Chemical Reactions and Equations

Chemical reactions are the processes in which new substances with new properties are formed.

• 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 due to the chemical reaction are called products.
  E.g. The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction.
  

Characteristics of Chemical reactions

The conversion of reactants into products in a chemical reaction is often accompanied by some features which can be easily observed easily.

The important characteristics of chemical reactions are:

• 1. Evolution of a gas
  2. Formation of a precipitate
  3. Change in colour
  4. Change in temperature
  5. Change in state

Evolution of a Gas:

Some chemical reactions are characterized by the evolution of a gas.

The chemical reaction between zinc and dilute sulphuric acid is characterized by the evolution of hydrogen gas.

• Take some zinc granules in a conical flask.
• Add dilute sulphuric acid over zinc granules.
• We will see the bubbles of hydrogen gas being formed around zinc granules.
• If we touch the conical flask with our hand, we will find that it is somewhat hot. So, a change in temperature also occurs in this chemical reaction.

Formation of a precipitate:

Some chemical reactions are characterized by the formation of precipitate. A precipitate is a ‘solid product’ which separates out from the solution during a chemical reaction.

The chemical between potassium iodide and lead nitrate is characterized by the formation of a yellow precipitate of lead iodide.

• Take some lead nitrate solution in test tube.
• Add potassium iodide solution to it.
• A yellow precipitate of lead iodide is formed at once.
• A change in colour also takes place in this chemical reaction.

Change in colour:

Some chemical reactions are characterized by a change in colour.

The chemical reaction between citric acid and purple coloured potassium permanganate solution is characterized by a change in colour from purple to colourless.

• Take some dilute potassium permanganate solution in a test tube. It has purple colour.
• Add some lemon juice (it contains citric acid) to it with the help of a dropper and shake the test tube.
• The purple colour of potassium permanganate solution goes on fading and ultimately it becomes colourless.

Change in temperature:

Some chemical reactions are characterized by a change in temperature.

The chemical reaction between quicklime and water to form slaked lime is characterized by a change in temperature.

• Take a little of quicklime in a hard-glass beaker.
• Add water to it slowly.
• Touch the beaker.
• The beaker feels to be quite hot.

Change in state:

• Some chemical reactions are characterized by a change in state.
• When wax is burned (in the form of wax candle), then water and carbon dioxide are formed.
• Now, wax is a liquid whereas carbon dioxide is a gas. This means that during the combustion reaction of wax, the physical state changes from solid to liquid and gas.

Chemical equations:

• The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation.
• Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.
  
• This is known as word equation. Putting the symbols and formulae of all the substances in the above word equation, we get the following chemical equation:
  
• The substances which combine or react are known as reactants.
• The new substances produced in a reaction are known as products.
• A chemical equation is a short-hand method of representing a chemical reaction.

Balanced and unbalanced chemical equations:

A balanced chemical equation has an equal number of atoms of different elements in the reactants and products.

Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.

Count the number of atoms of all the elements in the reactants and products separately.

• There are an equal number of atoms of different elements in the reactants and products, so the above chemical equation is a balanced equation.
• An unbalanced chemical equation has an unequal number of atoms of one or more elements in the reactants and products.
• Hydrogen reacts with oxygen to form water
  Count the number of atoms of all the elements in the reactants and products separately.

The above chemical equation contains an unequal number of oxygen atoms in reactants and products, so it is an unbalanced equation.