Conservation of mass and balanced chemical equations — AQA GCSE Combined Science
Test yourself on Conservation of mass and balanced chemical equations with AQA GCSE practice questions.
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Conservation of mass and balanced chemical equations explained
During a chemical reaction, bonds break and new bonds form, but the atoms themselves are simply rearranged.
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No atom disappears and no new atom appears, so the total mass of the products must equal the total mass of the reactants. For example, when 12 g of carbon reacts completely with 32 g of oxygen, the carbon dioxide formed has a mass of 44 g. This idea is the law of conservation of mass. It only appears to fail when a reactant or product is a gas that escapes from or enters the reaction vessel. In a sealed container, the total mass stays constant. The law allows you to calculate an unknown mass in a reaction by subtraction or addition, and it is the reason symbol equations must be balanced.
This means that chemical reactions can be represented by symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation.
Because atoms are conserved, a symbol equation must show the same number of atoms of each element on both sides. You balance an equation by writing correct formulae first, then placing whole-number multipliers in front of formulae until every element matches. For example, H₂ + O₂ → H₂O is unbalanced because there are two oxygen atoms on the left but only one on the right. Writing 2H₂ + O₂ → 2H₂O gives four hydrogen atoms and two oxygen atoms on each side. Never change a subscript inside a formula, because that changes the substance. Check each element in turn and recount after every change. Balanced equations are used to calculate reacting masses and to show the ratio in which substances react.
Students should understand the use of the multipliers in equations in normal script before a formula and in subscript within a formula.
Chemical equations use two kinds of multiplier, and confusing them changes the substance. A subscript sits inside a formula and multiplies only the atom or group immediately before it: in H₂O the 2 means two hydrogen atoms bonded to one oxygen, giving one water molecule. A normal-script number before a formula multiplies the whole formula: 2H₂O means two separate water molecules, so four hydrogen atoms and two oxygen atoms in total. When balancing, adjust the large number in front, never a subscript, because changing a subscript alters the identity of the substance. For example, 2H₂ + O₂ → 2H₂O is balanced, whereas writing H₂O₂ would describe hydrogen peroxide, a different compound.
Your focus
- State the law of conservation of mass in terms of atoms.
- Use the law to calculate an unknown mass in a reaction.
- Explain why mass appears to change when a gas escapes or enters.
Show all 9 objectives
- Write a balanced symbol equation from given formulae.
- Explain why an equation must be balanced in terms of atoms.
- Use a balanced equation to state the ratio in which substances react.
- State the meaning of a subscript within a chemical formula and a normal-script multiplier before a formula.
- Count the total number of each type of atom in a formula that includes both subscripts and a front multiplier.
- Balance a given chemical equation by adjusting normal-script multipliers only, leaving all formulae unchanged.
Conservation of mass and balanced chemical equations exam tips
Marking Points
- States that atoms are not created or destroyed in a chemical reaction, only rearranged.
- Explains that the total mass of reactants equals the total mass of products in a closed system.
- Applies the law to a calculation, for example finding the mass of a product from the masses of reactants.
- Recognises that an apparent mass change is caused by a gas entering or leaving the reaction vessel.
- Links the law to the need for balanced symbol equations.
- Writes correct chemical formulae before attempting to balance an equation.
- Uses whole-number multipliers in front of formulae to balance atoms of each element.
- Counts atoms of every element on both sides and confirms they are equal.
- Recognises that changing a subscript alters the substance and is not allowed.
- Uses a balanced equation to identify the ratio of substances reacting.
- A subscript within a formula multiplies only the atom or group immediately before it, as in H₂O or Ca(OH)₂.
- A normal-script number before a formula multiplies every atom in that formula, as in 2H₂O giving four H atoms and two O atoms.
- Changing a subscript changes the chemical identity of the substance, so subscripts are fixed by the formula of each reactant and product.
- Balancing is achieved by placing or adjusting normal-script multipliers in front of formulae until the number of each type of atom is equal on both sides.
- Counting atoms in a displayed equation requires multiplying the front number by each subscript in the formula that follows it.
- The same conventions apply to formulae containing brackets, where a subscript after a bracket multiplies everything inside the bracket.
Examiner Tips
- 💡Show the subtraction or addition clearly when calculating an unknown mass.
- 💡Mention the sealed container or trapped gas when explaining an apparent mass change.
- 💡Use the phrase 'atoms are rearranged' rather than 'atoms are used up'.
- 💡Write a tally of atoms for each element before and after balancing.
- 💡Balance the element that appears in the fewest formulae first, then leave hydrogen and oxygen until last.
- 💡Recheck the final equation by counting all atoms once more.
- 💡When asked to balance an equation, count each element on both sides after every change and record the totals so errors are easy to spot.
- 💡If an equation will not balance with whole-number multipliers, double all multipliers rather than introducing fractions or altering formulae.
- 💡In written answers, distinguish clearly between a subscript and a front number, for example by describing 2H₂O as two molecules of water.
Common Mistakes
- Thinking mass is lost because a gas escapes; correction: the gas still exists and must be included or the vessel sealed.
- Believing atoms change into energy during ordinary chemical reactions; correction: atoms are conserved and only rearranged.
- Adding the masses of reactants and products together; correction: reactant mass equals product mass, so compare the two totals.
- Changing a subscript, such as turning H₂O into H₂O₂, to balance atoms; correction: only place multipliers in front of formulae.
- Balancing only one element and ignoring the others; correction: check every element in turn.
- Forgetting to multiply through when a formula contains brackets; correction: treat the bracket as a group and multiply all atoms inside it.
- Treating a subscript as if it multiplied the whole formula, for example reading H₂O as two hydrogen molecules; correct this by noting the subscript belongs only to the atom or group immediately before it.
- Balancing by altering subscripts, such as changing H₂O to H₂O₂; correct this by keeping every formula unchanged and adjusting only the normal-script multipliers.
- Forgetting to multiply every atom when a front number is present, for example counting 2H₂O as only two hydrogen atoms; correct this by multiplying the front number by each subscript in turn.