Science Forum (continued)
Floating paperclips
Fill a glass with water and leave it to settle for a minute or so. How steady are your hands? Holding a paperclip horizontally between your finger and thumb, lower it toward the water surface. Keeping the paperclip level with the surface, wait until it just touches, then carefully let it go.The paperclip should float on the surface, held up by the thin skin of surface tension. If you can't do it, there's an easier way.Tear off a piece of paper towel about the same size as the paperclip. With the paperclip on top, lower the paper onto the surface of the water. At first the paper floats, along with the paperclip. When the paper sinks, the paperclip stays afloat.
Explanation
So how does surface tension explain how a toothpick can attract a tiny rod, or a pistachio shell? The water molecules are attracted to objects, the same way they are attracted to each other. Your floating piece of toothpick therefore attaches to a small part of the surface tension "skin." If you have a magnifier you'll be able to see this for yourself. Notice how the rod seems to dent the skin here and there, and how these dents stay attached to the rod as it moves. As you lower the toothpick into the water, notice how it dents the skin--it too has its own patch of attached skin. When you move the toothpick closer to the rod, the two patches joins together, so the rod gets dragged along by the toothpick.
Soap and surface tension
Ever wondered why you can blow bubbles using soapy water but not using plain water? It's because the surface tension of water is too high, preventing the surface skin from stretching out. Adding soap to water lowers its surface tension, allowing it to form a thin film that can be blown into a bubble. There are some neat experiments for showing this.
Speedy toothpicks
Float three toothpicks in a triangle, as shown in the photo, making sure that at least one of the three corners are not overlapping. Carefully aim a single drop of dish soap into the center of the triangle. As soon as the soap touches the water, the toothpicks fly apart. This is because the surface tension on the outside of the triangle remains high while suddenly dropping on the inside.
Sinking paperclips
Is your paperclip still afloat? If not, float another one. While holding a toothpick horizontally, add a small drop of dish soap to one end, keeping it turning so the droplet doesn't fall off. Still holding the toothpick level, line the tip up just above the floating paperclip. Now tilt the toothpick so a droplet of soap falls onto the water, beside the paperclip. The paperclip immediately sinks. This is because the soap lowers the surface tension of the water so much that the surface skin is no longer strong enough to support the paperclip's weight.
Fill a glass with water and leave it to settle for a minute or so. How steady are your hands? Holding a paperclip horizontally between your finger and thumb, lower it toward the water surface. Keeping the paperclip level with the surface, wait until it just touches, then carefully let it go.The paperclip should float on the surface, held up by the thin skin of surface tension. If you can't do it, there's an easier way.Tear off a piece of paper towel about the same size as the paperclip. With the paperclip on top, lower the paper onto the surface of the water. At first the paper floats, along with the paperclip. When the paper sinks, the paperclip stays afloat.
Explanation
So how does surface tension explain how a toothpick can attract a tiny rod, or a pistachio shell? The water molecules are attracted to objects, the same way they are attracted to each other. Your floating piece of toothpick therefore attaches to a small part of the surface tension "skin." If you have a magnifier you'll be able to see this for yourself. Notice how the rod seems to dent the skin here and there, and how these dents stay attached to the rod as it moves. As you lower the toothpick into the water, notice how it dents the skin--it too has its own patch of attached skin. When you move the toothpick closer to the rod, the two patches joins together, so the rod gets dragged along by the toothpick.
Soap and surface tension
Ever wondered why you can blow bubbles using soapy water but not using plain water? It's because the surface tension of water is too high, preventing the surface skin from stretching out. Adding soap to water lowers its surface tension, allowing it to form a thin film that can be blown into a bubble. There are some neat experiments for showing this.
Speedy toothpicks
Float three toothpicks in a triangle, as shown in the photo, making sure that at least one of the three corners are not overlapping. Carefully aim a single drop of dish soap into the center of the triangle. As soon as the soap touches the water, the toothpicks fly apart. This is because the surface tension on the outside of the triangle remains high while suddenly dropping on the inside.
Sinking paperclips
Is your paperclip still afloat? If not, float another one. While holding a toothpick horizontally, add a small drop of dish soap to one end, keeping it turning so the droplet doesn't fall off. Still holding the toothpick level, line the tip up just above the floating paperclip. Now tilt the toothpick so a droplet of soap falls onto the water, beside the paperclip. The paperclip immediately sinks. This is because the soap lowers the surface tension of the water so much that the surface skin is no longer strong enough to support the paperclip's weight.
