LAW OF CONSERVATION OF MASS

Explanations that how law of conservation of mass is obeyed in burning of a candle

When a candle burns, it forms carbon dioxide, water and some residue of melted wax. It seems that mass of candle is lost due to chemical change.
If we weigh the candle before burning and weigh all the products after
Burn the candle, we observe that there is no change in mass. We can verify this by Partigon experiment.

 A hard and a long glass tube open at both end is taken. Through the upper part a mixture of quick lime and soda lime is packed over a copper gauge.Through the lower end of the tube a candle placed over a porous cork placed  inside the tube. The entire arrangement (the tube with lime mixture and a candle) is placed on the left arm of the balanced and weighed carefully. The candle is now lighted and is quickly placed on the tube again, closing with porous cork.
After some time, when the candle burnt out completely, it has been found that the left arm of the balanced beam is tilted downward. It shows that there is a gain in mass during burning of the candle. This is because during combustion, carbon – dioxide and water produced are absorbed by the lime and sodalime. Again some oxygen from outside air through the porous of the cork has been used in the combustion increased the weight. Thus we observe that,
Mass of candle + mass of oxygen = mass of carbon dioxide + mass of water vapour
Or the total mass of the product = total mass of the reactants.
Thus, the law of conservation of mass is verified.

A little ash only remains as residue on burning a piece of magnesium wire.
Explain whether mass is conserved in this case.

Burning a piece of magnesium in air (or oxygen) is a chemical change. If we weight
the magnesium and the combined oxygen before reaction and after reaction, then there is no change in their mass.

The following experiment verify the law of conservation of mass.

Experiment : A hard test tube filled with oxygen and a tight fitted stopper is taken .
A small piece of magnesium is introduced in the closed tube contain oxygen and magnesium piece carefully. Now heat the test tube strongly by a Bunsen burner.
Magnesium piece burns brilliantly in the oxygen.

After cooling we find a white residue in the test tube. Weight the test tube again after reaction .We observe that the weight of the test tube before reaction and after complete reaction is the same. The magnesium piece reacts with oxygen form magnesium oxide.
                                                     2Mg + O₂ = 2MgO
Thus the weight of magnesium and oxygen before reaction and the weight of magnesium oxide after reaction is same. This verifies the law of conservation of mass.