Indicators and the pH scale
Your mother squeezes lemon onto a pancake on Shrove Tuesday, and later uses bleach to clean the bathroom. Both are strong-smelling and both can sting your skin, but they are opposites in chemistry. How can you tell which is an acid and which is a base without tasting them (never do that!)? And how can you give each one a number? By the end of this page you will be able to do both.
Acids, bases and neutral substances
Many substances in the kitchen, the garden and the lab belong to one of three groups.
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Acids taste sour (lemon juice, vinegar) and can be corrosive if strong.
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Bases feel slippery and are often found in cleaning products (bleach, baking soda). A base that dissolves in water is called an alkali.
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Neutral substances are neither acidic nor basic. Pure water is the best example.
| Substance | Type | Where you find it |
|---|---|---|
| Vinegar | Acid | Kitchen, on chips |
| Lemon juice | Acid | Kitchen |
| Pure water | Neutral | Tap (almost), lab |
| Baking soda | Base (alkali) | Baking, brown soda bread |
| Bleach | Base (alkali) | Cleaning |
| Lime | Base (only slightly soluble) | Spread on fields by farmers |
Name one acid and one base that are found in the home or in a kitchen. For each one, state where it is used and give an approximate pH value (above or below 7).
Name one acid and one base that are found in the home or in a kitchen. For each one, state where it is used and give an approximate pH value (above or below 7).
Indicators
An indicator is a substance that changes colour depending on whether it is in an acid or in a base. It lets us test a liquid safely.
Litmus is the simplest. It comes as a solution or as paper.
| Litmus | In acid | In neutral | In base |
|---|---|---|---|
| Colour | Red | Purple | Blue |
Litmus only tells you whether something is acidic or basic. It does not tell you how acidic or basic.
Universal indicator is a mixture of indicators that gives a whole range of colours. Each colour matches a pH value, so you can estimate the pH.
Roughly: red/orange = strong to weak acid (orange is about pH 3 to 4), yellow = very weak acid, green = neutral (pH 7), blue = weak base, purple = strong base.
Red cabbage juice is a natural indicator you can make at home: it is purple in neutral solutions, pink or red in acids, and green or yellow in bases.
In a school laboratory in Galway, a student adds a few drops of litmus solution to a sample of vinegar. a) What colour does litmus turn in an acid? b) What colour does…
In a school laboratory in Galway, a student adds a few drops of litmus solution to a sample of vinegar.
- a) What colour does litmus turn in an acid?
- b) What colour does litmus turn in a base?
- c) What is an indicator?
A student from Cork tests four unknown liquids with universal indicator and records these colours: Liquid P: red Liquid Q: green Liquid R: blue Liquid S: orange a) Put…
A student from Cork tests four unknown liquids with universal indicator and records these colours:
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Liquid P: red
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Liquid Q: green
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Liquid R: blue
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Liquid S: orange
- a) Put the liquids in order from most acidic to most basic.
- b) Which liquid is most likely to be pure water?
- c) Suggest a pH value for liquid R.
Red cabbage juice is purple in neutral solutions. It turns pink or red in acids and green or yellow in bases. A student tests three colourless liquids X, Y and Z with…
Red cabbage juice is purple in neutral solutions. It turns pink or red in acids and green or yellow in bases. A student tests three colourless liquids X, Y and Z with red cabbage juice. X turns pink, Y stays purple and Z turns green.
- a) Classify each liquid as acidic, neutral or basic.
- b) Mixing equal amounts of X and Z gives a purple mixture. Explain what has happened.
- c) Explain why red cabbage juice would be a less precise indicator than universal indicator for finding the exact pH of a liquid.
The pH scale
The pH scale gives a number to how acidic or basic a substance is. It usually runs from 0 to 14.
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pH below 7: acidic. The lower the number, the stronger the acid.
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pH equal to 7: neutral.
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pH above 7: basic. The higher the number, the stronger the base.
| Substance | Approximate pH |
|---|---|
| Stomach acid | 1.5 |
| Lemon juice | 2 |
| Black coffee | 5 |
| Pure water | 7 |
| Baking soda solution | 9 |
| Bleach | 13 |
Each step is tenfold
The pH scale is logarithmic. Moving down one pH unit makes a solution ten times more acidic. Moving down two units makes it $10 \times 10 = 100$ times more acidic.
The pH scale runs from 0 to 14. Copy and complete the table by stating whether each substance is acidic, neutral or basic. | Substance | pH | |---|---| | Lemon juice | 2…
The pH scale runs from 0 to 14. Copy and complete the table by stating whether each substance is acidic, neutral or basic.
| Substance | pH |
|---|---|
| Lemon juice | 2 |
| Pure water | 7 |
| Bleach | 13 |
| Black coffee | 5 |
A student claims: "A solution of pH 3 is just a little more acidic than a solution of pH 4." The pH scale is logarithmic, so each step of one pH unit represents a…
A student claims: "A solution of pH 3 is just a little more acidic than a solution of pH 4." The pH scale is logarithmic, so each step of one pH unit represents a tenfold change in acidity.
- a) How many times more acidic is a solution of pH 3 than a solution of pH 5? Show your reasoning.
- b) Evaluate the student's claim.
- c) A river in Wicklow has a pH of 6 after a spill, compared with a normal pH of 8. How many times more acidic is it than normal?
Using pH in everyday life
Neutralisation
When an acid reacts with a base, they cancel each other out. This is called a neutralisation reaction: acid + base → a salt + water. The pH moves towards 7, and you can check this by adding universal indicator, which turns green when the mixture is neutral. This is why an indigestion tablet (a base) raises the pH of stomach acid (for example from 1.5 to about 5 to 7).
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A farmer in County Kerry spreads lime (a base) on acidic soil to raise the pH towards the value that grass likes (about 6.5).
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Indigestion tablets contain a base that reacts with extra stomach acid and raises the pH a little.
Water treatment
A water treatment plant in Dublin must keep drinking water between pH 6.5 and 9.5. Too acidic water can corrode pipes; too basic water tastes unpleasant and can irritate skin. The plant uses a pH meter, which gives a precise number continuously, rather than litmus paper, which only shows acid or base.
A farmer in County Kerry finds that the soil in a field has a pH of 5. Grass grows best at a pH close to 6.5. The farmer spreads lime (a base) on the field. a) Is the…
A farmer in County Kerry finds that the soil in a field has a pH of 5. Grass grows best at a pH close to 6.5. The farmer spreads lime (a base) on the field.
- a) Is the soil acidic, neutral or basic before the lime is added?
- b) Explain what you expect to happen to the pH of the soil after the lime is added.
- c) Why is it not a good idea to add so much lime that the pH rises to 12?
Indigestion is caused by too much acid in the stomach. An indigestion tablet contains a base. a) What type of reaction takes place between the acid in the stomach and…
Indigestion is caused by too much acid in the stomach. An indigestion tablet contains a base.
- a) What type of reaction takes place between the acid in the stomach and the base in the tablet?
- b) The stomach contents have a pH of 1.5. What pH would you expect after a suitable tablet has been taken? Explain your answer.
- c) Explain why a student should not use a very strong base such as sodium hydroxide for this purpose.
A water treatment plant in Dublin must keep the pH of drinking water between 6.5 and 9.5. Four samples were measured: A pH 5.8, B pH 7.2, C pH 9.0, D pH 10.1. a) State…
A water treatment plant in Dublin must keep the pH of drinking water between 6.5 and 9.5. Four samples were measured: A pH 5.8, B pH 7.2, C pH 9.0, D pH 10.1.
- a) State which samples meet the standard.
- b) For each sample that does not meet the standard, state whether acid or base should be added to correct it.
- c) Explain why a plant would use a pH meter rather than litmus paper to monitor the water.
Back to the start: lemon juice (pH about 2) is an acid and bleach (pH about 13) is a base. An indicator would show them as red and purple, and the pH scale tells us they are far apart.