Laws of Chemical Combinations

Ø Law of Conservation of Mass: Antoine Lavoisier established the Law of Conservation of Mass. It states that matter can neither be created nor destroyed.
In other words, we can say that during any physical or chemical change, the total mass of reactants is equal to the total mass of products. For eg.
Consider the reaction 2H2 + O2 → 2H2O
Here 4 g of H2 combines with 32 g of O2 to form 36 g of water.
Total mass of reactants = 4 + 32 = 36g. Total mass of products = 36 g

Ø  Law of Definite Proportions: A given compound always contains the same elements in the same proportion by mass.
For Example: Carbon dioxide can be formed in the atmosphere by various methods like respiration, burning of fuels, reaction of metal carbonates and bicarbonates with acid etc. All these samples of CO2 contain only two elements Carbon and Oxygen combined in a mass ratio 3:8.

Ø  Law of Multiple Proportions: When two elements combine to form two or more compounds, then the different masses of one element, which combine with a fixed mass of the other, bear a simple ratio to one another.
Hydrogen combines with oxygen to form two compounds – water and hydrogen peroxide. Hydrogen + Oxygen → Water     
      2g            16g          18g

Hydrogen + Oxygen → Hydrogen Peroxide    
      2g               32g                34g
Here, the masses of oxygen (i.e. 16 g and 32 g) which combine with a fixed mass of hydrogen (2g) bear a simple ratio, i.e. 16:32 or 1: 2.

Ø  Gay Lussac’s Law: Under similar conditions of temperature and pressure whenever gases combine together, they do so in terms of volume.
e.g.,  2 H2 (g)  +  O2 (g)  →  2 H2O (g)     
          2 vol         1 vol           2 Vol     (at same T, P)

Ø  Avogadro’s Law: Under similar conditions of temperature and pressure equal volume of all gases contains equal number of molecules.
For example: If we take 10L each of NH3, N2, O2 and CO2 at the same temperature and pressure, all of them contain the same number of moles and molecules.