What You Need:
· Small Bowl
· Water
· Pepper
· Dish Soap
Method:
1. Fill your bowl ¾ full
2. Sprinkle enough pepper over the water so that it is mostly covered (make sure most of the pepper stays on the top of the water, some will sink to the bottom though)
3. Dip a clean finger in the center of the bowl. Nothing happens.
4. Squeeze a small drop of dish soap on your finger.
5. Lightly dip your finger in the center of the bowl.
6. Watch as the pepper scatters to the edge of the bowl in every direction and then slowly sink to the bottom.
Note: to do the experiment again you will need new materials and make sure you wash out the bowl.
Why and how does this work?
The motion results from the reduction in the water's surface tension when detergent is added. Detergents are members of a chemical family called surfactants (short for surface active agents). Every detergent molecule has two distinct ends which chemists call the head and the tail. The tail strongly repels water while the head is strongly attracted to it. When you first add detergent to water, the molecules scurry across the surface with their heads down and tails sticking up. Now, detergent heads are attracted to water, but not nearly as strongly as water molecules are attracted to each other. This is why detergents reduce the surface tension of water. Imagine a long line of people all holding hands and pulling each other together. The line is under tension. If a person near the middle lets go of both hands, everybody falls away from that person to either side. The tension has been broken. In a similar way, water molecules on the surface pull away from where you add detergent. This trick only works well once because the detergent molecules that cover the surface of water stay there.
· Small Bowl
· Water
· Pepper
· Dish Soap
Method:
1. Fill your bowl ¾ full
2. Sprinkle enough pepper over the water so that it is mostly covered (make sure most of the pepper stays on the top of the water, some will sink to the bottom though)
3. Dip a clean finger in the center of the bowl. Nothing happens.
4. Squeeze a small drop of dish soap on your finger.
5. Lightly dip your finger in the center of the bowl.
6. Watch as the pepper scatters to the edge of the bowl in every direction and then slowly sink to the bottom.
Note: to do the experiment again you will need new materials and make sure you wash out the bowl.
Why and how does this work?
The motion results from the reduction in the water's surface tension when detergent is added. Detergents are members of a chemical family called surfactants (short for surface active agents). Every detergent molecule has two distinct ends which chemists call the head and the tail. The tail strongly repels water while the head is strongly attracted to it. When you first add detergent to water, the molecules scurry across the surface with their heads down and tails sticking up. Now, detergent heads are attracted to water, but not nearly as strongly as water molecules are attracted to each other. This is why detergents reduce the surface tension of water. Imagine a long line of people all holding hands and pulling each other together. The line is under tension. If a person near the middle lets go of both hands, everybody falls away from that person to either side. The tension has been broken. In a similar way, water molecules on the surface pull away from where you add detergent. This trick only works well once because the detergent molecules that cover the surface of water stay there.