Science and Technology for UPSC Examinations [Chemistry] Part 2- Some Basic Laws

Law of Conservation of Mass

The law of Conservation of Mass states that matter can neither be created nor destroyed. This law was the result of exact measurement of masses of reactants and products, and carefully planned experiments performed by Lavoisier.

Law of Definite Proportions

French chemist, Joseph Proust gave another law which is called “Law of Definite Proportions“. This law stated that a given compound always contains exactly the same proportion of elements by weight.

Law of Multiple Proportions

Another law, called “Law of Multiple Proportions” was given by Dalton in 1803. This law says that if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers. For example, hydrogen combines with oxygen to form two compounds, namely, 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.

Law of Gaseous Volumes

The Gay Lussac’s Law of Gaseous Volumes said that when gases combine or are produced in a chemical reaction they do so in a simple ratio by volume provided all gases are at same temperature and pressure. Thus, 100 mL of hydrogen combine with 50 mL of oxygen to give 100 mL of water vapour.

Hydrogen + Oxygen → Water

100 mL 50 mL 100 mL

Thus, the volumes of hydrogen and oxygen which combine together (i.e. 100 mL and 50 mL) bear a simple ratio of 2:1.

In 1811, Avogadro proposed that equal volumes of gases at the same temperature and pressure should contain equal number of molecules.

Dalton’s Atomic Theory

Dalton proposed the atomic Theory which says:

• Matter consists of indivisible atoms.
• All the atoms of a given element have identical properties including identical mass.
• Atoms of different elements differ in mass. Compounds are formed when atoms of different elements combine in a fixed ratio.
• Chemical reactions involve reorganisation of atoms.
• These are neither created nor destroyed in a chemical reaction. Dalton’s theory could explain the laws of chemical combination.