Mass Matters
Mass is never created or destroyed in a reaction, yet the reading on a balance can still seem to change. Learn conservation of mass, how to work out relative formula mass, and why an open system can appear to gain or lose mass.
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Mass never disappears ⚗️
In any chemical reaction, mass is never created or destroyed. The atoms are just rearranged, so the total mass stays the same. Yet the reading on a balance can still seem to go up or down. This module explains why, and shows you how to work out the mass of a compound.
Words to know 🔑
Four terms run through this topic. Learn them first.
Open or sealed? ⚖️
Whether gas can get in or out decides what the balance shows.
Match the term 🔗
- Conservation of mass
- Reactant
- Product
- Relative formula mass
- The total mass does not change in a reaction
- A substance you start with
- A substance the reaction makes
- The sum of the relative atomic masses in a formula
Mass in a reaction 🎯
In a chemical reaction, how does the total mass of the products compare with the total mass of the reactants?
- They are equal
- The products always weigh more
- The products always weigh less
- Some mass is destroyed
Why the balance seems to change 💡
In an open system the balance can mislead you. If a gas escapes, such as carbon dioxide leaving when a carbonate reacts with acid, the mass seems to fall. If a gas is taken from the air, such as a metal gaining oxygen, the mass seems to rise. Mass is still conserved overall. Always name the gas that leaves or enters.
Match the measurement word 🔀
- Accuracy
- Precision
- Repeatability
- Reproducibility
- How close a result is to the true value
- How close repeated results are to each other
- The same person repeats it with the same method and equipment
- A different person or equipment gets the same result
Conservation of mass ✅
Which TWO statements about conservation of mass are true? Choose all that apply.
- The total mass of reactants equals the total mass of products
- No atoms are created or destroyed
- Some mass is destroyed in the reaction
- Atoms can simply disappear
Work out the Mr 🔢
An interactive activity.
How to find the Mr 🪜
An interactive activity.
Complete the sentences 🧩
In a reaction, the total mass of the _____ equals the total mass of the products, because no atoms are created or _____. The relative formula mass is the sum of the relative _____ masses. In an open system, the mass can seem to fall if a _____ escapes. In a sealed system, the measured mass stays the _____.
Spot the true statements 🖍️
An interactive activity.
Rise, fall or stay the same? 🧭
An interactive activity.
Explain the change ✍️
An interactive activity.