(ii) Rancidity of fishes and meat:
Fishes and meat contains high levels of polyunsaturated fatty acids.
These fatty acids undergo oxidation when exposed to air or light.
It results in bad odour. This process is called rancidity.
(iii) Apples and fruits turn brown when cut:
It is due to chemical reaction with oxygen in air.
The cells of apples contain an enzyme called tyrosinase that when in contact with oxygen catalyses a biochemical reaction of plant's phenolic compounds to brown pigments known as melanins.

a) burning of crackers during festivals
Heat
b) Fading of coloured clothes on drying under sunlight
Heat
c) Cooking of eggs.
Heat
d) Charging of batteries.
electricity

No, all the fruits and vegetables do not undergo browning. The cells of certain fruits like apples contain an enzyme called tyrosinase when in contact with oxygen catalyses a biochemical reaction of plant's phenolic compounds to brown pigments. So, the fruits and vegetables which contain tyrosinase undergo browning.

No, rusting is not good for iron materials. Rust is formed when an iron surface comes into contact with water and oxygen. It results in the degradation of iron. Thus the metal will be weakened and become brittle and flaky.

Burning fossil fuels is a cause of global warming because it produces Co, and methane. These gases give the greenhouse effect. This increase the Earth's average temperature and results in the phenomenon called Global warming.

Yeast is a biocatalyst when yeast is added while making cakes
i) it triggers fermentation
ii) it releases carbondioxide which results in the expansion of dough.
iii) it is responsible for the soft porous texture of the cake
iv) It is the keyfactor behind the characteristic bakery flavour of a cake.