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Chemical Reactions and Equations

Chemical Changes and their Representation in the Form of Chemical Equations

Chemical Changes



A chemical change can be confirmed by any or all of the following observations:
  • change in state
  • change in colour
  • change in temperature
  • evolution of gas

A chemical change is always accompanied by a chemical reaction. Reaction is the term used for depicting a change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances. Let us see how we can represent a chemical change using a reaction.

Thus, a chemical equation is an easier and more concise method for representing a chemical reaction. It involves writing symbols and formulae (instead of words) for all substances involved in the reaction. A chemical equation also indicates the number of atoms of each element involved in a reaction.

Try to represent the statements given below as chemical equations.

(a) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

(b) Hydrogen gas combines with nitrogen to form ammonia.

Symbols of elements:

Potassium = K

Hydrogen =H

Nitrogen = N

 

Questions asked in previous years’ board examinations


Ques.

Write any two observations in an activity which may suggest that a chemical reaction has taken place. Give an example in support of your answer.

(2 marks)

2010 CBSE Delhi

Sol: A chemical change can be confirmed by any of the following observations:

  • Change in temperature
  • Evolution of gas

For example: Calcium oxide reacts vigorously with water to produce calcium hydroxide. During this process, a large amount of heat is also evolved, which increases the temperature of the system. This confirms that a chemical reaction has taken place.

Also, when calcium carbonate is heated, it decomposes to form calcium oxide and carbon dioxide.

In this reaction, calcium carbonate breaks down to form calcium oxide and carbon dioxide. Here, evolution of the gas (carbon dioxide) confirms that a chemical reaction has taken place.

To describe a chemical reaction more concisely, equations of the reactions are written. 

Chemical equation

A chemical equation is a concise form which uses symbols and formulae of the chemical compounds or elements involved in the reaction. It also indicates the number of atoms of each element involved in a reaction.

In a chemical reaction, ...

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