🔥 Chemical Reactions & Word Equations: From Ingredients to New Substances
Learn how to describe chemical reactions using word equations, identifying reactants and products in everyday changes like rusting, cooking, and respiration.
Learning Goals
By the end of this lesson, you will be able to:
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Distinguish between reactants and products in a chemical reaction
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Write accurate word equations for common everyday reactions
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Identify evidence that a chemical reaction has occurred
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Explain why mass is conserved during a chemical reaction
What You Should Already Know
Before this lesson, you should know:
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The difference between a physical change (like melting) and a chemical change (like burning)
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That substances are made of particles and have measurable mass
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Basic names of common substances such as oxygen, water, and carbon dioxide
Notation & Symbols
Key notation and symbols used in this topic:
+
Means 'reacts with' or 'and' when placed between substances on the same side of an equation
→
The reaction arrow — means 'produces' or 'yields', pointing from reactants to products
Reactants
The starting substances, written on the left of the arrow
Products
The new substances formed, written on the right of the arrow
(s), (l), (g), (aq)
State symbols for solid, liquid, gas, and aqueous (dissolved in water)
Understanding Chemical Reactions and Word Equations
A chemical reaction is a process in which one or more substances (reactants) are changed into one or more new substances (products) with different properties. A word equation describes this change using the names of the substances.
Reactants and products: Reactants are written on the left of the arrow and products on the right. The arrow → shows the direction of change and is read as 'produces'.
Evidence of a reaction: Signs include colour change, gas bubbles, a temperature change, light, or a precipitate (new solid). Rusting iron turns reddish-brown; burning wood releases heat and light.
Conservation of mass: Atoms are never created or destroyed in a reaction, only rearranged. The total mass of reactants equals the total mass of products.
Structure of a word equation: General form: reactant+reactant→product+product. Each '+' separates different substances on the same side.
Example
When iron rusts, iron reacts with oxygen in damp air to form iron oxide (rust). As a word equation: iron+oxygen→iron oxide Iron and oxygen are the reactants; iron oxide is the product.
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The arrow separates reactants (iron and oxygen) from the product (iron oxide) in a rusting reaction.
Worked Example
Basic
Write the word equation for the rusting of iron, where iron reacts with oxygen to form iron oxide.
Worked Example
Intermediate
During respiration in your body cells, glucose reacts with oxygen to release energy, producing carbon dioxide and water. Write the word equation.
Worked Example
Advanced
When you heat sodium hydrogencarbonate (baking soda) during baking, it breaks down into sodium carbonate, water, and carbon dioxide. Write the word equation and identify the type of reaction.
Why It Works
Understanding the reasoning behind the rule:
Word equations rest on one powerful idea: the law of conservation of mass. Understanding this explains why reactants turn into products without matter vanishing.
Your Turn
Try this one yourself — use the hint if you get stuck:
During photosynthesis, a plant uses carbon dioxide and water, with light energy, to make glucose and oxygen. Write the word equation.
Watch Out For...
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Writing the arrow in a word equation as an equals sign (=).
A chemical reaction uses an arrow → meaning 'produces' or 'turns into'. The equals sign is for mathematics; the arrow shows a one-way change from reactants to products.
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Putting the products on the left and reactants on the right.
Always write reactants (starting substances) on the left of the arrow and products (new substances) on the right: reactants → products. For rusting: iron + oxygen → iron oxide.
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Thinking new atoms are created or mass disappears during a reaction.
Atoms are only rearranged, never created or destroyed. The total mass of reactants equals the total mass of products — this is conservation of mass.
In the Real World
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Baking bread: the word equation for respiration in yeast, glucose → ethanol + carbon dioxide, explains why dough rises as bubbles of gas form.
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A rusting bicycle chain follows iron + oxygen + water → hydrated iron oxide, which is why engineers coat or oil metal to keep oxygen and water away.
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Photosynthesis in leaves, carbon dioxide + water → glucose + oxygen, is the reaction that supplies almost all the oxygen you breathe.
Extended Practice
Test yourself with a mix of multiple choice and short-answer questions:
Q1.
In the word equation magnesium + oxygen → magnesium oxide, which substances are the reactants?
MCQ
Q2.
Which symbol correctly separates the products in a word equation?
MCQ
Q3.
Write the word equation for the burning of carbon (in a barbecue) which combines with oxygen to form carbon dioxide.
carbon + oxygen → carbon dioxide
Q4.
A student heats 10 g of a metal with 4 g of oxygen and it all reacts to form a metal oxide. By conservation of mass, what mass of metal oxide forms?
10+4=14 g of metal oxide, because mass is conserved.
Q5.
In photosynthesis, carbon dioxide + water → glucose + oxygen, name one product.
Glucose (or oxygen) — both are products on the right of the arrow.
Extension
Advanced
A harder challenge that connects to the next skill:
When a candle burns, wax (which contains carbon and hydrogen) reacts with oxygen to produce carbon dioxide and water vapour. Write the word equation, and explain why the candle seems to 'lose mass' even though mass is conserved.
Key Takeaways
1.A word equation shows reactants → products, using + to separate substances and an arrow to mean 'produces'.
2.Reactants are the starting substances (left of the arrow); products are the new substances formed (right of the arrow).
3.In every chemical reaction atoms are rearranged, not created or destroyed, so total mass is conserved: mass of reactants = mass of products.
4.Everyday reactions like rusting, burning, respiration and photosynthesis can all be described with word equations.
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Ready to Practice?
Now that you've learned Chemical Reactions & Word Equations, put it to the test with a quick quiz!