Water - detergent

Detergents are big molecules that make it easy for water and oil to mix. Water and oil do not mix and the water molecules can not come close to the grease particle to dissolve it. The detergent molecule allows the water to come really close to the grease particles and dissolve them. To do this the detergent molecule has two parts to it, a water attracting part (head) and a long oil loving tail. Detergent molecules also act to lower the surface tension of water. Water tension is the surface energy of water. The water molecules are held together with a certain amount of energy. Detergents act to decrease this amount of energy. To see a 120kb movie click here.

Notice how the water molecules now pull apart (taking the oil droplet with it) as the detergent is added. The force between the water molecules is decreased.

Water attracting head Water repelling tail.

Water molecules do not mix with grease particles.
When detergent is added to a mixture of oil and water the detergent molecules go to work. Their tail digs into the oil while the head stays exposed to the water.
As the detergent molecules dig their tail into the grease it breaks up large grease particles into smaller ones. It also makes the surface of the grease particle full of water attracting heads.
The grease molecules burry their tail into the grease exposing their heads to the water, attracting water molecules. Click to see
The detergents now make the small oil droplets soluble in the water. The detergents now make the small oil droplets solub le in the water. The detergents now make the small oil droplets solub le in the water.

The water now moves closer to the small grease particles and surrounds them. The small grease particles, covered in detergent molecules, now dissolve in the water.

Detergents have been around for many thousands of years. The early cave man found its uses when animal fat fell into the ashes of the fire to create the first primitive soap.

You can make soap using animal fat and caustic soda.
Exercises