In an identification task each test produces an observation and, after reasoning, an inference. Keeping them in separate columns protects your marks and prevents jumping to a conclusion the evidence cannot support.
This lesson belongs to chemical reasoning in mixed tasks. The results below are invented for teaching and are labelled as such. Only the reasoning is taught here, and test procedures are done in a laboratory under teacher supervision.
How do I keep the two apart?
- An observation uses words like “formed”, “turned”, “dissolved” or “gave off”. It describes what happened without naming a chemical.
- An inference names a substance, ion or class, and starts with “so” or “this shows”.
A useful check: if someone with no chemistry knowledge could write it down, it is an observation.
Worked example (invented results)
A student has an unknown blue solid, W, and carries out two tests on separate samples of its solution.
| Test | Observation | Inference |
|---|---|---|
| 1. Add sodium hydroxide solution | A blue precipitate formed | Copper(II) ions are present |
| 2. Add dilute nitric acid, then silver nitrate solution | A white precipitate formed | Chloride ions are present |
Question: What is the identity of W, and which evidence supports each part?
Step 1, test 1. The blue precipitate with sodium hydroxide is the result expected for copper(II) ions. This is an inference about the positive ion.
Step 2, test 2. Acid is added first so that other ions do not give a false result. The white precipitate with silver nitrate is the result expected for chloride ions, an inference about the negative ion.
Step 3, combine. Copper(II) ions with chloride ions point to copper(II) chloride, whose formula is CuCl₂.
Step 4, check the formula. Copper(II) has a 2+ charge and chloride has a 1− charge, so two chloride ions balance one copper ion: 2+ and 2 × (1−) = 0.
Step 5, state the limit. The two tests are consistent with copper(II) chloride. A stronger answer says which extra check would confirm it, such as a second test for chloride or copper(II).
The mistake to watch for
Mistaken observation: “Copper(II) hydroxide formed.”
The student wrote an inference in the observation column. What was seen is “a blue precipitate formed”. The name copper(II) hydroxide comes from knowing the chemistry, so it belongs in the inference column.
The correction is to describe colour and state in the observation, and to name the ion in the inference. A second slip is giving only one half of the identity when both tests were done.
Check yourself
1. Sort into observation (O) or inference (I): (a) the gas turned limewater milky; (b) the gas is carbon dioxide; (c) the solid dissolved to give a colourless solution.
Show answer
(a) O, because it describes what was seen. (b) I, because it names a substance. (c) O, because it describes what was seen.
2. A solid gives bubbles when dilute acid is added, and the gas turns limewater milky. What is the likely negative ion, and what is the formula of the gas?
Show answer
The likely negative ion is carbonate, and the gas is carbon dioxide, CO₂. State it as “suggests”, since another test would make it more secure.
3. Write the formula of a compound made from a 2+ metal ion and chloride ions.
Show answer
Charges must balance, so two 1− chloride ions are needed for each 2+ metal ion. The formula pattern is MCl₂ (for example, CuCl₂).
Where this leads next
You have now covered each skill in this module. Try the mixed practice set, then use the scientific investigation critic to check whether your test plans give clear evidence. You can also return to acidity evidence and substance class to compare two ways of reasoning from evidence.
If you want someone to check your identification answers line by line, our teachers can do that with you in online one-to-one Combined Science tuition.