Soap is very important to modern life. Hand soaps allow us to keep clean and remove dirt from our hands before eating. Washing powder is a type of soap used to clean clothes. Soaps save lives by stopping the spread of harmful bacteria in dirt.
THE USES OF WATER
Water is good at removing large bits of dirt from our skin or clothes. But a lot of dirt is invisible. Most dirt on our skin and clothes is oil based. Harmful bacteria can also live in the oils on our skin. Water and oil cannot mix on their own. Water cannot remove dirty oil from our skin or clothes.
Soap is an emulsifier. This means that it allows water and oil to mix. You can test this at home:
- Put oil and water into a bottle.
- Shake the bottle – what do you see?
- The water and oil will split into layers with the oil on top.
- Now put a small amount of liquid soap into the bottle with the oil and water.
- Shake the bottle again – what do you see?
- You should have a cloudy mixture. The oil and water do not separate.
THE COMPOSITION OF SOAP
Soaps are actually a type of salt. A salt is an ionic compound with a positive ion and a negative ion. Cooking salt is made of two ions of only one element each – sodium and chlorine.
Soaps also have small positive ions like sodium (Na+) or potassium (K+). But their negative ions are very large and contain more than one atom. Their negative ions are fats or oils that have lost an electron.
Soap is made by reacting animal fat or plant oils with a strong base. Industrially, the base used is NaOH (sodium hydroxide). NaOH also has the common name “lye”. Traditionally lye is made using ash from plants such as trees or plantains. This is often called “potash”.
Soap particles have two ends. One end is hydrophilic (loves water) and the other is hydrophobic (hates water).
The hydrophilic end is negatively charged and loves water because water is polar. This end of the molecule likes to be surrounded by water molecules.
The hydrophobic end is neutral and prefers to be surrounded by other neutral oil molecules. This causes the soap molecules to make tiny balls of oil within the water. Each molecule has its hydrophilic end in the water and the hydrophobic end in the oil. These little balls are called micelles. They are spread out throughout the water. This is what makes a soap and oil and water mixture look cloudy.
Soap is an emulsifier meaning it allows water and oil to mix. This allows it to remove dirt, making it a useful aid in maintaining good health every day.