How do shampoos work? Hair care, acids and bases | 14-16 years

This practical activity capitalises on teenagers’ hair obsessions to explore the acid‑base chemistry behind how shampoos work. Students bring in their own shampoos to test and use a database to find out more about the ingredients present.

Learning objectives

Students will be able to:

• Explain that most shampoos are weak acids.
• Make experimental observations that produce reliable evidence.

Sequence of activities

Before the lesson

Prior to the session, invite students to bring their shampoo bottles from home. They need the ingredients list and the price.

If this is not possible, buy six to eight different shampoos for class use. Keep the receipt for the prices.

Introduction

Introduce the topic: ’Shampoos are substances we use regularly – but what is in them? How do they work?’

Explain that the session will provide a way of finding out.

What is in a shampoo?

1. Organise students into groups of three or four.
2. Give each group at least one copy of the ’Cosmetic ingredients database’.
3. Give each student a copy of the ’Wash in style’ worksheet (two parts).
4. Circulate and support as groups find out what the ingredients are in their shampoos.
5. Gather their results together producing lists of the ingredients that are surfactants, preservatives, emulsifiers, salts, thickeners and perfumes.

Allow about 20 minutes for the research.

Plenary 1

In a plenary, use questions to prompt a discussion about the roles of the different ingredient types (see the ‘Cosmetic ingredients database’):

• ‘Why are all these substances needed in a shampoo?’
• ‘Do all shampoos work in the same way?’
• ‘What are the differences between shampoos for greasy hair, dry, coloured, dandruff, etc?’

How do shampoos work?

Introduce the next activity, to find out more about how shampoos work.

To each group, give out a set of equipment and a variety of shampoo samples. Circulate and supervise as groups:

1. Test the shampoos they are given.
2. Test extra shampoos if time is available.
3. Develop the idea of reliable and accurate testing (the same shampoo will be tested three or four times) by looking at how similar the results are.

Allow about 25 minutes for the testing. Collect test results on an interactive whiteboard or overhead transparency as they are completed, to build a complete set for the whole class to see.

After the testing is complete, circulate and support as groups:

1. Agree answers to the questions on the worksheet.
2. Elect a spokesperson to feedback answers to the class.

Plenary 2

In a plenary:

1. Look at the data.
2. Hear the answers to the questions from each group.
3. Widen the discussion, using questions such as:
   • ‘Why are shampoos acids, not alkalis?’
   • ’What would happen if we used an alkaline shampoo?’
   • ’Is it worth spending more money on expensive shampoos?’

Feedback

Collect the worksheets and give written feedback reinforcing the good points in responses and giving pointers for improvement.

Commentary

Students question and assess their basic knowledge of shampoos and the behaviour of acids. The database task makes them look further at what they understand, with the discussion relating to the practical tests increasing their vocabulary and understanding.

Additionally, students develop their listening skills and coherent thinking as they receive and give feedback.

An alternative approach with this material is to organise the class to work entirely in groups, minimising discussion between groups until the end of the session. After agreeing the criteria, the class can assess the work of each group as they present their findings.

Practical notes

Apparatus

For each group of students:

• Eye protection
• Petri dishes, preferably one per shampoo
• Dropping pipettes, for the shampoos and for oil
• Ruler
• Universal indicator chart
• Test tubes, preferably one per shampoo
• Test tube rack
• Square of dark paper, about 10 cm x 10 cm

Chemicals

For each group of students:

• Shampoo samples, x4–6
• Distilled water
• Cooking oil, about 30 cm³
• Universal indicator solution

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection.
• It is the responsibility of the teacher to carry out an appropriate risk assessment.

Answers

Wash in style, part 1: what is in a shampoo?

1. This depends on the shampoos investigated.
2. Each ingredient has a different purpose – see ‘Cosmetic ingredients database for details’.
3. The ingredients have specific roles so some are designed for certain types of hair or to create specific effects, eg boosting volume, adding shine, reducing dandruff.

Wash in style, part 2: how do shampoos work?

1. They are generally weak acids.
2. This depends on the results – shampoos with added thickeners tend to spread out less easily than those with no thickeners. Students could discuss whether thickeners ‘mask’ the effects of the shampoo. In practice, standard laboratory tests show that all shampoos have similar cleaning abilities.
3. Soap or detergent gives the cleaning properties; other chemicals give thickness, control pH or act as a preservative.
4. Answers depend on the products.
5. It could be – more expensive products tend to use milder, more expensive detergents. However, they also use tiny amounts of expensive perfumes which add nothing to the cleaning abilities. It’s probably best to buy a mid‑price shampoo that matches hair type.