Chemical Reactions: What Changes and What Stays the Same
Extension topic – this whole chapter is Extension content in the 2025 junior secondary science curriculum (Unit 8 Atomic World, sub-topic 8.5).
Lighting a gas stove, frying an egg, a rusty railing by the sea – chemical reactions are all around you! In this chapter you will learn that matter is never lost in a chemical reaction, how to write balanced chemical equations, and why some processes release energy while others absorb energy. Take it one step at a time – you can do this!
1. What Happens in a Chemical Reaction?
A chemical reaction is a process that makes new substances (see the chapter Physical and chemical changes).
Reactants and products
The substances we start with are the reactants. The new substances that form are the products.
\(\text{reactants} \rightarrow \text{products}\)
The arrow means "react to form". A plus sign (+) separates the reactants, or the products.
Atoms are rearranged, not created or destroyed
All matter is made of tiny atoms. In a reaction, the atoms in the reactants break away from their old partners and join new partners to make the products. No atom appears or disappears.
Think of LEGO bricks: take apart a LEGO car and build a house with the same bricks. The model looks totally different, but the bricks are exactly the same – same number, same kinds, same total mass.
Key takeaway: In a chemical reaction, atoms are rearranged to turn reactants into products. No atoms are lost or made.
2. Matter Is Conserved in a Chemical Reaction
The law of conservation of mass
Because the atoms are the same before and after a reaction, the total mass stays the same too. This is the law of conservation of mass:
\(\text{total mass of reactants} = \text{total mass of products}\)
Example: 12 g of carbon burns completely in 32 g of oxygen. What mass of carbon dioxide forms?
\(\text{mass of carbon dioxide} = 12\ \text{g} + 32\ \text{g} = 44\ \text{g}\)
Testing it in a closed system
To test the law, nothing must get in or out. A sealed set-up that no substance can enter or leave is a closed system.
A simple test:
1. Stand a small tube of copper(II) sulphate solution inside a conical flask of sodium hydroxide solution.
2. Close the flask with a stopper and weigh the whole set-up on an electronic balance.
3. Tip the flask so that the two solutions mix. A new blue solid forms (no gas is made, so the stopper is safe).
4. Weigh the set-up again. The reading stays the same.
Now mix vinegar and baking soda in an open beaker on a balance. It fizzes and the reading goes down. Has matter been destroyed? No! The bubbles are carbon dioxide gas, which escapes into the air. If we caught and weighed the gas too, the total mass would not change.
Why does burning seem to change mass?
Burning usually happens in open air, which is not a closed system. The burning substance takes in oxygen from the air, and some products may be gases that escape. So the reading can go down or up.
Burning paper: the mass seems to go down
\(\text{paper} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water vapour}\)
These products are gases that escape into the air, so the little ash left behind is much lighter than the paper. The "missing" mass has gone into the air, not out of existence.
Burning iron wool: the mass goes up
\(\text{iron} + \text{oxygen} \rightarrow \text{iron oxide}\)
Iron joins with oxygen from the air to form iron oxide, a solid that stays on the balance. The extra mass is the mass of oxygen that joined the iron.
Did you know? In the 1700s, the French chemist Antoine Lavoisier weighed substances before and after reactions in sealed containers. His results helped scientists accept that mass is conserved.
Common mistake: "Burning makes matter disappear." Wrong! The atoms are all still there – some have just moved into the air as gases.
Key takeaway: In a closed system, total mass stays the same. In an open system, mass only seems to change because gases escape or oxygen joins from the air.
3. Writing Balanced Chemical Equations
A chemical equation is a short way to show a reaction. We build it in three steps.
Step 1: Write a word equation
Write the names of the reactants on the left and the products on the right. In this chapter we study three reactions in which elements join to form compounds:
\(\text{hydrogen} + \text{chlorine} \rightarrow \text{hydrogen chloride}\)
\(\text{hydrogen} + \text{oxygen} \rightarrow \text{water}\)
\(\text{carbon} + \text{oxygen} \rightarrow \text{carbon dioxide}\)
Step 2: Change the names into formulae
Hydrogen, oxygen and chlorine gases are made of molecules with two atoms joined together, so we write them as \(\text{H}_2\), \(\text{O}_2\) and \(\text{Cl}_2\). Carbon is written simply as \(\text{C}\).
The products are compounds: hydrogen chloride is \(\text{HCl}\), water is \(\text{H}_2\text{O}\) and carbon dioxide is \(\text{CO}_2\) (see Atoms and elements).
Step 3: Count the atoms and balance
An equation is balanced when each kind of atom has the same number on both sides – just as the law of conservation of mass says.
The golden rule: you may only change the big number in front of a formula, called the coefficient. Never change the small numbers inside a formula, called subscripts, because that would change the substance itself.
A coefficient multiplies everything in the formula after it. For example, \(2\text{H}_2\text{O}\) means 2 water molecules, which contain \(2 \times 2 = 4\) hydrogen atoms and \(2 \times 1 = 2\) oxygen atoms.
Memory aid – "Count, Fix, Check": Count the atoms of each kind on each side, Fix any difference by changing a coefficient, then Check every kind of atom again.
Example 1: Carbon + oxygen
\(\text{C} + \text{O}_2 \rightarrow \text{CO}_2\)
Left: C = 1, O = 2
Right: C = 1, O = 2
The numbers already match, so this equation is balanced as it is. (A coefficient of 1 is never written.)
Example 2: Hydrogen + chlorine
Unbalanced: \(\text{H}_2 + \text{Cl}_2 \rightarrow \text{HCl}\)
Left: H = 2, Cl = 2
Right: H = 1, Cl = 1 (not balanced)
We need 2 H and 2 Cl on the right, so we put a 2 in front of HCl:
\(\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}\)
Check: H = 2 on both sides and Cl = 2 on both sides. Balanced!
Example 3: Hydrogen + oxygen
Unbalanced: \(\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}\)
Left: H = 2, O = 2
Right: H = 2, O = 1 (oxygen not balanced)
Fix oxygen: put a 2 in front of \(\text{H}_2\text{O}\). Now the right side has H = 4 and O = 2. Oxygen is fixed, but hydrogen is not (2 on the left, 4 on the right).
Fix hydrogen: put a 2 in front of \(\text{H}_2\):
\(2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\)
Check: H = 4 on both sides and O = 2 on both sides. Balanced!
Notice: 3 molecules react to make 2 molecules. The number of molecules can change, but the number of each kind of atom cannot.
Common mistake: Changing \(\text{H}_2\text{O}\) into \(\text{H}_2\text{O}_2\) to "balance" the oxygen. \(\text{H}_2\text{O}_2\) is hydrogen peroxide, a different substance used as a bleach – not water! Only change coefficients.
Key takeaway: Word equation, then formulae, then balance by changing coefficients only until every kind of atom has the same count on both sides.
4. Energy Changes: Releasing and Absorbing Energy
Many processes involve an energy change, usually noticed as heat. There are two groups:
Energy-releasing processes give out energy to the surroundings, so the surroundings get warmer. Scientists also call them exothermic.
Energy-absorbing processes take in energy from the surroundings, so the surroundings get cooler. Scientists also call them endothermic.
Memory aid: EXothermic – heat EXits. ENdothermic – heat ENters.
Energy-releasing processes
1. Combustion (burning) – A fuel reacts with oxygen and releases heat and light. A gas stove, a candle flame and charcoal at a barbecue site in a Country Park all release energy. The three reactions in Section 3 also release energy.
2. Rusting – Iron reacts with oxygen and water to form rust, for example on old railings near the sea. Rusting releases heat, but so slowly that we usually cannot feel it.
3. Neutralisation – When an acid reacts with an alkali, a salt and water form and the mixture gets warmer – a thermometer shows the rise (see the chapter Neutralisation).
Did you know? Disposable hand warmers contain iron powder. When you open the pack, air gets in and the iron rusts very quickly, releasing enough heat to warm your hands on a cold winter day!
Energy-absorbing processes
1. Evaporation – A liquid must take in heat energy to turn into a gas. When sweat evaporates from your skin, it takes heat from your body, so you feel cooler – that is why you feel cold climbing out of a swimming pool on a windy day.
2. Dissolving table salt – When table salt (sodium chloride) dissolves in water, it takes in a little energy, so the water becomes slightly cooler. The temperature drop is small, so read the thermometer carefully.
Important: Evaporation and dissolving are physical changes – no new substance forms (water vapour is still water, and the salt can be got back by evaporating the water). So energy changes happen in physical processes as well as in chemical reactions. Some chemical reactions absorb energy too, such as photosynthesis, which takes in light energy.
How can we tell? Use a thermometer
Measure the temperature of the mixture before and after the process:
Temperature rises \(\rightarrow\) energy is released
Temperature falls \(\rightarrow\) energy is absorbed
Common mistake: Thinking an energy-absorbing process feels hot because it "takes in heat". It is the opposite! It takes heat away from the beaker and your hand, so it feels cold.
Key takeaway: Combustion, rusting and neutralisation release energy, so the surroundings get warmer. Evaporation and dissolving table salt absorb energy, so the surroundings get cooler.
Quick Review
1. In a chemical reaction, reactants change into products because their atoms are rearranged.
2. Law of conservation of mass: total mass of reactants = total mass of products.
3. In a closed system, the mass before and after a reaction stays the same.
4. Burning paper in the open seems to lose mass because gases escape. Burning iron wool gains mass because oxygen from the air joins the iron.
5. To write a chemical equation: word equation, then formulae, then balance.
6. Balance by changing coefficients only – never subscripts.
7. Energy-releasing: combustion, rusting, neutralisation. Energy-absorbing: evaporation, dissolving table salt.
Great work – keep practising balancing and it will soon feel easy!