​Chemical Reaction:

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The transformation of a chemical substance into another chemical substance is known as Chemical Reaction. For example: Rusting of iron, the setting of milk into curd, digestion of food, respiration, etc.

​Chemical Reaction:

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The transformation of a chemical substance into another chemical substance is known as Chemical Reaction. For example: Rusting of iron, the setting of milk into curd, digestion of food, respiration, etc.

Chemical reaction, a new substance is formed which is completely different in properties from the original substance, so in a chemical reaction, a chemical change takes place.

Only a rearrangement of atoms takes place in a chemical reaction.

• The substances which take part in a chemical reaction are called reactants.

• The new substances produced as a result of a chemical reaction are called products

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Example: The burning of magnesium in the air to form magnesium oxide is an example of a chemical reaction.

2Mg(s) + O2(g)     △     →     2MgO(s)

Before burning in the air, the magnesium ribbon is cleaned by rubbing with sandpaper.

This is done to remove the protective layer of basic magnesium carbonate from the surface of the magnesium ribbon.

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Characteristics of Chemical Reactions :

(i) Evolution of gas: The chemical reaction between zinc and dilute sulphuric acid is characterized by the evolution of hydrogen gas.

Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) ↑

(ii) Change in Colour: The chemical reaction between citric acid and purple-colored potassium permanganate solution is characterized by a change in color from purple to colorless.

The chemical reaction between sulfur dioxide gas and acidified potassium dichromate solution is characterized by a change in color from orange to green.

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(iii) Change in state of substance: The combustion reaction of candle wax is characterized by a change in state from solid to liquid and gas (because the wax is a solid, water formed by the combustion of wax is a liquid at room temperature whereas, carbon dioxide produced by the combustion of wax is a gas). There are some chemical reactions that can show more than one characteristic.

(iv) Change in temperature: The chemical reaction between quick lime water to form slaked lime is characterized by a change in temperature (which is a rise in temperature).

The chemical reaction between zinc granules and dilute sulphuric acid is also characterized by a change in temperature (which is a rise in temperature).

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