What Does the Law of Conservation of Mass Say?
The Law of Conservation of Mass
The law of conservation of mass states that:
In a chemical reaction, mass is neither created nor destroyed.
Instead:
- The atoms present at the start are rearranged to form new substances.
- The total mass of the reactants equals the total mass of the products.
We can change how atoms are bonded, but we do not create or destroy atoms in ordinary chemical reactions.
Antoine Lavoisier first clearly demonstrated this principle in the late 18th century by carefully weighing reactants and products.
Experimental Evidence
We can test this law in the lab by weighing substances before and after a reaction.
Neutralisation in a closed container
- Consider the reaction of hydrochloric acid with sodium hydroxide: $$\mathrm{HCl}(\mathrm{aq})+\mathrm{NaOH}(\mathrm{aq}) \rightarrow \mathrm{NaCl}(\mathrm{aq})+\mathrm{H}_2 \mathrm{O}(\mathrm{l})$$
- Suppose you mix:
- 36.5 g of HCl(aq)
- 40.0 g of NaOH(aq)
- After the reaction is complete, you obtain:
- 58.5 g of NaCl(aq)
- 18.0 g of H₂O(l)
- Total mass of reactants: $$36.5 \mathrm{~g}+40.0 \mathrm{~g}=76.5 \mathrm{~g}$$
- Total mass of products: $$58.5 \mathrm{~g}+18.0 \mathrm{~g}=76.5 \mathrm{~g}$$
- The masses match → mass is conserved.
To show conservation of mass clearly, it’s best to carry out the reaction in a closed container (for example, a conical flask with a bung) so that no gas can escape and nothing can enter.
Why Does Mass Sometimes Appear Not to Be Conserved?
- In everyday experiments, we often use open systems (e.g. beakers open to the air).
- In such cases, gases can escape or enter, making it seem like mass has been lost or gained.
Escape of Gases – Apparent Loss of Mass
When a reaction produces a gas that escapes into the air, the measured mass of the container + contents decreases, even though the total mass of the system (including the surrounding air) is unchanged.
Reaction of calcium carbonate with hydrochloric acid
$$\mathrm{CaCO}_3(\mathrm{~s})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{CaCl}_2(\mathrm{aq})+\mathrm{H}_2 \mathrm{O}(\mathrm{l})+\mathrm{CO}_2(\mathrm{~g})$$
- If this reaction is done in an open beaker, carbon dioxide gas, CO₂(g), bubbles out and escapes into the room.
- If you weigh the beaker before and after the reaction, the final mass is smaller.
- It might seem that mass has been “lost”, but in reality the missing mass is simply the CO₂ that left the container and mixed with the air.
To see that mass is truly conserved, you can perform the same reaction in a sealed flask with a balloon or bung. The mass of the flask + contents (including the gas) will stay the same.
Absorption from the Surroundings – Apparent Gain of Mass
Sometimes a substance can gain mass because it reacts with something from the air (often oxygen or water vapour).
Burning magnesium in air
$$2 \mathrm{Mg}(\mathrm{~s})+\mathrm{O}_2(\mathrm{~g}) \rightarrow 2 \mathrm{MgO}(\mathrm{~s})$$
- If you weigh a strip of magnesium before burning and then weigh the white magnesium oxide formed, you will find the mass has increased.
- The “extra” mass comes from oxygen in the air that has combined with the magnesium.
- Again, no mass has been created. The system gaining mass (the solid) has taken in atoms from the surroundings (the gas).
Closed vs Open Systems
- In a closed system, no reactants or products can enter or leave.
- It is easier to show that mass is conserved.
- In an open system, gases can escape or be absorbed.
- The mass of what you are weighing may change, even though the overall mass of the universe is unchanged.
When it looks like mass is lost or gained, ask:
- “Is a gas escaping (open system)?”
- “Is something from the air being added to the system?”
How Does Conservation of Mass Help Us Understand Reactions?
The idea that mass is conserved helps us to:
- Balance chemical equations.
- Predict how much product will form from given reactants.
- Check whether our reaction descriptions make sense.
Balancing Chemical Equations
- Because atoms are neither created nor destroyed, a correct chemical equation must have:
- The same number of each type of atom on both sides.
- For example, the reaction of hydrogen and oxygen to form water:
- Unbalanced: $$\mathrm{H}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g}) \rightarrow \mathrm{H}_2 \mathrm{O}(\mathrm{l})$$
- Reactants: 2 H atoms, 2 O atoms
- Products: 2 H atoms, only 1 O atom
- To obey conservation of mass, we balance it: $$2 \mathrm{H}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g}) \rightarrow 2 \mathrm{H}_2 \mathrm{O}(\mathrm{l})$$
- Reactants: 4 H, 2 O
- Products: 4 H, 2 O → balanced
Balancing is based on conservation of atoms, which directly reflects conservation of mass.
Predicting Reactant and Product Quantities (Stoichiometry)
Once an equation is balanced, we can use it to calculate how much product will form from given masses of reactants.
Idea (no full calculation here):
- For the reaction: $$\mathrm{CaCO}_3(\mathrm{~s})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{CaCl}_2(\mathrm{aq})+\mathrm{H}_2 \mathrm{O}(\mathrm{l})+\mathrm{CO}_2(\mathrm{~g})$$
- If you know the mass of CaCO₃ you start with, you can:
- Use the balanced equation to find the mole ratio.
- Calculate the mass of CO₂ produced.
- Predict the mass of CaCl₂ formed.
- This works because the total mass of reactants equals the total mass of products and because atoms are conserved.
- What is the difference between a closed system and an open system in the context of conservation of mass?
- Give a real-life example where mass appears to be lost during a chemical reaction.
- Give another example where mass appears to be gained.
- Why must chemical equations be balanced to obey the law of conservation of mass?
- How does knowing that mass is conserved help chemists predict how much product will form in a reaction?
