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Class seven science Physical and chemical change

Chapter 6: Physical change and chemical change

3. Single displacement reaction


In this reaction, one element replaces another element from a compound. It is found that a more reactive element replaces a less reactive element from its compound.

(i) Zinc is a more reactive element than copper. If pieces of zinc are added to a solution of copper sulphate, zinc sulphate is formed. Copper metal gets deposited on the zinc piece.

(ii) If a piece of copper is dipped in silver nitrate solution, it gets coated with silver, and the colour of the solution changes to bluish due to the formation of copper nitrate.

4. Double displacement reaction


In this reaction, two compounds react by exchanging, their radicals to form two new compounds by mutual exchange of their ions.

When a solution of barium chloride is added into a solution of zinc sulphate, barium exchanges its chloride ions with sulphate ions of zinc sulphate and form a precipitate of barium sulphate.

There are two types of double displacement reactions.
(a) Precipitate reactions: In these types of double displacement reactions, two compounds react in their aqueous (solution) state and form an insoluble product which is called a precipitate.
(i) When aqueous sodium chloride is mixed with aqueous silver nitrate, they react and form aqueous sodium nitrate and a white precipitate of silver chloride.

(ii) When aqueous calcium chloride is mixed with aqueous sodium carbonate, they react and form aqueous sodium chloride and a precipitate of calcium carbonate.

(b) Neutralisation reactions: When an acid reacts with a base to form a salt and water, such a reaction is known as an acid-base reaction or a neutralisation reaction.

(i) When sulphuric acid reacts with sodium hydroxide solution which is a
base, they form sodium sulphate (salt) and water.

(ii) When hydrochloric acid reacts with potassium hydroxide which is a base, they form potassium chloride (salt) and water.

Oxidation and reduction reactions

1. Oxidation


A chemical reaction that involves the addition of oxygen to a substance or removal of hydrogen from a substance is known as an oxidation reaction.
When magnesium reacts with oxygen to form magnesium oxide it is said
that magnesium is oxidised by gaining oxygen.

Oxidising agent: The substance which oxidises other substances by providing oxygen to them or by removing hydrogen from them is known as an oxidising agent. Hydrogen sulphide reacts with sulphur dioxide to form sulphur by oxidation.

In this reaction, sulphur dioxide acts as an
oxidising agent because it oxidises hydrogen
sulphide to sulphur.


2. Reduction


A chemical reaction that involves the addition of hydrogen to a substance or removal of oxygen from a substance is known as a reduction reaction.
When hydrogen is passed over heated copper oxide, they react to form copper and water by reduction. Here, copper oxide is reduced to copper by the loss of oxygen.

Reducing Agent: The substance which reduces other substances by providing Hydrogen to them or by removing oxygen from them is known as a reducing agent. Iron oxide reacts with carbon to form iron and carbon dioxide. Here iron oxide is reduced to form iron by the removal of oxygen.

Difference between oxidation and reduction

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