CHEMICAL REACTIONS

CHEMICAL REACTIONS

Physical and chemical changes

 Physical changes

  • A change in which a substance undergoes a change in its physical properties (shape, density, volume ,size, colour and state of a substance) is called a physical change.
  • Most of the physical changes are temporary and reversible.
  • In such a change no new substance is formed.
  • Composition of the constituents of matter remains same during physical change.

Examples-

  • Kicking a football – When we kick a football, its position change but it remains a football .No new substance is formed. The molecules that make up the football remain the same.
  • Tearing of paper - On tearing, a paper changes its shape and size but it remains a paper. No new substance is formed. The molecules of paper remain the same.
  • Changing of size with use - The size of a pencil or eraser changes with use, but no new substance is formed. The molecules that make up the pencil and eraser remain the same.
  • Freezing of water - We can easily get ice by freezing water. The molecules of ice and water are exactly the same.
  • Heating substances - If a balloon is kept in the sun, its size increases because the air in it expands on heating. This is called expansion. When a piece of iron is heated, it expands. Similarly liquids expand on heating. If the heated substances are cooled, they regain their original size. The molecules of the substance do not change.
  • Some physical changes such as tearing of paper or changing of size of a pencil with use cannot be reversed. However, most physical changes are temporary and can be easily reversed to return substances to their previous form. Most physical changes are reversible.

 Chemical changes

  • A change in which one or more new substances are formed is called a chemical change.
  • A chemical change is also called a chemical reaction.
  • During chemical changes , composition of substance change due to change in the constituent particle of substance
  • Chemical change is permanent and irreversible.

 Examples-

  • Burning - When you burn a piece of paper, smoke and gases like carbon dioxide and water vapour are given out. The ash left behind differs from paper in its appearance and properties. Thus, new substance with different properties and different molecules are formed.
  • Cooking - When we boil an egg, it becomes hard and tastes very different. The molecules change to give new substances. Similarly cooking of rotis , vegetables or rice results in new substances being formed.
  • Rusting - When an iron chain is kept in humid air for some days, a brown substance deposits on it. This slowly eats up or corrodes the iron. It is found that the brown substance called rust has very different properties from those of iron.

  • When iron nail is kept in copper sulphate solution  -When iron nail is kept in copper sulphate solution, blue colour of copper sulphate changes to green colour due to formation of ferrous sulphate (new compound) and brown substance is deposit on iron nail is copper .

  • When vinegar is mixed with Baking soda, there is evolution of carbon dioxide gas. Vinegar (Acetic acid) + Baking soda (Sodium hydrogen carbonate) → Carbon dioxide + other substances

  • When carbon dioxide is passed through lime water, calcium carbonate is formed, which makes lime water milky. The turning of lime water into milky is a standard test of carbon dioxide.
  • Carbon dioxide (CO2 ) + Lime water [Ca(OH)2 ] → Calcium Carbonate (CaCO3 ) + Water (H2O)

CHEMICAL REACTIONS

 The process involving a chemical change is called a chemical reaction.

or

A chemical reaction is a process which transforms one or more substances into new substances.

or

The process in which a substance or substances undergo change, to produce new substances with new properties, is known as chemical reaction.

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

Products: The new substances formed as a result of the chemical reaction are called products.

For example

(Reactant)                (Reactant)                      (Product)

In the above chemical reaction hydrogen and oxygen which are written on the left hand side are reactants and water which is written on the right hand side is a product.

A chemical equation not only provides a qualitative description but also quantitative description of the chemical reaction.

For example: Consider the following chemical equation.

2CO + O2-→ 2CO2

If a chemical change takes place, it usually leads to following -

  • Change of state-When hydrogen gas reacts with oxygen gas it results in formation of liquid water.
  • Change in colour-When iron nail is dipped in blue coloured copper sulphate solution, the blue colour of solution changes to green.
  • Evolution of gas-When sodium reacts with water, bubbles of hydrogen gas is seen.
  • Change in temperature-When water is added to quick lime the vessel becomes warm due to evolution of heat. When potassium hydroxide is added to ammonium chloride the vessel becomes cold due to absorption of heat.
  • Formation of a precipitate-When silver nitrate solution reacts with sodium chloride solution it gives white precipitate of silver chloride and sodium nitrate solution.