Acids and Bases
From the tangy bite of lemon juice to the slippery feel of soap, acids and bases are everywhere. They are two major classes of substances with characteristic properties that chemists have studied for centuries. Understanding acids and bases is the key to understanding many of the reactions that shape our world.
Before You Begin
Think about these common substances: lemon juice, vinegar, baking soda, drain cleaner, and water.
Write down your answers before reading on:
- Which of these would taste sour? Which would taste bitter or feel slippery?
- What do you already know about the words "acid" and "base"?
- How might scientists tell whether an unknown liquid is acidic or basic?
Know
- That acids and bases are classes of substances with characteristic properties
- The pH scale and what acidic, neutral and alkaline mean
- Common acids and bases found in everyday life
Understand
- How pH indicators detect acidity and alkalinity through colour change
- That pH meters give a numerical reading of acidity/alkalinity
- Why universal indicator is more useful than single-colour indicators
Can Do
- Classify common substances as acidic, neutral or basic using pH
- Interpret indicator colour changes to estimate pH
- Select the appropriate tool (indicator paper or pH meter) for a given situation
Acids and Bases in Everyday Life
Two classes of substances all around you
Chemical Equations
Acids and bases are classes of substances with characteristic properties that you can observe and test.
Acids typically taste sour, react with metals to produce hydrogen gas, and turn blue litmus paper red. Common examples include lemon juice (citric acid), vinegar (acetic acid), and fizzy drinks (carbonic acid). Your stomach produces hydrochloric acid to help digest food.
Bases (also called alkalis when dissolved in water) typically taste bitter, feel slippery or soapy, and turn red litmus paper blue. Common examples include baking soda (sodium bicarbonate), soap, detergent, and drain cleaner. Many cleaning products are basic because bases dissolve grease and oils.
Common acids and bases
| Common acids | Where found | Approximate pH |
|---|---|---|
| Hydrochloric acid (dilute) | Stomach acid | 1–2 |
| Lemon juice (citric acid) | Citrus fruit | 2–3 |
| Vinegar (acetic acid) | Kitchen | 2.5–3.5 |
| Cola (carbonic/phosphoric acid) | Soft drinks | 2.5–3.5 |
| Tomato juice | Food | 4 |
| Common bases | Where found | Approximate pH |
| Baking soda (sodium bicarbonate) | Kitchen | 8–9 |
| Soap | Bathroom | 9–10 |
| Ammonia solution | Cleaning products | 11–12 |
| Drain cleaner | Hardware store | 13–14 |
| Bleach | Cleaning products | 12–13 |
The pH Scale
Measuring acidity and alkalinity
The pH scale is a way of measuring how acidic or alkaline a substance is. It runs from 0 to 14:
- pH 0–6: Acidic — the lower the number, the more acidic
- pH 7: Neutral — pure water is neutral
- pH 8–14: Alkaline (basic) — the higher the number, the more alkaline
pH Indicators
Colour-changing chemical detectives
A pH indicator is a substance that changes colour depending on the pH of the solution it is in. Indicators let us "see" pH without needing electronic equipment.
Litmus paper
Litmus is one of the oldest indicators. It comes in two colours:
- Blue litmus paper turns red in acid
- Red litmus paper turns blue in base
Litmus can only tell you whether a substance is acidic or basic — it cannot tell you how acidic or basic. It also does not change in neutral solutions.
Universal indicator
Universal indicator is a mixture of several indicators that produces a range of colours across the full pH scale:
| pH | Universal indicator colour |
|---|---|
| 0–3 | Strong red |
| 4–6 | Orange / yellow |
| 7 | Green |
| 8–10 | Blue |
| 11–14 | Purple / violet |
Because universal indicator shows a different colour for different pH values, it gives a much better estimate of pH than litmus alone.
Phenolphthalein
Phenolphthalein is another common indicator used in schools. It is colourless in acidic and neutral solutions, but turns a vivid pink in alkaline solutions (pH above ~8.3). It is often used to show when a solution has changed from acidic to basic during a reaction.
pH meters
A pH meter is an electronic device that measures pH and gives a numerical reading. It is more accurate than indicator paper and is used when precise pH values are needed — for example, in swimming pool maintenance, wine making, water quality testing, and medical laboratories. The probe must be calibrated regularly to stay accurate.
Common Misconceptions
"Acids are dangerous and bases are safe." No — both strong acids and strong bases can be corrosive and cause serious burns. Concentrated drain cleaner (a strong base) is just as dangerous as concentrated acid.
"pH measures strength." No — pH measures acidity or alkalinity. A substance can be a weak acid at a low pH (like vinegar) or a strong acid that has been diluted to a higher pH. Concentration and pH are related but different concepts.
pH in Australian Industries
Australia's wine industry relies heavily on pH measurement. Wine grapes contain natural acids (mainly tartaric acid) that give wine its crispness and help preserve it. Winemakers monitor pH throughout fermentation because it affects taste, colour stability, and microbial safety. Australian Shiraz typically has a pH between 3.3 and 3.6.
Swimming pools across Australia are also tested regularly for pH. The ideal pool pH is between 7.2 and 7.6 — slightly alkaline. If the pH is too low, the water corrodes metal fittings and irritates swimmers' eyes. If the pH is too high, chlorine becomes less effective at killing bacteria.
✍ Copy Into Your Books
▾Acids
- Taste sour, turn blue litmus red
- pH less than 7
- Examples: lemon juice, vinegar, stomach acid
Bases (Alkalis)
- Bitter taste, slippery feel, turn red litmus blue
- pH greater than 7
- Examples: baking soda, soap, ammonia
The pH Scale
- 0–6 = acidic; 7 = neutral; 8–14 = alkaline
- Universal indicator: red (acid) → green (neutral) → purple (alkaline)
Everyday pH Detective
Indicator Match-Up
Test Your Understanding
1. Which of the following substances is a base?
2. What colour does universal indicator turn in a neutral solution?
3. A student tests an unknown solution with blue litmus paper and it turns red. Which statement is correct?
4. Which tool would a swimming pool technician MOST likely choose to check that the pool water has a safe pH?
5. A solution turns universal indicator purple and phenolphthalein pink. A student concludes the solution has a pH of exactly 10. Is this conclusion valid?
Short Answer Questions
1. Describe three characteristic properties of acids and three characteristic properties of bases. Include at least one property that can be safely tested at home. 4 MARKS
2. A farmer wants to test whether the water in their dam is suitable for irrigation. Explain why a pH meter would be more useful than litmus paper for this purpose, and describe what pH range would be considered acceptable for most crops. 4 MARKS
3. Compare and contrast litmus paper, universal indicator and phenolphthalein as tools for measuring pH. In what situations would each be most appropriate? 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain why some substances taste sour while others feel slippery?
- What is the difference between an acid and a base in terms of pH?
Answers
▾MCQ 1
C — Baking soda (sodium bicarbonate) is a base. Lemon juice, vinegar and cola are all acidic.
MCQ 2
B — Universal indicator turns green at pH 7 (neutral).
MCQ 3
D — Blue litmus turning red tells you the solution is acidic, but litmus cannot tell you the exact pH. You would need universal indicator or a pH meter for that.
MCQ 4
A — A pH meter gives a precise numerical reading, which is needed to ensure pool water is in the safe range (7.2–7.6). Litmus paper only shows acidic vs basic, not the exact pH.
MCQ 5
C — Both results show the solution is alkaline, but neither gives an exact pH value. Universal indicator purple covers pH 11–14, and phenolphthalein turns pink at any pH above ~8.3. A pH meter would be needed for an exact reading.
Short Answer 1
Model answer: Acids taste sour, turn blue litmus red, and have a pH less than 7. Bases taste bitter, feel slippery, turn red litmus blue, and have a pH greater than 7. A safe test at home is using red cabbage juice (a natural indicator) to test kitchen substances: it turns pink in acid and green/yellow in base. (Any safe test described appropriately is acceptable.)
Short Answer 2
Model answer: A pH meter is more useful than litmus paper because it gives a precise numerical pH reading, whereas litmus paper can only tell you if the water is acidic or basic. For irrigation, most crops grow best in slightly acidic to neutral water with a pH between 6.0 and 7.5. If the pH is too low (acidic), it can damage roots and leach toxic metals. If the pH is too high (alkaline), nutrients become less available to plants.
Short Answer 3
Model answer: Litmus paper is simple and cheap but only shows whether a substance is acidic or basic, not the degree. It is best for quick go/no-go tests. Universal indicator shows a range of colours across the full pH scale, making it useful for estimating approximate pH in classroom investigations. Phenolphthalein only changes in alkaline conditions (colourless to pink), so it is best for detecting when a solution has become basic, such as in neutralisation reactions.
Boss Battle
Test your knowledge in a rapid-fire quiz battle. Defeat the boss by answering questions correctly!
Mark lesson as complete
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