AQA Combined Science (8464) 5.3.1.1 · Quantitative
Conservation of mass and balanced equations
Bonds break, new substances form, a flame flashes inside a sealed flask. What happens to the mass, and how does a balanced equation prove it?
Combined Science · Higher tier
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A spark in a sealed flask.
A thick glass flask holds methane and oxygen. The stopper is clamped shut, and the flask sits on a balance reading 312.48 g. A spark sets the methane burning. A flash, and then droplets of water run down the inside of the glass.
If you had to guess, once the flask has cooled, does the balance read less than 312.48 g, or exactly the same?
This is the law of conservation of mass: no atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants. Bonds break and new bonds form, but the same atoms are only rearranged into new substances. That is why a symbol equation must be balanced: it shows the same number of atoms of each element on both sides.
Inside the flask
Count the atoms. Then fire the spark.
CH₄ + 2O₂ → CO₂ + 2H₂O
Commit first. How many oxygen atoms are on the right-hand side?
Key fact
A big number in front multiplies the whole formula: 2H₂O is 4 H and 2 O. A small number inside belongs to the substance, and you never change it to balance.
Think again
“2H₂ + O₂ → 2H₂O has three molecules on the left and only two on the right. So it is not balanced, and the water must weigh less.”
Spot the flaw
ExplainWhat is wrong with that reasoning?
A word equation names the substances. A symbol equation uses their formulae, and can add state symbols: (s) solid, (l) liquid, (g) gas and (aq) aqueous, meaning dissolved in water. The balancing numbers give the ratio of formula units that react, not the number of grams.
HigherThe same numbers are amounts in moles: 2H₂ + O₂ → 2H₂O means 2 moles of H₂ react with 1 mole of O₂ to give 2 moles of H₂O.
Word to symbol · three moves
Marble chips fizz in hydrochloric acid.
calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
1 · Which line has the right formulae?
Equation
Learn it · no chemistry equation sheettotal mass of reactants = total mass of products
Rearranged, it finds any one missing mass: add up the side you know, then subtract the masses you have on the other side.
Command words in this lesson
- Balance
- Put numbers in front of formulae until every element has the same number of atoms on each side.
- Write
- For a symbol equation: correct formulae, balancing numbers, and state symbols when the question asks.
- Calculate
- Show each line of working and finish with the unit.
- Explain
- Give reasons as linked steps, each one leading to the next with “so” or “because”.
Key note · AQA 5.3.1.1 (8464)
Conservation of mass and balanced equations
Photograph this card. Tomorrow, cover it and say each line first.
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Not sure why the balance does not move?
The flask draws each atom as a ball that slides to its new partner in one smooth move, and draws calcium hydroxide as single formula units. In a real reaction countless particles react at once, and ionic substances are not separate molecules.