Water Investigations

Thursday, May 14th, 2009
Today we learned more about water. We tried some investigations. They were very interesting. We know that when water is poured it always falls straight down to the ground. We can get it to fall diagonally by getting it to run along a piece of string. The water holds on to the string using surface tension.
Next we tried some bath tricks. If you have a bottle full of water, and you turn it upside down, all the water will run out. If you turn the same bottle of water upside down, but you do this under water, the water will not run out. This is because the water in the bottle is trying to get out, but air pressure is trying to force water in the basin to go into the bottle. Neither the water in the basin nor the water in the bottle end up moving at all. We can see air pressure at work even more clearly when we get a cup filled with water, turn it upside down on cardboard, and let go. We all expected the cardboard to fall down and the water in the cup to run out, but this didn't happen because the air pressure on the cardboard prevented the cardboard from moving.
Surface Tension and Detergent: This is a great investigation you can try in the bath at home. Fill the bath with water. Make a cardboard boat and float it on the water. It will not move if the water is still. Now drop one drop of wash up liquid (or shampoo) into the water at the back of the boat. This causes the surface tension on the water to be broken and the boat will shoot forward. Be warned though: This works only once, until the bath is washed and dried again, getting rid of any detergent.
Water Races: We tried to figure out the fastest way to empty a bottle. We tried racing bottles, we tried pouring, squeezing and shaking a bottle, while racing it against a control bottle. None of these ways got the water out of the bottle more quickly than others. What we discovered is that you need to swirl a bottle of water to get the water to pour out more quickly. When you swirl a bottle, the water runs to the outside of the bottle, allowing air to push up inside the bottle. This air pushes down on the water, pushing it quickly out of the bottle.