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IB DemystifiedMYP Sciences

Word and symbol equations; conservation of mass

When wood burns to a small pile of ash, where does the rest go? Chemical equations are the language chemists use to describe reactions, and they rest on one simple rule: atoms are never created or destroyed, only rearranged.

Recommended for MYP 3 · About 3 lessons · Criteria A, B, C and D

HHHH+OO→OHHOHH2H₂ + O₂2H₂O4 H atoms, 2 O atoms4 H atoms, 2 O atomsAtoms are rearranged,not created or destroyed.
Figure 1. The same atoms appear on both sides of a balanced equation.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Balancing step by step
  6. Equations in the real world
  7. Worked examples
  8. Assessment tips
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • write word equations and identify reactants and products
  • interpret formulas and state symbols
  • balance symbol equations by counting atoms
  • apply conservation of mass to open and closed systems
  • calculate masses of reactants and products
  • discuss carbon dioxide from cement and recycling

Before you start

You will use these skills. If any feel shaky, review them first.

  • elements, compounds and formulas (see Elements, compounds and the periodic table)
  • physical and chemical changes (see Physical and chemical changes)
  • acids and alkalis (see Acids, alkalis and indicators)

Key vocabulary

Reactant
A substance that reacts, shown on the left of an equation.
Product
A substance formed, shown on the right of an equation.
Balanced equation
An equation with the same number of each type of atom on both sides.
State symbols
(s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
Conservation of mass
The total mass of products equals the total mass of reactants.
Closed system
A container from which nothing can escape.

Understanding the ideas

  1. Word and symbol equations

    A word equation names the reactants and products: magnesium + oxygen → magnesium oxide. A symbol equation uses formulas: 2Mg + O₂ → 2MgO. State symbols show whether each substance is solid, liquid, gas or dissolved in water.

  2. Balancing equations

    In a reaction, atoms are rearranged into new substances, but none are created or destroyed. So a balanced equation must have the same number of each type of atom on both sides. We balance equations by changing the large numbers in front of formulas, never the small numbers inside them.

  3. Conservation of mass

    Because atoms are conserved, the total mass of products always equals the total mass of reactants. In an open container, a gas may escape and the mass seems to fall; if a gas from the air joins in, the mass seems to rise. In a closed system, the mass stays the same.

  4. Why does it matter?

    Conservation of mass lets chemists calculate how much product a reaction will make and how much waste, such as carbon dioxide, it will release.

  5. What does it connect to?

    This topic underpins all of chemistry, including reactivity, neutralisation, rates and quantitative chemistry.

Balancing step by step

  • Write the correct formulas for every reactant and product.
  • Count each type of atom on both sides.
  • Add large numbers in front of formulas to balance one element at a time (often leaving O or H until last).
  • Never change the small numbers inside a formula; that would change the substance.
  • Check every element again; add state symbols.

Equations in the real world

Cement works calculate the carbon dioxide released from limestone using equations. Engineers use balanced equations to work out how much fuel and oxygen an engine needs. Recycling keeps valuable atoms, such as aluminium, in use.

Worked examples

Example 1: balancing

Balance: Na + Cl₂ → NaCl.

  1. Left has 2 Cl; put 2 in front of NaCl: 2NaCl.
  2. Now 2 Na are needed: 2Na + Cl₂ → 2NaCl.

Example 2: conservation of mass

12 g of carbon burns to form 44 g of carbon dioxide. What mass of oxygen reacted?

  1. Mass of reactants = mass of products.
  2. Oxygen = 44 − 12 = 32 g.

Assessment tips

Questions on this topic often ask you to complete, balance and use equations. Expect to:

  • Write word and symbol equations with state symbols.
  • Balance equations and show that atoms are conserved.
  • Calculate unknown masses using conservation of mass.
  • Explain mass changes in open and closed systems.

Common mistakes: changing small numbers in formulas to balance; forgetting that O₂ has two atoms; saying mass is lost when a gas escapes; and mixing up reactants and products.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: burning steel wool

Partially guided investigation · about 20 minutes · teacher demonstration

Research question
What happens to the mass of steel wool when it burns in air?
Scientific background
Steel wool is mostly iron. When it burns, iron atoms combine with oxygen atoms from the air to form iron oxide.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
A ball of fine steel wool, heatproof mat, top-pan balance reading to 0.01 g, a 9 V battery or Bunsen burner (teacher), tongs, safety screen.
Method
  1. Place the steel wool on the heatproof mat on the balance and record the mass.
  2. The teacher touches the steel wool with the battery terminals (or a flame) so that it glows and burns.
  3. Record the mass after the reaction has finished and the product has cooled.
  4. Compare the masses and write word and symbol equations for the reaction.

Safety. Teacher demonstration only, behind a safety screen. Burning steel wool gets very hot and can throw sparks; keep flammable materials away and use tongs.

Then evaluate: explain why the mass increased, using conservation of mass.

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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