7.1.2Acids, Bases and Salts
Comparing Acids and Alkalis — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 3 of 8 Unit quiz
Section 1 of 7The pH scale
What the scale measures
- What the scale measures The pH scale is a number line that tells you how acidic or alkaline an aqueous solution is.
- In school work it runs from 0 to 14, although very concentrated acids can have a pH below 0 and very concentrated alkalis can exceed 14.
- Each number is a measurement of the concentration of hydrogen ions in the solution: more hydrogen ions means a lower pH.
Acids
- Acids Any solution with a pH below 7 is acidic.
- The lower the number, the more acidic the solution.
- Lemon juice and vinegar have a low pH, and battery acid is lower still.
Neutral solutions
- Neutral solutions A solution with a pH of exactly 7 is neutral.
- Pure water at room temperature is the standard example.
- Neutral solutions are neither acidic nor alkaline.
Alkalis
- Alkalis Any solution with a pH above 7 is alkaline.
- The higher the number, the more alkaline the solution.
- Soap, toothpaste, household bleach and sodium hydroxide solution sit progressively higher up the scale.
Section 2 of 7Strong and weak
| Typical pH range | Description | Example |
|---|---|---|
| 0 to 2 | Strongly acidic | Hydrochloric acid, sulfuric acid |
| 3 to 6 | Weakly acidic | Ethanoic acid (vinegar), citric acid |
| 7 | Neutral | Pure water |
| 8 to 11 | Weakly alkaline | Ammonia solution, sodium hydrogencarbonate solution |
| 12 to 14 | Strongly alkaline | Sodium hydroxide solution |
Section 3 of 7Using universal indicator
1
Place a small sample of the solution to be tested on a white tile or in a clean test tube.
2
Either add a few drops of universal indicator solution, or dip a strip of universal indicator paper into the liquid using clean forceps or a glass rod.
3
Look at the colour produced straight away, before it has time to change.
4
Compare that colour with the colour chart supplied with the indicator. Each colour on the chart matches a pH value.
5
Record the pH and use it to say whether the solution is acidic, neutral or alkaline, and roughly how strong it is.
Section 4 of 7Colour and pH
1Red and orange
- Red and orange These colours show a very low pH, so the solution is strongly acidic.
2Yellow
- Yellow Yellow shows a pH of about 5 to 6, which is a weak acid.
3Green
- Green Green means the pH is 7, so the solution is neutral.
4Blue
- Blue Blue shows a pH of about 9 to 11, which is a weak alkali.
5Purple
- Purple Purple shows a very high pH, so the solution is strongly alkaline.
Section 5 of 7Comparing solutions
Ranking by acidity
- Ranking by acidity To rank several acids, test each one and put them in order of pH.
- The solution with the lowest pH is the most acidic.
- Using the same indicator and the same volume of each solution makes the comparison fair.
Why indicator papers are useful
- Why indicator papers are useful Universal indicator gives a range of colours,
- so it shows how acidic or alkaline a solution is, not just which side of neutral it lies on.
- Litmus only shows acid (red) or alkali (blue), so it cannot be used to compare strengths.
Limits of the method
- Limits of the method Matching colours by eye is a judgement and different people may disagree by a pH unit.
- A coloured solution can hide the indicator colour, so a pH meter is more reliable and gives a precise numerical reading.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| pH scale | A scale from 0 to 14 that measures how acidic or alkaline a solution is. |
| Acid | A solution with a pH below 7. |
| Alkali | A solution with a pH above 7; a soluble base. |
| Neutral | A solution with a pH of exactly 7, neither acidic nor alkaline. |
| Universal indicator | A mixture of dyes that changes through a range of colours as the pH changes. |
| Hydrogen ion | The ion, H+, whose concentration in a solution determines its acidity. |
Section 7 of 7Exam technique
1
Saying that a higher pH means a more acidic solution. It is the other way round: lower pH means more acidic.
2
Thinking that pH 7 is 'weakly alkaline'. A pH of 7 is neutral and has its own colour, green, with universal indicator.
3
Confusing the strength of an acid with its concentration. Strength describes how many hydrogen ions each molecule releases, and the pH describes the resulting solution.
4
Describing universal indicator as only turning 'red or blue'. It shows a full range of colours and that range is what lets you estimate the pH.
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