Chemical Reactions and Equation

Types of Changes in Matter

• Physical Change
• Chemical Change (Chemical Reaction)

Evidences that hint that a chemical change has happened:

• Change in state:
  • \(\ce{$\underset{\mathrm{Gas}}{\ce{Hydrogen}}$ + $\underset{\mathrm{Gas}}{\ce{Oxygen}}$ ->[burning] $\underset{\mathrm{Liquid}}{\ce{Water}}$ }\)
• Change in temperature:
  • \(\ce{Coal + Oxygen -> Carbon dioxide + Heat}\)
• Change in colour:
  • \(\ce{$\underset{\mathrm{red- orange}}{\ce{Copper}}$ + $\underset{\mathrm{colourless}}{\ce{Oxygen}}$ ->[] $\underset{\mathrm{black}}{\ce{Copper Oxide}}$ }\)
  • \(\ce{$\underset{\mathrm{Silver-Grey}}{\ce{Iron}}$ + $\underset{\mathrm{colourless}}{\ce{Oxygen}}$ ->[rusting] $\underset{\mathrm{reddish-brown}}{\ce{Iron Oxide}}$ }\)
• Evolution of a gas:
  • \(\ce{Paper -> [burning] Smoke (Gas)}\)
• Formation of precipitate:
  • \(\ce{$\underset{\mathrm{}}{\ce{Lead nitrate}}$ + $\underset{\mathrm{}}{\ce{Potassium iodide }}$ ->[] $\underset{\mathrm{ppt.}}{\ce{Lead iodide }}$ + Potassium nitrate }\)

Note:

These observations indicate a possibility of chemical change. They are not confirmatory evidences of a chemical reaction. Example: In melting of ice, state changes from solid to liquid, but it is a physical change. Dissolving glucose in water cools down the water, but it is also a physical change.

The only confirmatory observation that a chemical change has taken place is the change in identity of the matter.

Reactants and Products

In a chemical reaction, one or more substances convert into another.

• Reactants: The matter which undergoes conversion during a chemical change.
• Products: The substance that forms during a chemical reaction.

Example: in the reaction Carbon + Oxygen → Carbon dioxide, Carbon and Oxygen are reactants. Carbon dioxide is the product.

What happens in a Chemical Reaction?

In a chemical reaction, atoms regroup themselves to form new compounds by breaking old bonds and making new ones. For example, in the reaction, Hydrogen + Oxygen → Water