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Law of Conservation of Mass – Grade 7 Science Worksheets

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What is the Principle of Law of Conservation of Mass?

The Law of Conservation of mass states that “mass can neither be created nor destroyed”.

Systems can be classified into two categories – Open System and Closed System.

Example: When some hot drink is poured in a mug, the heat escapes within a few minutes and the mug cools down. So, it is an example of open system.

Law of Conservation of Mass

Open System is a system where a quantity can enter or leave the system to a certain degree.

Example: When a liquid is poured in a thermos flask, there is no exchange of heat or matter. So, it an example of closed or isolated system.

Law of Conservation of Mass

The universe is also an example of isolated system.

Closed System is the opposite of an open system. It is a system where quantity cannot enter or leave the system. Closed system is also called as “isolated system”.

Note: Practically, there is no system which is absolutely closed. Closed systems have limited interaction with the environment.



How is the Law of Conservation of Mass shown by a balanced Equation?

In a chemical reaction, the mass of the reactants (at the start) = mass of the products (at the end)

Law of Conservation of Mass


A better glance at the Law of Conservation of Mass


Let‘s say A potter uses 150 grams of clay to make a pot.


When you weigh the pot after its done it is equal to 150 grams.

Weight of the clay used to make the pot = weight of the pot.

Hence the law:  Mass can never be created nor is it destroyed.

It just changes its form.

Law of Conservation of Mass

Let’s say a cube of ice weighs “x” grams.


When this is weighed after it melts, it weighs the same,

that its weight after melting is “x” grams.

So, weight of the ice cube = weight of the water

Nothing changes with respect to the weight of the ice or water.

It just changes its form.

Mass is neither created not it is destroyed, it’s constant.

Law of Conservation of Mass

Experiments involving chemical change also prove the law of conservation of mass.


Weight of (wood + burnt in oxygen) = Weight of the (ashes +carbon dioxide+ water vapor)

Law of Conservation of Mass




How to Prove Law of Conservation of Mass?

4. Mass of the Reactants = Mass of the Product

Law of Conservation of Mass

Mass of 32 g of oxygen plus 12 g of carbon (before Reaction) = Mass of carbon dioxide (after reaction)

Check Point

A. State True or False 

  1. The law of conservation of mass means, in a closed system, matter cannot be created or destroyed. It changes forms, but is constant.
  2. Mass of the reactants at the start is always greater than the mass of the product in the end.
  3. Mass is sometimes viewed as a measure of inertia, the opposition that free bodies offer to forces.
  4. Matter is always created and destroyed in a closed system.
  5. The principle of mass conservation, different measurements of the mass of an object taken under various circumstances should always be the same.


B. Explain what does the following mean :

  1. Mass
  2. Matter
  3. Open System
  4. Closed System
Answer Key
A. True or False:

  1. True
  2. False
  3. True
  4. False
  5. True


B. Definitions:

  1. Mass: Mass is the amount of matter in an object.
  2. Matter:  Matter is anything that has mass and takes up space.
  3. Open system: a system in which mass or energy can be lost to or gained from its surroundings.
  4. Closed system: a system in which energy or matter is   not exchanged with its surroundings.

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