1. Law of conservation of mass : This law was given by French chemist A. Lavoisier (1774) which states that "during any physical or chemical change, the total mass of products is equal to the total mass of reactants". It is also called law of indistinctibility. It does not hold good for nuclear reaction.
2. Law of definite proportions : This law was given by Proust (1799) and states that "a chemical compound always contains some elements combined together in same proportion by mass". For example different samples of pure CO2 always have carbon and oxygen in 3 : 8 ratio by mass.
3. Law of multiple proportions : This law was given by John Dalton (1803) and states that "when two elements combine to form two or more compounds, the different mass of one of the elements and the fixed mass of the one with which it combines always form a whole number ratio". This law explains the concept of formation of more than one compound by two elements.
4. Law of reciprocal proportions : This was given by Richter (1792) and states that "when two elements combine separately with a fixed mass of third, the ratio of masses in which they do so is same or whole number multiple of the ratio in which they combine with each other." This law is also called law of equivalent proportions and is helpful in determining equivalent weights.
5. Gay Lussac's law of combining volumes : This law states that when gases react with each other, their volumes bear a simple whole no. ratio to one another and to volume of products (if gases) and similar conditions of pressure and temperature.
6. Dalton's atomic theory :
   • Proposed by John Dalton in 1808. Main points are :
   • Matter is made up, by indivisible particles called atoms
   • Atoms of same elements are identical in physical and chemical properties.
   • Atoms of different substances are different in every respect
   • Atoms always combine in whole numbers to form compounds
   • Atoms of resultant compounds possess similar properties

Drawbacks of Dalton's theory :

• Does not explain structure of atom.
• Fails to explain binding forces between atoms in compounds.
• Does not explain Gay Lussac's law.
• Does not differentiate between atom and molecule.

7. Avogadro's law :

It states that "equal volumes of all gases, under similar conditions of temperature and pressure contain equal number of molecules". Applications are:

• Deducing atomicity of elementary gases
• Deriving relationship between molecular mass and vapour density
• Deriving formula of substances
• Determining molecular wt. of a gas
• Deducing the gram molecular volume.