Teacher Tip: To make sure your count is accurate, hold the eyedropper far enough above the coin so that the drop has to fall a short distance before fusing with the droplet on the coin. Details Activity Length mins.
Objectives Describe the cohesive and adhesive properties of water. What To Do Place a coin on a table or desk. Holding the eyedropper close to the surface of the coin, carefully squeeze water droplets onto the coin, one at a time. The droplets should pool up on the coin, creating a big droplet of water. Get the students to count the drops. Stop squeezing when the droplet on the coin breaks up and overflows. The count is the number of drops that the coin could hold before the one that caused the coin to overflow.
Wipe off the coin or use a new one. As before, gradually add drops of oil or syrup to the surface of the coin using the eyedropper. Extensions Perform this as a student activity, rather than just a demo. It reduces the surface tension causing a dramatic reduction in the number of drops that will fit on the coin. Start with a full glass of plain water with a dry rim to prevent the water from dripping down the side of the glass.
How many coins can we add to the water without the glass overflowing? Because water molecules at the surface of the water puddle attract more to one other than they do to the air molecules above them, they cling together and form a dome shape on the coin. You can keep adding water drops until the surface tension is not strong enough to counter the gravitational pull on the water. In the exploration part of the experiment, you see that the sizes of drops coming out of the dropper are different for different types of liquid.
So the surface tension of each liquid is different. The number of drops you can put on a coin without spilling is also different. Surface tension is one of many interesting properties of water.
Check out these other experiments on water properties. By accessing the science activity ideas on Rookieparenting. In addition, your access to Rookieparenting. Now carefully add one drop of water at a time to the top of the penny. Hold the medicine dropper just above the top of the penny not touching it so each new drop has to fall a short distance before it merges with the drop on the penny.
You can write down the number of drops you add if you like. How many drops of water do you think will fit on top of the penny? Watch the drop on top of the penny carefully as it grows. It should keep getting bigger and bigger until it touches the edges of the penny. Keep adding drops refill your medicine dropper as necessary one at a time. How big does the drop on the penny get before it finally spills over the edges?
Mix a small amount of dish soap with your tap water. Now, repeat the experiment using soapy water. Do you think you will be able to add more drops or less before the liquid spills over the sides of the penny? Again, slowly add one drop at a time. How big does the drop of water on top of the penny get before it breaks and flows over the edges?
Extra : Try the experiment with different liquids or other things you can find in your kitchen. Make sure you have an adult's approval to use any liquids before you handle them. How do different soaps and detergents like hand soap or laundry detergent compare with one another? What about other liquids like milk or juice? Which ones make the biggest or smallest drops? With the most or least number of drops?
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