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As the remnants of Hurricane Isaac dropped lots of much needed rain on our drought stricken area last week, our minds were on water. Drops of rain stuck to the window. Streams of water rushed through yards making new trenches. The visible power and properties of water created lots of questions. So, we experimented and investigated.
In today’s Lab Report, sponsored by Apologia Science, we’ll experiment with the surface tension and cohesion properties of water.
Related post: Properties Of Liquids Worksheet
Surface Tension
Water molecules like to stick together. On the surface where the water meets the air, water molecules cling even more tightly to each other. This causes a “skin” to form on the surface of the water. This skin is so strong that it can hold a weight that normally would sink in water. This is called surface tension.
When the surface tension is disrupted, the heavy object that is floating on the skin will sink. A light object on the surface will be pulled by the attraction of the water molecules if the surface tension is disrupted. This easy experiment will demonstrate that phenomenon.
Cohesion
The surface tension of water is cause by cohesion. Cohesion means that the water molecules like to stick to each other. This is caused by the slightly negative charge of the oxygen atom of one water molecule being attracted to the slightly positive charge of the hydrogen atoms of another water molecule. You can, also, test the cohesion properties of water using an eyedropper, water and a coin.
Slowly, drop water onto a coin. Watch as the drops of water stick together to form a larger drop.
The water molecules will stick together and form a dome over the coin. Keep adding drops until the drop breaks and spills off of the coin. This is caused by gravity overcoming the force of the cohesion. How many drops of water can you fit on a coin?
More Water Experiments
Charcoal Water Purifying Experiment
We did the get a needle to float on the water experiment when we were studying Flying Creatures, and the kids thought that was the coolest thing ever.
We’ve also done some experiments with water and air pressure that the kids were amazed by!