CLASS 10 SCIENCE · CHAPTER 1 · STUDY NOTES
Chemical Reactions and Equations
Understand what changes, learn to balance equations, and connect reaction patterns to the world around you.
EXPLORE
What actually changes?
A reaction is a rearrangement of atoms that makes new substances.WHAT CHANGES?
Same atoms. Different substances.
Bonds change; H and O atoms are conserved. Hydrogen and oxygen become water.
- Chemical reaction
- A change in which reactants form one or more new substances. Atoms are rearranged; their identities are retained in an ordinary chemical reaction.
- Reactants and products
- Reactants are the starting substances. Products are the substances formed by the reaction.
Picture a set of building blocks. You take a model apart and rebuild it into a different model. The blocks are still there, but their arrangement has changed. Atoms behave a little like those blocks: old bonds break and new bonds form.
The materials you start with are reactants. The materials you end with are products. A chemical reaction changes the identity of at least one substance; a change in shape or state alone does not necessarily do that.
Melting ice still gives water, so it is a physical change. When magnesium burns, magnesium oxide forms. That is a chemical change.
A new material appears
2Mg(s) + O₂(g) → 2MgO(s)Magnesium burns with a bright white light and produces a white solid, magnesium oxide. Cleaning the ribbon removes its surface oxide layer so magnesium can react more easily.
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Question
Does cutting paper into tiny pieces make a new substance?
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EXPLORE
Look for clues
What you notice can help you infer that a reaction has happened.OBSERVE THE EVIDENCE
What can a reaction look like?
Zn + 2HCl → ZnCl₂ + H₂
Hydrogen bubbles form when zinc reacts with dilute hydrochloric acid. The gas is a product, not boiling water.
- Evidence of a chemical reaction
- Observations such as gas evolution, a precipitate, or a colour or temperature change can indicate a reaction. Check whether new substances have formed: an observation alone is not always proof.
| Clue | Example | What it tells you |
|---|---|---|
| A new colour | A copper deposit forms on iron in copper sulphate solution. | A different material may have formed. |
| Gas appears | Zinc reacts with dilute acid and hydrogen bubbles form. | A gaseous product is being produced. |
| A solid appears in a liquid | Two solutions form an insoluble solid. | That solid is called a precipitate. |
| The mixture warms or cools | Quicklime reacts with water and releases heat. | Energy is being transferred. |
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Question
Two clear solutions make a cloudy solid. What is the solid called?
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EXPLORE
Read the language of equations
An equation is a compact story: what goes in, what comes out, and under what conditions.READ THE SCIENTIFIC LANGUAGE
A chemical equation is a compact story.
Reactants are the substances you start with. Magnesium and oxygen are written on the left.
- Aqueous, (aq)
- The substance is dissolved in water. This is different from (l), which labels a substance in the liquid state.
| Symbol | Meaning | Read it as |
|---|---|---|
| (s) | Solid | Iron as a solid metal |
| (l) | Liquid | Liquid water |
| (g) | Gas | Steam or hydrogen gas |
| (aq) | Dissolved in water | A salt solution, not a pure liquid salt |
| Heat / light / electricity above the arrow | A reaction condition | Energy is supplied in that form |
Read the arrow as “reacts to form”. A plus sign separates substances on the same side. Reactants go on the left; products go on the right. A skeletal equation has correct formulae but may not yet have equal atom counts.
Words → formulae → balanced equation
Magnesium + oxygen → magnesium oxideThe word equation tells the story. Mg + O₂ → MgO uses formulae. 2Mg + O₂ → 2MgO balances the atoms.
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Question
Is H₂O(g) the same physical state as H₂O(l)?
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EXPLORE
Balance without changing the substance
Atoms are conserved. Count each element separately on both sides.COUNT · CHANGE · CHECK
Can you conserve every atom?
1H₂ + 1O₂ → 1H₂O
Not balanced yet. Change the coefficients until both atom counts match.
- Balanced chemical equation
- An equation with the same number of atoms of each element on both sides. Change coefficients, never the formula of a substance.
| Element | Reactant atoms | Product atoms |
|---|---|---|
| Fe | 3 | 3 |
| H | 4 × 2 = 8 | 4 × 2 = 8 |
| O | 4 × 1 = 4 | 4 |
In an ordinary chemical reaction, atoms are rearranged rather than created or destroyed. A balanced equation respects conservation of mass. Balance by changing coefficients in front of formulae—not the subscripts inside them.
A first balancing example
2H₂(g) + O₂(g) → 2H₂O(l)There are 4 hydrogen atoms and 2 oxygen atoms before and after. Two hydrogen molecules and one oxygen molecule rearrange into two water molecules.
3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
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Question
Balance Mg + O₂ → MgO.
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Why is 2H₂O different from H₂O₂?
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EXPLORE
Combination: several become one
Two or more reactants form one product.TWO BECOME ONE
Quicklime + water → slaked lime
CaO + H₂O → Ca(OH)₂ + heat
One product substance forms from two reactants. “Combination” describes the pattern; “exothermic” describes its energy transfer.
- Combination reaction
- Two or more reactants form a single kind of product.
Reaction labels describe different aspects of a change. “Combination” describes the reactants and products. “Exothermic” describes the energy transfer. A reaction can have both labels.
In whitewashing, calcium hydroxide later reacts with carbon dioxide in air: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The calcium carbonate layer gives the finish. Do not confuse quicklime, CaO, with slaked lime, Ca(OH)₂.
Quicklime → slaked lime
CaO(s) + H₂O(l) → Ca(OH)₂(aq)Calcium oxide combines with water to make calcium hydroxide. The reaction releases heat, so it is both a combination reaction and an exothermic reaction.
Burning carbon
C(s) + O₂(g) → CO₂(g)Carbon and oxygen form a single product, carbon dioxide.
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Question
Is 2H₂ + O₂ → 2H₂O a combination reaction even though it makes two molecules?
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EXPLORE
Where does the energy go?
Reactions may transfer heat to their surroundings or absorb energy.FOLLOW THE ENERGY
Does heat enter or leave the system?
The system releases energy to its surroundings. Heat flows out.
- Exothermic reaction
- A reaction that releases heat to its surroundings.
- Endothermic reaction
- A reaction that absorbs energy from its surroundings.
Exothermic reactions release heat to the surroundings. Endothermic reactions absorb energy from the surroundings. Think of the reaction mixture as the system: does energy leave it or enter it?
Burning fuel releases energy. Cellular respiration also releases energy from glucose; it is not simply breathing air in and out. Decomposition often needs an input such as heating, light or electricity.
Energy from a fuel
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)Burning methane is exothermic.
Energy in living cells
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂ORespiration releases energy that cells can use.
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Question
A reaction mixture releases heat and warms its surroundings. Exothermic or endothermic?
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