An observation is something your senses can record. An explanation says why it happened, and it needs chemical knowledge. The mark scheme keeps them apart.
This lesson is part of interpreting unfamiliar chemical evidence. Use it before choosing supported observations.
How can I tell which is which?
Ask whether a camera could record the statement. If it could, the statement is an observation. If it needs you to know a reaction, it is an explanation.
Colour change, bubbles, a solid appearing, a solid disappearing, a temperature rise and a smell are observations. Names of products, names of gases, “displaced”, “neutralised” and “decomposed” are explanations.
Worked example: heating a green powder
A student heats green copper(II) carbonate in a boiling tube. The gas passes into limewater.
| Wrong version | Marked version | |
|---|---|---|
| Observation | Copper(II) oxide forms and carbon dioxide is released | Green solid turns black. Limewater turns cloudy |
| Explanation | (left blank) | Copper(II) carbonate decomposes to copper(II) oxide and carbon dioxide |
The wrong version is correct chemistry in the wrong column. It names two products that nobody saw. The marked version reports the colour change and the limewater result, then uses them to explain.
A second example: a displacement
A strip of zinc is placed in blue copper(II) sulfate solution for ten minutes. The strip is then taken out.
Observation: the blue solution becomes paler, and a brown solid coats the zinc strip.
Explanation: zinc is higher in the reactivity series than copper, so zinc displaces copper from the solution. The brown solid is copper.
Notice that “brown solid” is enough for the observation. Calling it copper turns the line into an inference.
The mistake that loses marks
The common slip is using a verb that carries a reaction inside it: “the solution is neutralised”, “copper is displaced”, “the salt dissolves completely”. Replace each one with what was seen, such as “the red colour of the indicator disappears”.
Check yourself
Sort each statement as observation (O) or explanation (E).
- (a) Magnesium ribbon shrinks and bubbles form.
- (b) Hydrogen gas is produced.
- (c) The tube feels warm.
- (d) The reaction is exothermic.
Answer
(a) O. Shrinking and bubbles can be seen.
(b) E. The gas is named, which needs a test or knowledge.
(c) O. A temperature change is felt directly.
(d) E. “Exothermic” interprets the warmth as heat released by the reaction.
What to study next
Move to choosing supported observations, then try the practice set. The claim, evidence and explanation revision desk helps you rewrite answers that mixed the two.
To have a teacher go through your written answers with you, see online one-to-one Chemistry tuition.