A chemical reaction occurs only when the substances are brought in _____________ with each other.

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Solution

Chemical reaction:
When one or more substances (elements or compounds) undergo a chemical change with the absorption or evolution of energy to give new substances (called products), this change is called a chemical reaction.
A chemical reaction occurs only when the substances are brought in close contact with each other.
For example Sodium reacts with water only when it is brought in contact with water.
$\underset{sodium}{2 Na (s)} + \underset{water}{2 H_{2} O (l)} \to \underset{sodium hydroxide}{2 NaOH (aq)} + \underset{hydrogen gas}{H_{2} (g)}$
Therefore, a chemical reaction occurs only when the substances are brought in contact with each other.

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(a) A reaction in which two or more substances combine to form a single substance is called a ___ reaction.

(b) A ___ is a substance which changes the rate of a chemical reaction without undergoing a chemical change.

(d) The reaction between an acid and a base is called ___

(e) Soluble bases are called ___

(f) The chemical change involving iron and hydrochloric acid illustrates a ___ reaction.

(g) In the type of reaction called ___ two compounds exchange their positive and negative radicals respectively.

(h) A catalyst either ___ or ___ the rate of a chemical change but itself remains ___ at the end of the reaction.

(i) The chemical reaction between hydogen and chlorine is a ___ reaction

(j) When a piece of copper is added to silver nitrate solutuions, it turns ___ in colour.

In a chemical reaction, the substances which take part in a chemical reaction are called products.

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