Grade 8
Chemical reactions and how to recognise them
A chemical reaction rearranges atoms into new substances. Observable changes can provide evidence, but a change of state alone is not a reaction.
IntuitionIntuition: particles and change
A candle burning gives off heat and light and produces gases; melting candle wax changes only its state. The key question is whether new substances form.
SchoolSchool: models and rules
Definition: Key idea
A chemical reaction is a process in which reactant particles are converted into product particles through rearrangement of atoms and changes in chemical bonds.
Common clues include gas bubbles, a precipitate, a lasting colour change, light, or a temperature change. None is conclusive by itself: boiling also makes bubbles, but it is a physical change.
| Observation | Suggests reaction when… | Physical look-alike |
|---|---|---|
| Bubbles | a new gas forms in the liquid | boiling water |
| Colour change | it persists without dilution or light change | diluting a coloured drink |
| Heat or light | the mixture warms or glows on its own | a bulb filament glowing |
| New solid in liquid | a precipitate appears from clear solutions | chalk dust suspended in water |
Example: Worked example
Two clear solutions form a cloudy solid when mixed. What does this suggest?
Solution
A precipitate has formed, evidence that a new, poorly soluble substance was produced. Bubbles or colour alone are less decisive because physical changes can also cause them.
Example: Second example
Quicklime CaO is dropped into water: the mixture heats and clouds. Is it a reaction?
Solution
Yes — the temperature rises spontaneously and a new substance, Ca(OH)₂, forms: CaO + H₂O → Ca(OH)₂.
Record observations before, during and after mixing. Compare with controls when possible, and use tests—such as checking whether a gas relights a glowing splint—only under suitable safety rules.
In a reaction, the total numbers of each kind of atom stay the same before and after. Rearranged atoms mean changed substances — this is why reactants and products have different properties.
Example: Calculation / application example
When copper wire is dipped in AgNO₃ solution, the solution turns blue and silvery crystals grow. What are the signs of reaction?
Solution
Two clear signs: a persistent colour change (colourless to blue due to Cu²⁺ ions) and the deposition of a new solid (silver metal crystals).