Chemical Reactions and Conservation of Mass
4. Conservation of Mass
Learning outcomes
- I can state the law of conservation of mass.
- I can explain conservation of mass using the particle model.
- I can describe how atoms are rearranged but not created or destroyed during reactions.
- I can use chemical equations to demonstrate conservation of mass.
- I can apply conservation of mass to simple reaction problems.
Introduction
When a chemical reaction takes place, it may seem as though matter disappears or is created. For example, when wood burns, only a small amount of ash remains, while an effervescent tablet seems to disappear when dropped into water. However, careful scientific measurements show that matter is never lost or created during ordinary chemical reactions.
This idea is known as the Law of Conservation of Mass. It is one of the most important principles in chemistry because it explains why chemical equations must be balanced and why the same atoms are present before and after every reaction. Understanding conservation of mass helps scientists predict the amounts of substances produced during chemical reactions.
The Law of Conservation of Mass
The Law of Conservation of Mass states:
Mass is neither created nor destroyed during a chemical reaction.
This means:
- The total mass of the reactants equals the total mass of the products.
In every ordinary chemical reaction:
Total mass before reaction = Total mass after reaction
Figure 1. The total mass remains the same before and after a chemical reaction.
Why Mass Is Conserved
Chemical reactions do not create or destroy atoms.
Instead:
- Existing chemical bonds break.
- Atoms are rearranged.
- New chemical bonds form.
The atoms are simply reorganised into different combinations.
Since the same atoms are present before and after the reaction, the total mass remains constant.
The Particle Model
The particle model explains conservation of mass.
Before the reaction:
- Atoms are arranged in the reactant particles.
During the reaction:
- Bonds break.
- Atoms rearrange.
After the reaction:
- The same atoms are arranged into product particles.
No atoms disappear.
No new atoms appear.
Only their arrangement changes.
Figure 2. The particle model shows that atoms are rearranged, not created or destroyed.
Rearrangement of Atoms
Consider the reaction:
Hydrogen + Oxygen → Water
At the particle level:
- Hydrogen atoms remain hydrogen atoms.
- Oxygen atoms remain oxygen atoms.
The atoms simply combine differently to form water molecules.
Every hydrogen atom present before the reaction is still present afterwards.
The same is true for every oxygen atom.
This is why mass is conserved.
Chemical Equations Show Conservation of Mass
Balanced chemical equations demonstrate conservation of mass.
Example:
2H₂ + O₂ → 2H₂O
Count the atoms.
Before:
- Hydrogen atoms = 4
- Oxygen atoms = 2
After:
- Hydrogen atoms = 4
- Oxygen atoms = 2
The number of each type of atom is the same on both sides.
Therefore, mass is conserved.
Figure 3. Balanced chemical equations contain the same number of each type of atom on both sides.
Why Chemical Equations Must Be Balanced
Chemical equations are balanced because:
- Atoms cannot be created.
- Atoms cannot be destroyed.
Only coefficients may be changed when balancing equations.
Subscripts must never be changed because they would change the identity of the substance.
Example:
Correct:
2H₂ + O₂ → 2H₂O
Incorrect:
H₂ + O₂ → H₂O₂
Changing the subscripts creates a different compound.
Applying Conservation of Mass
The law can be used to solve simple problems.
Example
A reaction starts with:
- 12 g of carbon.
- 32 g of oxygen.
Total reactant mass:
44 g
If all the reactants form carbon dioxide,
Mass of carbon dioxide:
44 g
The total mass remains unchanged.
Open and Closed Systems
Sometimes it appears that mass changes.
Example:
A burning candle loses mass.
Why?
Some products are gases that escape into the air.
If the reaction occurred in a closed container, the total mass would remain exactly the same.
The law always applies, provided all reactants and products are included.
Figure 4. In a closed system, the total mass remains constant because no substances can enter or leave.
Everyday Examples
Conservation of mass occurs in:
- Rusting iron.
- Burning fuels.
- Photosynthesis.
- Cellular respiration.
- Cooking.
- Digestion.
Although substances change, the total amount of matter remains the same.
Why Conservation of Mass Is Important
The law helps scientists:
- Balance chemical equations.
- Predict product masses.
- Design industrial processes.
- Understand chemical reactions.
- Calculate quantities in chemistry.
It is one of the fundamental laws of science.
Figure 5. Conservation of mass is the foundation of balancing chemical equations and quantitative chemistry.
Worked Example
Question
A reaction begins with:
- 18 g of hydrogen.
- 144 g of oxygen.
Assuming all the reactants react completely, what is the total mass of water produced?
Solution
Total mass of reactants:
18 g + 144 g = 162 g
According to the Law of Conservation of Mass:
Total mass of products = Total mass of reactants
Answer: 162 g of water
Real-World Connection
Chemical engineers rely on the Law of Conservation of Mass when designing factories that manufacture medicines, fertilisers, plastics, and fuels. By knowing the masses of the reactants, they can predict how much product will be formed and how much waste may be produced. This helps reduce costs, improve efficiency, and minimise environmental impacts.
Did You Know?
The Law of Conservation of Mass was first clearly stated in the late 1700s by the French chemist Antoine Lavoisier, often called the "Father of Modern Chemistry." His careful experiments using sealed containers showed that the total mass of substances remains constant during chemical reactions, helping transform chemistry into a precise quantitative science.
Key Terms
Balanced chemical equation – A chemical equation with the same number of each type of atom on both sides.
Closed system – A system in which no matter can enter or leave.
Coefficient – A number placed before a chemical formula indicating the number of particles involved in a reaction.
Conservation of mass – The principle that mass is neither created nor destroyed during ordinary chemical reactions.
Particle model – A model describing matter as tiny particles that can rearrange during chemical reactions.
Product – A substance formed during a chemical reaction.
Reactant – A starting substance in a chemical reaction.
Key Takeaways
- The Law of Conservation of Mass states that mass is neither created nor destroyed during ordinary chemical reactions.
- During a reaction, atoms are rearranged, but no atoms are created or destroyed.
- The particle model explains conservation of mass because the same atoms are present before and after the reaction.
- Balanced chemical equations demonstrate conservation of mass by showing equal numbers of each type of atom on both sides.
- In a closed system, the total mass of the reactants always equals the total mass of the products.
- Conservation of mass is a fundamental principle used throughout chemistry to predict reaction outcomes and balance chemical equations.