A pH measurement gives evidence about a solution. To link it to a class of substance, you must say what was dissolved and how that substance behaves in water.
This lesson belongs to chemical reasoning in mixed tasks. The data below is invented for teaching and is labelled as such. Hazards are discussed conceptually only.
How do I read acidity evidence?
Use pH as the first clue.
- pH below 7: acidic. A dissolved non-metal oxide, such as sulfur dioxide, is a common reason.
- pH 7: neutral. Pure water and many salt solutions fall here.
- pH above 7: alkaline. A soluble metal hydroxide, or a metal oxide that dissolves, is a common reason.
Then ask what second piece of evidence would support it, such as neutralisation by a known alkali or acid.
Worked example (invented data)
A student collects rainwater near a factory and bubbles the exhaust gas through distilled water. Pure distilled water has pH 7.0. The measurements are:
| Sample | pH |
|---|---|
| Distilled water, before | 7.0 |
| Water after exhaust gas, reading 1 | 3.1 |
| Water after exhaust gas, reading 2 | 3.0 |
| Water after exhaust gas, reading 3 | 3.2 |
The acidic solution is then neutralised with a standard alkali. Three titrations need 20.1 cm³, 19.9 cm³ and 20.0 cm³.
Question: What class of substance does the exhaust gas contain, and how secure is the conclusion?
Step 1, state the evidence. The pH fell from 7.0 to about 3.1. The three readings differ by 0.2 at most, so they agree.
Step 2, interpret. A pH of about 3 means the solution is acidic, so the gas reacted with water to form an acid. This supports an acidic oxide, which is a non-metal oxide.
Step 3, check the repeats. The mean titre is (20.1 + 19.9 + 20.0) ÷ 3 = 60.0 ÷ 3 = 20.0 cm³. The values are close together, so the result is reliable.
Step 4, limit the conclusion. The pH shows the gas forms an acid. It does not name the exact gas. A further test would be needed to identify it.
The mistake to watch for
Mistaken answer: “The pH is 3, so the gas is sulfur dioxide.”
The student named one gas, but several non-metal oxides give acidic solutions, and the data does not separate them. The correction is to state the class the evidence supports (“an acidic oxide”) and say that the exact gas needs another test. A second slip is saying “the gas is an acid”, when it is the solution that is acidic.
Check yourself
1. Solution A has pH 12.4 and solution B has pH 6.8. Describe each.
Show answer
A is alkaline because its pH is above 7. B is weakly acidic because its pH is below 7 but close to it.
2. Three titres are 21.4 cm³, 21.0 cm³ and 21.2 cm³. Find the mean titre.
Show answer
Sum = 21.4 + 21.0 + 21.2 = 63.6. Mean = 63.6 ÷ 3 = 21.2 cm³.
3. A metal oxide is added to water and the pH rises to 12. What class does this suggest, and why is it not proof?
Show answer
It suggests a metal oxide that dissolves to form an alkali, because the solution is alkaline. It is not proof because another alkaline substance could give the same pH, so a second test would make the conclusion more secure.
Where this leads next
Next, distinguish observation from inference across two chemical tests to practise separating what you saw from what you conclude. You can revisit interpreting a separation sequence for the other kind of method reasoning, and the mixed practice set includes an acidity case.
If your conclusions go further than the evidence allows, our teachers can help you set the limit in online one-to-one Combined Science tuition.