Acids and alkalis: read pH and explain neutralisation
Classify solutions using pH, distinguish an alkali from a base, and use evidence to decide whether an acid has been neutralised.
Before you begin
- Solutions and dissolving
- Chemical reactions form new substances
Read the pH scale
For the aqueous solutions near room temperature used here, pH below 7 is acidic, pH 7 is neutral and pH above 7 is alkaline. A smaller pH means a more acidic solution. The familiar school scale runs from 0 to 14, but these are not absolute limits for every solution. pH has no unit. It is not a linear scale: a pH of 4 does not mean twice the acidity of pH 8. Do not average two pH readings to predict the pH after mixing.
Distinguish a base from an alkali
A base reacts with an acid in a neutralisation reaction. An alkali is a base that dissolves in water and produces an alkaline solution. Sodium hydroxide is an alkali; copper oxide is a base with very low solubility, so it is not classed as an alkali. Every alkali is a base, but not every base is an alkali. The colourless appearance of a solution tells you neither its pH nor whether it is safe.
Use indicators as evidence
An indicator changes colour over a particular pH range. Universal indicator is a mixture that gives a range of colours; compare the observed colour with the chart supplied for that indicator. This gives an approximate pH, not an exact decimal measurement. A calibrated pH meter can give a more detailed reading, but calibration, contamination and temperature still affect results. Colour is evidence about pH, not proof of a chemical's identity.
Explain neutralisation without guessing the endpoint
When hydrochloric acid reacts with sodium hydroxide, sodium chloride and water form. Sodium chloride is one example of a salt; the word salt in chemistry does not mean only table salt. If acid remains in excess, the mixture can still be acidic even though some neutralisation has occurred. With these reactants, neither in excess and near room temperature, the solution is approximately pH 7. Adding more alkali makes it alkaline. Equal volumes alone do not guarantee neutralisation: the amounts and concentrations matter. Other acid–base combinations need not have pH 7 at completion.
Worked example
Solution P has pH 3 and solution Q has pH 9. Classify each. A student mixes equal volumes and predicts pH 6 by averaging. Is the prediction justified?
Show the worked solution
- P is acidic because 3 is below 7. Q is alkaline because 9 is above 7.
- The arithmetic average is 6, but mixing does not average pH: pH is not a linear measure of the reacting amounts.
- The reactant identities, concentrations and amounts are needed to predict the final pH. Measure the mixture with suitable equipment rather than claiming it is pH 6.
P is acidic; Q is alkaline. The final pH cannot be found by averaging.
Try it yourself
An acid–alkali mixture changes from pH 2 to pH 5. Which conclusion is supported?
Common mistakes
- An alkali is a soluble base; an insoluble base need not produce an alkaline solution.
- Do not average pH values or assume equal volumes react exactly.
- Neutralisation may be partial; a reaction is not evidence that the final pH is 7.
- An indicator chart estimates pH; it does not identify an unknown substance.
Read the scale and the evidence
Interpret a neutralisation table
Original illustrative readings for adding dilute sodium hydroxide to a fixed starting sample of dilute hydrochloric acid, near room temperature. These are constructed practice data, not measured experimental results or instructions for mixing chemicals.
| Total alkali added (mL) | Measured pH |
|---|---|
| 0 | 2.0 |
| 5 | 2.5 |
| 9 | 3.3 |
| 10 | 7.0 |
| 11 | 10.7 |
| 15 | 11.3 |
Write answers before opening the guidance. These five self-check points are learning guidance, not official marks or an automatically awarded score.
- Classify the mixture at 9 mL, 10 mL and 11 mL.
- Which recorded addition gives a neutral reading? What does the next reading suggest is in excess?
- Can the table locate the neutral point more precisely than the recorded additions? Explain.
- Suggest a measurement improvement and a control for comparing repeated runs.
- Explain why the value at 10 mL cannot be used as a rule for every acid sample.
Compare my data reasoning
- 9 mL: acidic (pH 3.3); 10 mL: neutral (pH 7.0); 11 mL: alkaline (pH 10.7).
- 10 mL gives the recorded neutral reading. At 11 mL, added alkali is in excess in this model.
- No. The table contains only selected readings; the displayed decimal does not establish an exact neutral volume or the full uncertainty.
- Take smaller additions near the large pH change and repeat using a calibrated meter, rinsing the probe between samples. Keep the initial acid volume and concentration, alkali concentration and temperature comparable.
- The amount needed depends on the reactants and their amounts and concentrations. A larger or more concentrated acid sample need not require 10 mL.
Challenge a conclusion
A different experiment records pH 4.6 after 6 mL of alkali and pH 9.2 after 8 mL. A student claims that exactly 7 mL must give pH 7. Explain the flaw and choose the next measurement.
Compare my transfer reasoning
The change suggests the neutral reading lies between those additions if the pH rises continuously, but the midpoint of the volumes need not give pH 7. pH does not change linearly with added volume. A measurement at 7 mL can narrow the interval; it is a test of the claim, not proof in advance.
Plan a supervised comparison
This lesson uses supplied data. Any real investigation needs teacher-selected chemicals, quantities, eye protection and a risk assessment. Do not taste unknown solutions or mix household cleaners. Explain how you would prevent contamination and keep initial samples comparable; completing this written task does not demonstrate supervised practical competence.
Specialist review has not been completed. This introductory lesson does not cover salt preparation or all acid reactions. Grade 8 is StudyForA’s suggested placement. Curriculum reference: England KS3 science: acids, alkalis, pH and neutralisation.
See how this lesson connects to its questions and practice.
Connect to conservation: reacting matter is rearranged, not destroyed.
What can you explain now?
An unknown solution gives pH 11. After some acid is added, the measured pH is 8. Classify both readings and explain whether this establishes complete neutralisation.
Compare with the explanation
Both readings are alkaline. The decrease does not establish complete neutralisation.
Both 11 and 8 exceed 7. The solution has become less alkaline, but alkali may still be in excess. A changed pH alone does not identify the substances or prove that exactly the required amounts have reacted.
After trying it yourself, choose your next review. This is your self-assessment.
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