Samacheer Class 8 Science - Changes Around Us

Chemical changes are otherwise called as chemical reactions, because one or more substances (Reactants) undergo a reaction to form one or more new substances (Products).
Reactant(s) → Product(s)

When iron nail is dipped in the solution of copper sulphate, copper is displaced by iron and forms ferrous sulphate. Thus, the blue colour of copper sulphate changes into green because of formation of ferrous sulphate.

Tarnishing of metal articles:
Shiny metal surfaces and other articles lose their shining appearance due to chemical reactions on the surface. For example, silver articles become black on exposure to atmospheric air.
Similarly, brass vessels which contain copper as one of constituents develop a greenish layer on exposure to air for a long time. This is due to a chemical reaction between copper and moist air to form basic copper carbonate and copper hydroxide.

A concentrated solution of sodium chloride called Brine is electrolysed to produce chlorine and hydrogen gases along with sodium hydroxide. This is a very important reaction to produce chlorine industrially.

Substances used to alter the speed of a chemical reaction, are called catalysts. For example, metallic iron is used as a catalyst in the manufacture of ammonia by Haber’s process.

1. Photosynthesis is a process in which light energy from the sun is used by the plants to prepare starch from carbon dioxide and water.
2. The sunlight uses the chemical reaction between carbon dioxide and water, which finally ends up in the production of starch (photo means light and synthesis means production). These chemical reactions in used by light are called as photochemical reactions.
Hot Questions

Yeast is a key ingredient in the production of baked goods.
Yeast is a bio – organic catalyst carbon dioxide is generated by the yeast as a result of the breakdown of fermentable sugars in the dough and makes cakes to rise.

The combustion of fossil fuels also releases a large amount of carbon dioxide into the atmosphere. Carbon dioxide is a greenhouse gas which is responsible for global warming. Humans bum fossil fuels, releasing huge amounts of carbon pollution and trapping more and more heat in the atmosphere.

Rain becomes acidic in nature due to the presence of certain pollutants in the air released by cars and industrial processes.
Acid rain is caused by emissions of sulphur dioxide and nitrogen oxide which react with the water molecules in the atmosphere.

No. Rusting is not good for iron materials. Rust is a form of iron oxide. It occurs when iron combines with the oxygen in the air causing it to corrode. Rust can affect iron and its alloys. It makes them weaker, by replacing the strong iron with flaky powder.

No, not all the fruits. E nzymic browning can be observed in fruits such as apricots, pears, bananas, grapes and avocados and vegetables such as aubergines, potatoes, lettuce. The enzyme polyphenol oxidase or tyrosinase that when in contact with oxygen catalyses a biochemical reaction of plants phenolic compounds to brown pigments known as melanin, which results in the fruit or vegetable turning brown.

1. 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 taste and odour, deterioration and loss of nutrients.
Example 1 : Rotten eggs develop a bad smell due to formation of hydrogen sulphide gas.
Example 2 : Decaying of vegetables and fruits due to microbes.
2. Rancidity of fishes and meat:
Fishes and meat containing high levels of polyunsaturated fatty acids that undergo oxidation causes bad odour when exposed to air or light. This process is called rancidity.
3. Apples and fruits turn brown when cut:
Apples and some fruits turn brown due to chemical reaction with oxygen in air. This chemical reaction is called browning.
The cells of apples, fruits and other vegetables contain an enzyme called polyphenol oxidase or tyrosinase, when come in contact with oxygen catalyses a biochemical reaction of plants, phenolic compounds to brown pigments known as melanins.

Conditions required for a chemical reaction to take place:
Chemical reactions can be done through:
Physical contact
Solution of reactants
Electricity
Heat
Light
Catalyst
1. Chemical Reactions Based On Solution Of Reactants:
When we mix two substances (Reactants) in solution form, the chemical reaction takes place to form new substances (Products).
Example:
Take small amount of solid silver nitrate and sodium chloride in a test tube. The reactants in solid state have no reactions.
Dissolve the same reactants in water in separate test tubes.
Mix both the solutions. Silver nitrate solution reacts with sodium chloride solution to form a white precipitate of silver chloride and sodium nitrate solution.
From the above reaction, we infer that some chemical reactions proceed only in solution form not in solid form.
2. Chemical Reaction Based On Electricity:
When electricity is passed through water containing small amounts of sulphuric acid, hydrogen and oxygen gases are liberated.
Similarly, a concentrated solution of sodium chloride called Brine is electrolysed to produce chlorine and hydrogen gases along with sodium hydroxide.
This is a very important reaction to produce chlorine industrially.
From the above two reactions, we infer that some chemical reactions proceed only by the passage of electricity.
Hence, such reactions are called as electrochemical reaction or electrolysis.
3. Chemical Reactions Based On Light:
Sunlight is important not only for us but also for plants as welll. As you know photosynthesis is a process in which light energy from the sun is used by the plants to prepare starch from carbon dioxide and water.
Example:
The sunlight uses the chemical reaction between carbon dioxide and water, which finally ends up in the production of starch (photo means light and synthesis means production). These chemical reactions is used by light are called as photochemical reactions.

:
Yes, Ramesh is right in his suggestion because old iron rods were rusted.
The older rods are brownish and they are rusted. Rusting destroys the quality of the iron rods and harmful for construction.
Responsible behaviour, correct decision, caring and awareness.

Yes, Rajasekar is more concerned over his family health than saving money.
Rajasekar know that the air pollution from nearby chemical industry would be harmful and affect his family health.
Environmentally sensitive, awareness, caring and correct decision.