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Laws of Chemical Combination – Law of Conservation of Mass

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What is matter? What is its Characteristics?

States of matter

Mole Concept – Importance and formulas


There are four laws of chemical combination. They are –

1. The law of conservation of mass
2. The law of constant (definite) proportions
3. The law of multiple proportions
4. The law of reciprocal proportions

Joseph Louis Proust (1754-1826) was popular for the law of definite proportion, otherwise called the Proust Law.

A. Law of conservation of mass

The law of conservation of mass or matter states thatmatter is neither created nor destroyed during chemical reaction but changes from one form to another. This law is a derivation of a part of the Dalton’s atomic theory which stated that, atoms can neither be created nor be destroyed but change from one form to another.The substances which combine or react together in a chemical reaction are called ‘reactants’ and the new substances formed or produced are called ‘products’.  According to the law of conservation of mass or matter, the total mass of the products formed during a chemical reaction is equal to the total mass of the reactants.  It means, there will be no change in mass during a chemical reaction.
  Mass of reactants = Mass of products
The law has become the foundation of all chemical reactions and is supported by several laboratory experiments.
For e.g. –
1. Burning a piece of paper or coal or wood in the presence of oxygen. It seems that the mass shrinks as the paper (or wood or coal) burns. However, what actually happens, is that the paper is changing its form to ash, carbon dioxide, and water vapor. If we could capture all of those bits i.e. reactants –  paper or coal or wood and reacted oxygen; and the products – carbon dioxide, water vapor, ash, soot and other components during the burning, and weigh them, reveal that mass is actually conserved.
2. Calcium carbonate  →  Calcium oxide   +     Carbon dioxide
100 g                                56g                               44g
According to law of conservation of mass –
Mass of reactants  =  Mass of products
100  =  56 + 44
3. In an experiment, 63.5g of copper combines with 16g of oxygen to give 79.5g of cupric oxide (a black oxide of copper). This is in agreement with the law of conservation of mass.


Experimental verification of Law of conservation of mass
The law can be verified by the below mentioned experimental setup.

1. Take a conical flask and test tube. Take barium chloride (X) solution in the conical flask and sodium sulphate (Y) solution in test tube. Other combinations can be taken such as – (i) copper sulphate and sodium carbonate (ii) lead nitrate and sodium chloride (iii) calcium chloride and sodium sulphate
2. Hang the test tube in the flask carefully. The solutions X and Y should not be mixed. Put a cork on the flask and make it air-tight.
3.Weigh the flsk with its contents carefully and note it.
4. Now mix the solutions together by tilting the flask. Sodium sulphate reacts with barium chloride to give a white precipitate of barium sulphate and sodium chloride solution.
5. Weigh the flask with products formed. It is observed that the mass of reactants before the reaction and the products formed after the reaction are equal which verifies the law of conservation.of mass.

Science today knows that matter can be converted into energy (and vice-versa). Hence, during all chemical and physical changes, the total mass + energy before the change is equal to the total mass + energy after the change. Still, as there is no detectable change in mass in an ordinary chemical reaction, the law of conservation of mass is still valid.


Read next –

Problems Based on Law of Conservation of Mass

Law of constant (definite) proportions

Problems Based on Law of constant (definite) proportions


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