Indicators and the pH scale
In this topic you will:
- learn how to tell an acid from an alkali
- learn how to make and use indicators
- use the pH scale to find out more about acids and alkalis.
Key words
- indicator
- litmus
- neutral
- pH scale
- universal indicator
Which is which?
These three containers all look the same. One contains water, one contains acid and one contains alkali.
You can tell them apart when you add a few drops of an indicator. An indicator turns one colour in an acid and a different colour in an alkali. Red cabbage juice can be used as an indicator.
Red cabbage indicator turns acids red in acids, blue in water and yellow in alkalis. So, you now know what was in each beaker.
Indicators can be made from the brightly coloured berries, flowers and other parts of plants. These include:
- red cabbage
- blackcurrant
- beetroot
Questions
Show Answer
An indicator shows different colours in acids, alkalis, and neutral solutions. This colour change helps identify the type of substance.
Show Answer
Red cabbage juice turns red when added to lemon juice because lemon juice is acidic.
Litmus
Litmus is a very common indicator. It is a dye. You usually use litmus paper, which is made by soaking absorbent paper in litmus solution.
Litmus turns red in acids. Litmus turns blue in alkalis. Litmus turns purple when it is in a neutral substance. A neutral substance is one that is neither acid nor alkali.
Litmus turns purple in water. Water is neutral. This means water is neither an acid nor an alkali.
This table shows the colours litmus goes in some substances and what those colours mean.
| Substance | Litmus colour | Type of substance |
|---|---|---|
| hydrochloric acid | red | acid |
| sodium hydroxide | blue | alkali |
| water | purple | neutral |
| lemon juice | red | acid |
| calcium hydroxide | blue | alkali |
Questions
Show Answer
Litmus turns blue when added to sodium hydroxide because it is an alkali.
Show Answer
Litmus turns red in an acid.
Show Answer
Water is neutral because litmus turns purple in it, and it is neither acidic nor alkaline.
Think Like a Scientist
Making and Using Your Own Indicator Solution
In this task you will make your own indicator solution and use it to test various chemicals in the laboratory.
You will need: some plant material, such as red cabbage or beetroot, a knife and cutting board, a pestle and mortar, two dropper pipettes, test tubes and a test-tube rack, safety glasses, a range of laboratory chemicals, ethanol
Safety
Make sure you are careful and read all the hazard warning labels. Ethanol is flammable.
Step 1. Cut up the plant material.
Step 2. Place some of the material into a pestle and mortar and crush it.
Step 3. Use a pipette to add a little ethanol.
Step 4. Crush the plant material again.
Step 5. Use a different pipette to transfer some of the liquid from the mortar to a test tube.
Step 6. Use the liquid you collect to test some everyday liquids and laboratory chemicals.
Step 7. Make a table to record the chemicals you test and the colours you see.
Self-Assessment
Compare your indicator with litmus. Does your indicator turn the same colour as litmus? Does it clearly show which is an acid and which is an alkali?
Other indicators
Litmus and other simple indicators just show if a substance is an acid or an alkali. Universal indicator shows how acidic or alkaline a substance is. The acidity or alkalinity of a substance is one of its chemical properties.
Universal indicator can change to many different colours. Universal indicator is made up of a mixture of different indicators.
| Type of substance | Colour of universal indicator |
|---|---|
| strongly acid | red |
| weakly acid | yellow |
| neutral | green |
| weakly alkaline | blue |
| strongly alkaline | purple |
The strength of acids and alkalis is measured on the pH scale.
Universal indicator changes colour and shows the pH of a substance. The pH of a substance is one of the chemical properties of that substance.
Questions
Show Answer
The pH scale measures how acidic or alkaline a substance is.
Show Answer
The pH of a neutral solution is 7.
Show Answer
It is a strongly acidic liquid.
Show Answer
Strong alkalis have a pH between 12 and 14.
Show Answer
It turns blue, indicating a weak alkali.
Show Answer
It turns red in strong acids and yellow in weak acids.
Think Like a Scientist
Investigating the pH of Different Substances
In this task you will test various laboratory chemicals with universal indicator to measure the pH and what type of chemical it is.
You will need: universal indicator papers, a range of liquids, test tubes and a test tube rack, safety glasses
Safety
Read any hazard warning labels and take care not to spill substances on your skin. Make sure you know what to do if you do spill anything.
Step 1. Put on your safety glasses.
Step 2. Pour a small amount of liquid from a bottle of liquid into a clean test tube.
Step 3. Test with universal indicator.
Step 4. Record the colour of the indicator and the pH.
Step 5. Record the type of each liquid, such as strongly or weakly acidic, neutral, strongly or weakly alkaline. You could use a table like this one:
| Liquid | Colour of universal indicator | pH | Type of liquid |
|---|---|---|---|
| lemon juice | red | 4 | weakly acidic |
| salt water | green | ||
| soap solution | 8 | weakly alkaline | |
| cola drink | yellow | 4 |