Conservation of mass

 

What is the conservation of mass?

Mass does not change when something changes or reacts. The mass of the chemicals before a change is the same as the mass of the chemicals after a change. This is the Law of Conservation of Mass.

For example, when a log of wood burns, the mass of the wood and oxygen that burn is the same as the mass of the smoke and ashes that are left.

 

 

Conservation of mass

 

What is the difference between weight and mass?

Mass tells us how much matter something has. We use kilograms or grams to measure mass.

Weight is how much the Earth pulls on something. We use newtons to measure weight.

 

Weight changes when mass or gravity changes. Mass does not change when something moves to a different place in the universe, but weight can change. For example, this can happen if something goes to a place where gravity is different from Earth’s gravity, like space or another planet.

 

The mass of a gas

Gases have mass, but it is hard to measure how much they weigh.

If you boil 100 grams of water in a pan, it will turn into steam and disappear. But if you catch all the steam and weigh it, it will still be 100 grams.

 

🧪 Knowledge Check: Conservation of Mass

Test your understanding of the Law of Conservation of Mass and how atoms behave during chemical reactions.

1. According to the Law of Conservation of Mass, what happens to atoms during a chemical reaction?

• Some atoms are destroyed.
• New atoms are created.
• Atoms are neither lost nor made; they are only rearranged.

2. If the total mass of the reactants is 50g, what will be the total mass of the products?

• Exactly 50g
• Slightly less than 50g
• Slightly more than 50g

3. Why is the total mass of the products equal to the total mass of the reactants?

• Because the number of atoms changes.
• Because the number of atoms of each element stays the same.
• Because atoms get smaller during a reaction.

4. In a balanced symbol equation, what must be true about the atoms?

• There are more atoms on the products side.
• There are fewer atoms on the products side.
• There is the same number of atoms of each element on both sides.

5. A chemical reaction involves the formation of new substances. Where do the atoms in these new substances come from?

• They are newly made during the reaction.
• They are the same atoms that were in the reactants, just rearranged.
• They come from the air around the reaction.

Click to Reveal Answers

1. Atoms are neither lost nor made (No atoms are lost or made during a chemical reaction).
2. Exactly 50g (The total mass of the products is equal to the total mass of the reactants).
3. The number of atoms of each element stays the same (Mass is conserved because atoms are not destroyed).
4. Same number of atoms of each element on both sides (Chemical equations must be balanced to show conservation of mass).
5. The same atoms that were in the reactants (Chemical reactions only involve the rearrangement of existing atoms).

 

Summary:

 

• No atoms are made or lost when chemicals change or change state. The mass of chemicals stays the same before and after a change. This is called the Law of Conservation of Mass.