Topic Sections

## Bleach and percentages

The concentration of some household materials is sometimes quoted as '% by mass' or '% by volume'. Let's take a closer look at bleach.

A typical household bleach solution is often quoted as being in the range of 3-5% bleach. Sometimes you may see 3.5 w/w solution sodium hypochlorite written in the ingredients.

How would you make up a 3.5% bleach solution using sodium hypochlorite?

Measure out 3.5 g of sodium hypochlorite per 100 g of water.

What does 3.5 w/w solution sodium hypochlorite mean?

3.5 g of sodium hypochlorite made up to 100 g with water.

Finding out which brand of bleach is best makes a nice classroom investigation, and the problem based practical activity cleaning solutions provides full experimental details and technical support.

To change hair colour permanently, hair salons use hydrogen peroxide which also acts as a bleach. 'Hairdressing hydrogen peroxide' is usually mixed with other liquid based or cream based tints. In order to achieve the desired hair colour it is vital that the correct concentration of hydrogen peroxide is used. However, when it comes to looking at the bottle, it can be quite confusing as two very different figures are often quoted and it is important to get the correct one.

What does 3% mean?

In every 100 ml of a 3% solution, 3 ml will be pure hydrogen peroxide and 97 ml will be water.

What does 10 volume mean?

When applied to the hair, hydrogen peroxide decomposess according to the equation:

2H2O2(aq)  2H2O(l) + O2(g)

The 10 volume means that 1 cm3 of H2Owill produce 10 cm3 of oxygen gas.

The oxygen gas then goes on to bleach the pigment in the hair.

The Faces of Chemistry: hair colourant video provides further background information for those students interested in hairdressing.

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