Laws of Chemical Combination-
1. Law of conservation of mass: It was proposed by Lavoisier and verified by Landolt. According to this, in a chemical reaction total mass of the products is equal to the total mass of the reactants or mass is neither created nor destroyed in a chemical reaction. Chemical equations are based on this law. However, remember that this law does not hold good for nuclear reactions.
2. Law of definite proportions: It was proposed by Proust. This states that elements always combine in fixed ratio of their weights, or a pure compound contains the same elements in the fixed ratio of their weights whatever its methos of preparation may be. For example, pure water (H2O) always contains 1: 8 ratio of hydrogen and oxygen by weight.
3. Law of multiple proportions: It was proposed by Dalton and verified by Berzelius. According to this, different weights of an element that combine with a fixed weight of the other bear a simple numerical ratio. For example, weight of oxygen that combines with 12 g of carbon in CO and CO2 are the ratio of 1: 2. Other examples are H2O, H2O2, SO2, SO3, SnCl2, SnCl4 etc. This law explains the formation of more than one compound by two elements.
4. Law of reciprocal proportions: This law was proposed by Ritcher. According to this, weights of two different elements which are combining with a fixed weight of a third element, are also the weights with which they combine with one another or their multiples or submultiples. Examples:
(i) NaCl, NaH and HCl
(ii) CH4, CO2 and H2O
(iii) NH3, H2O and N2O3
Law of reciprocal proportions can be used to obtain equivalent weights of elements. Hence it is also called law of equivalent proportions according to which elements always combine in terms of their equivalent weights.
5. Law of combining volumes: This was proposed by Gay-Lussac and is applicable only for gases. According to this, when gases combine they do so in simple ratio of their volumes. This law is not accurate as gases deviate from ideal behaviour.
