Purpose: Observe some interesting properties of water.

Performing the experiments – Is Surface Tension important?

? How many drops of water can a penny hold?

• Place a clean dry penny on a tray, plate, or paper towel.
• Partially fill a dropper with water.
• Hold the plastic dropper vertically about 1 centimeter above the penny in order to place drops of water on the penny one by one.
• Count the drops carefully as you drop them on the penny. Watch from the side as the water builds up. The surface tension of the water will finally break, causing the water to spill off the penny.
• Dry off the penny and repeat. Can you get more this time?

?? Is pepper afraid of soap?

• Fill a small paper cup ½ full of water.
• Sprinkle a small amount of pepper in the cup.
• Place a small dot of dishwashing soap on your finger or on the tip of a toothpick.
• Lightly touch the surface of the water with your soapy toothpick. What happens? Can you do it again?
• Pour the water into the pitcher marked ”Discard” and throw away the cup.

??? Does milk have magic?

• Pour ½ cup of whole milk into a plate or flat bowl.
• Add 2 drops of food coloring in four different areas of the bowl (pretend the bowl is a clock and add the drops at 12:00, 3:00, 6:00 and 9:00). Use different colors for each area.
• Dip a toothpick into dishwashing liquid like Dawn. Stick the dishwashing liquid end of the toothpick into the center of the bowl of milk. What happens?

???? Can water magnify images?

• Place a washer down over a word on the Science is Fun phrases at the end of these pages.
• Using a dropper, drop a couple of drops of water in the center of the washer.
• Look at the letters through the water. Do they look bigger?

????? Can a few drops of water move things?

• Bend 5 wooden toothpicks in half – gently enough that each toothpick forms a “V” but do not break it apart.
• Place the bent toothpicks on the center of a dry, smooth surface, arranging them so that the points of the Vs touch in the center of the plate. It will look like a flower, *
• Partially fill a dropper with water.
• Drop several drops of water in the center of the toothpick pattern.
• Do the toothpicks form a star?

??? Questions to think about

• Could you coat the penny with something that would help you add more drops of water before it spills off?
• Could you coat the penny with something that would make the water spill off when fewer drops are added?
• Why does the pepper experiment work well only once?
• How would different kinds of milk (powered, skim, low-fat, soy) work in place of whole milk?
• What would happen if you used maple syrup or another liquid instead of water for magnification?
• Could you form other shapes with toothpicks and water?
• In magnification, what would you see if you were observing from the other side of the drop of water?

Explanation of Surface Tension and Results

Water has a high surface tension, which means the surface of water acts like a thin, invisible “skin.” Surface tension results from attractive forces or Hydrogen bonding (H-bonding) between hydrogen and oxygen atoms from different water molecules. A difference in the amount of H-bonding in the interior of a drop of water and the surface of the drop creates the force called surface tension. Insects that have legs that repel water ( hydrophobic) do not break the surface tension and are able to walk on the surface of water. The ability to not disturb the surface tension of the forming drop affects the number of drops of water that fit on a penny. Thus, factors such as the height of the dropper above the penny, the placement of the drops, the angle of the dropper, the size of the drops, and the condition of the penny will also have an affect.

The surface tension is also what keeps the hydrophobic pepper suspended in the water. Adding a hydrophilic substance like liquid soap that wants to spread rapidly over the water forces the pepper to the sides of the cup.

In the milk experiments, milk is mostly a mixture of fat and water. The food coloring mixes easily in the water but not in the fat. So, the more fat content in the milk the less mixing occurs. Surface tension helps keep the drops on the surface. The detergent causes the colors to move because of the way the detergent interacts with the fat and the water.

The water magnifies because the surface tension causes it to form drops, which have round or spherical surfaces. The light coming out of the drop bends as it breaks the surface of the drop due to the change in medium from water to air. Because of the spherical shape, the bending is different at different parts of the drop resulting in the magnification observed. Different liquids have different light bending properties or refractive indices and may produce different levels of magnification.