Acids, Bases and Salts · Lesson 1 of 12
Understanding The Chemical Properties Of Acids And Bases
“Acids and bases reveal themselves with more colour drama than a school art room.”
• Recall familiar acidic and basic substances from everyday life. • Distinguish acids and bases using natural and synthetic indicators. • Explain how litmus, turmeric, phenolphthalein and methyl orange respond to acidic and basic media. • Explain the meaning of an olfactory indicator. • Use indicator observations to identify whether an unknown solution is acidic or basic.
A slice of lemon tastes sour, while soap feels slippery and has a bitter character. These differences are not accidental. They arise because the substances contain compounds that behave as acids or bases. Tasting unknown laboratory substances is unsafe, so chemistry relies on observable changes produced by special substances called indicators.
Acids And Bases In Familiar Substances
Acids are commonly associated with a sour taste, while bases are commonly associated with a bitter taste and a slippery or soapy feel. Lemon juice and vinegar are familiar acidic materials. Soap solution and baking soda solution show basic behaviour. These sensory descriptions are useful for familiar food materials, but they should never be used as a test for unknown chemicals.
An important property of acids and bases is that they can oppose each other's effects. For example, excessive acid in the stomach can be relieved using a mild base. This cancellation of acidic and basic effects develops later into the idea of neutralisation.
Acid-Base Indicators
An Acid-Base Indicator is a substance that shows a detectable change, usually a colour change, in acidic and basic media.
Natural Indicators
Natural indicators are obtained from natural materials. Litmus is obtained from lichens. Turmeric, red cabbage and coloured flower petals can also behave as indicators because their pigments respond differently in acidic and basic surroundings.
Litmus is a natural indicator that appears purple in a neutral medium. Acids turn blue litmus red, while bases turn red litmus blue.
Turmeric remains yellow in acidic and neutral conditions but develops a reddish-brown colour in a basic medium. A curry stain on cloth may therefore turn reddish-brown when rubbed with soap because soap is basic.
Synthetic Indicators
Synthetic indicators are manufactured chemical indicators. Two important examples are phenolphthalein and methyl orange. Their colour changes are useful because they are clear and repeatable.
| Indicator | Acidic Medium | Basic Medium |
|---|---|---|
| Blue litmus | Turns red | Remains blue |
| Red litmus | Remains red | Turns blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric | Yellow | Reddish-brown |
Testing Acids And Bases In The Laboratory
A systematic indicator test can compare several acids and bases. Acidic solutions may include hydrochloric acid, sulphuric acid, nitric acid and acetic acid. Basic solutions may include sodium hydroxide, calcium hydroxide, potassium hydroxide, magnesium hydroxide and ammonium hydroxide. A drop of each solution can be tested separately with litmus, phenolphthalein and methyl orange.
| Type | Examples |
|---|---|
| Acids | Hydrochloric acid, sulphuric acid, nitric acid, acetic acid |
| Bases | Sodium hydroxide, calcium hydroxide, potassium hydroxide, magnesium hydroxide, ammonium hydroxide |
Problem
An unknown solution turns red litmus blue and phenolphthalein pink. What can you conclude?
- 1.Red litmus turning blue is characteristic of a basic medium.
- 2.Phenolphthalein becoming pink also indicates a basic medium.
- 3.Both observations agree.
- 4.The unknown solution is basic.
Olfactory Indicators
An Olfactory Indicator is a substance whose odour changes or becomes difficult to detect in acidic or basic media.
Onion, vanilla and clove oil can be tested for changes in smell after contact with acidic or basic solutions. Their behaviour allows acidity or basicity to be detected without relying on colour.
Quiz
Which observation correctly identifies a basic solution?
What happens to blue litmus paper in an acidic solution?
Which indicator becomes pink in a basic solution?
Which of the following can act as an olfactory indicator?
Practice Problems
- Explain how blue and red litmus can be used together to distinguish an acid from a base.
- A solution turns phenolphthalein pink and methyl orange yellow. Identify its nature and justify your answer.
- Classify litmus, turmeric, phenolphthalein and methyl orange as natural or synthetic indicators.
- Explain why tasting an unknown substance is not a suitable method for deciding whether it is acidic or basic.
- Explain what is meant by an olfactory indicator and name three examples.
Key Takeaways
• Acids and bases can be distinguished safely using indicators. • Acids turn blue litmus red, while bases turn red litmus blue. • Phenolphthalein and methyl orange are synthetic indicators. • Litmus, turmeric, red cabbage and coloured flower pigments can act as natural indicators. • Onion, vanilla and clove oil can act as olfactory indicators.
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Next · Lesson 2
How Do Acids And Bases React With Metals?