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What is Law of Conservation of Mass? – Examples and Problems

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Table of Content –

  • What is Law of Conservation of Mass?
  • Examples
  • Experimental verification of Law of conservation of mass
  • Problems based on Law of Conservation of Mass
  • Frequently Asked Questions – FAQs
  • Test your understanding – Quiz

Read more –

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.

 Law of conservation of mass

What is Law of Conservation of Mass?

The law of conservation of mass or matter states that “matter 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

Examples

The law has become the foundation of all chemical reactions and is supported by several laboratory experiments.
For e.g. –
1. Combustion Process –
 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. Chemical reactions –
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

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 flask 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.

Problems Based on Law of Conservation of Mass
  1.  10 grams of calcium carbonate (CaCO3) produces 3.8 grams of carbon dioxide (CO2) and 6.2 grams of calcium oxide (CaO). Represent this reaction/ statement in terms of law of conservation of mass.
    Sol. – According to law of conservation of mass –
    Mass of reactants = Mass of products
    10 gram of CaCO3 = 3.8 grams of CO2 + 6.2 grams of CaO
    10 grams of reactant = 10 grams of products.
    Hence, it is proved that the law of conservation of mass is followed by the above reaction.
  2.  In an experiment, 63.5g of copper combines with 16g of oxygen to give 79.5g of cupric oxide (a black oxide of copper). Is this statement in agreement with the law of conservation of mass?
    Sol. – According to law of conservation of mass –
    Mass of reactants = Mass of products
    63.5g of copper + 16g of oxygen = 79.5g of cupric oxide
    79.5  grams of reactant = 79.5 grams of products.
    Hence, it is proved that the law of conservation of mass is followed by the above reaction.

For more Problems –

Problems Based on Law of Conservation of Mass

 

Frequently Asked Questions – FAQs
  • Why is there no change in mass or no new matter during chemical reactions?
    According to law of conservation of mass, in a chemical reaction, atoms are neither created nor destroyed. The atoms of the reactants are just rearranged to form products. Hence, there is no change in mass in a chemical reaction.
  • Verify law of conservation of mass with an experiment.
  • What happens in a combustion reaction?
  • What are other laws of chemical combination?
  • Nuclear fusion reactions violate the law of conservation of mass. Explain the statement.
    According to the law of conservation of mass, during any physical or chemical changes,
    total mass of the products = total mass of the reactants.
    But in nuclear fusion reaction , for example, energy that sun emits in its core is due to collision of hydrogen nuclei and formation of helium nuclei. Here conservation of mass is not obeyed as certain part of mass is converted into energy. So, law of conservation of mass is violated here.

  • What are the importance of the law of conservation of mass?
     The law of conservation of mass is important to the study and production of chemical reactions or in the balancing of a chemical reaction.  or By using this law, we can calculate the quantities of reactants and products.

  • Who founded the law of conservation of mass?

    The law of conservation of mass was founded by French chemist Antoine Lavoisier in 1789 after performing experimental studies for combustion reactions.

     

  • Lavoisier carried out his experiments inside a sealed glass jar. Why was sealing the jar important for his results? What might his results have been if he hadn’t sealed the jar?Sealing the jar was very important so that any gases produced in the reactions were captured and could be measured. If he hadn’t sealed the jar, gases might have escaped detection. Then his results would have shown that there was less mass after the reactions than before. In other words, he would not have been able to conclude that mass is conserved in chemical reactions.

Read next –

Problems Based on Law of Conservation of Mass

Law of constant (definite) proportions

Problems Based on Law of constant (definite) proportions

Test your understanding – Quiz

 

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