[Physics Class Notes] on Law of Conservation of Mass Pdf for Exam

The law of conservation of mass is a very important concept, it says that we cannot create or destroy the mass; however, this mass can be transformed into another form.

In Physics, we express the law of conservation of mass as the differential form by continuity equation of the mechanics of fluids and continuum mechanics with the following equation:

∂ρ/∂t +⛛(ρv) = 0

Where,

ρ = density

t = time

v = velocity

⛛ = divergence

The law of conservation of mass states that in all the physical and chemical processes, the total mass of products in a chemical reaction is equal to the mass of reactants.

What is the Law of Conservation of Mass?

The Law of conservation of mass was studied by a French Chemist named Antoine Lavoisier in 1789.

This law states that in a chemical reaction, the mass of products in chemical reactions equals the mass of reactants.

According to this law, the matter cannot be created nor be destroyed. We call this law the law of indestructibility of matter. Let’s study the following experiments as the law of conservation of mass examples to get clarity in the conservation of mass definition:

When Matter Undergoes a Physical Change

Take a piece of ice (ice is solid water) and place it in a conical flask. This flask is properly corked and weighted and it is now heated gently to melt the ice into water.

Ice      → Heat    → Water

(solid)                    (Liquid)

The flask is weighed again, we notice that the weight of the flask remains the same because the mass of ice does not change after it undergoes a physical change.

When Matter Undergoes a Chemical Change

A Swiss Chemist named Hans Heinrich Landolt took two test tubes joined with a common line as shown below:

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One tube contains a solution of sodium chloride and another contains a silver nitrate solution; the tube containing these two solutions is called Landolt’s tube.

Both tubes were corked and weighed. Now, the above arrangement is tilted to let these solutions mix. The chemical reaction occurs, resulting in a curdy white precipitate of silver chloride. The reaction is as follows:

Nacl (s)          +        AgNO₃ (aqueous)       →         AgCl (s)            +     NaNO₃ (aqueous)

Sodium chloride           Silver Nitrate                   Silver Chloride            Sodium Nitrate

                                                                               (White precipitate)

After this reaction, the above arrangement was weighed again, it was found that the weight remains the same.

Decomposition of Mercuric Oxide (HgO)

When 100 g of HgO is heated,  it decomposes into two compounds viz: one mole of Hg and a half mole of O₂. The reaction occurs in the following manner:

HgO (s)                 →  Hg (l)       +       ½ O₂

Mercuric Oxide         Mercury         Oxygen 

      100 g                       92.6 g             7.4 g

Here, if we calculate the mass of products viz: Hg and O₂, i.e., 92.6 + 7.4 = 100g, which is equivalent to the mass of the reactant, i.e., HgO. So, the law of conservation of mass verifies here.

Combustion Process

When we burn pieces of wood, these pieces turn to ashes, water vapor, and carbon dioxide. 

If we weigh the piece of wood and after burning it, ashes, water vapors, and CO₂ are heated, there will not be any change in the mass before and after the reaction.

Now, let’s look at some more examples of the law of conservation of mass:

Law of Conservation of Mass Examples

Example 1:

Take a container and place 16 g of methane or CH₄ and 64 g of O₂. After the container is closed, CH₄ and O₂ remain closely packed. Now, ad the reaction proceeds, i.e., the combustion reaction, we get the following products:

CH₄ (gas)           +       2 O₂ (g)        →       CO₂ (g)            +    2 H₂ O (g)

Methane                     Oxygen               Carbon Dioxide          Water vapor

16 g                   64 g           44 g

Here, before the reaction, the total mass of reactants was:  

16 + 64 = 80 g

So, here what do you expect could be the mass of water vapor after the reaction?

Well, by the law of conservation of mass, the total mass of products must be 80 g. 

44 +  mass of H₂O = 80 g.

So, we get the mass of 2 moles of H₂O as 36 g.

Therefore, our final balanced equation after applying the principle of conservation of mass is:

CH₄ (gas)          +     2 O₂ (g)         →       CO₂ (g)           +    2 H₂O (g)

Methane                Oxygen                  Carbon Dioxide     Water vapor

16 g                           64 g                            44 g                        36 g

From here, we conclude that the sum of masses of reactants and products remain constant.

Example 2:

Take 10 g of CaCO₃. Now, after the decomposition, CaCO₃ decomposes to 6.2 g of CaO and 3.8 g of CO₂. So, let’s represent this equivalence of mass as the conservation of mass:

CaCO₃                     — decomposition →     CaO              +      CO₂

Calcium Carbonate                                 Calcium Oxide      Carbon dioxide

          10 g                                                          6.2 g                        3.8 g