An observation is what you can see, hear, smell or read off an instrument. An inference is what you decide it means. Exam questions ask for one or the other, and mixing them up costs marks.
A quick test for any sentence
Ask: could someone with no Science knowledge write this sentence just by watching? If yes, it is an observation. If they would need to know a fact first, it is an inference.
“The limewater turned cloudy” passes the test. “Carbon dioxide was released” fails it, because you need to know what turns limewater cloudy.
Sorting a worked example
A student heats a blue crystal in a test tube. Here are five statements about it.
| Statement | Type |
|---|---|
| The blue crystal turned white | Observation |
| Droplets formed on the cooler part of the tube | Observation |
| The crystal contained water | Inference |
| The tube felt hot | Observation |
| A chemical change took place | Inference |
The inferences are all sensible, but “state two observations” does not ask for them.
The mistake and the fix
A typical weak answer to “State one observation” is: “Water was driven off.” That is an inference, because nobody saw water leave.
The rewarded version is: “Colourless droplets appeared on the wall of the tube.” It names what was visible, using a colour, a place and a physical state.
Good observations are specific. “It changed” is too vague, while “turned from blue to white” says exactly what changed.
When one question asks for both
Some questions ask you to observe and then infer. Write two labelled sentences.
Observation: the wet red litmus paper turned blue. Inference: the gas is alkaline, which fits ammonia. Keeping them apart shows the marker that you know the difference.
Try it yourself
Sort these into observation or inference: (a) the solution became warmer, (b) the reaction is exothermic, (c) a brown solid formed at the bottom, (d) a precipitate was formed.
Answer
(a) Observation, because a thermometer or your hand can record it. (b) Inference, because it explains why the temperature rose.
(c) Observation, colour and position are visible. (d) Inference, “precipitate” names what the solid is, which takes knowledge of solubility. If you chose (d) as an observation, “a solid formed” is the safe wording.
What to read next
The next skill is choosing an independent and dependent variable. The scientific reasoning hub lists the other skills. If you want a teacher to go through your own answers, try SPM Science tuition.