Surface Tension
Surface Tension is the thin, stretchy skin that forms on top of water because water pulls itself together.
Water has a thin, stretchy layer on its surface, like plastic wrap stretched across a bowl. Small, light things like a paper clip can rest on top without sinking. The water at the top pulls toward itself, forming a skin that holds the weight. Push through that skin, and the object drops to the bottom.
Explaining surface tension by grade level
Water has a thin skin on top. That skin is strong enough to hold up small things. When you add soap to water, it breaks that skin. That is why soap bubbles pop when the skin gets too weak.
Projects that explore surface tension
Water pulls itself together at the surface, forming a thin stretchy skin that can support light objects. Adding salt or raising the temperature weakens this invisible film. You can measure the change by floating aluminum foil on water and counting how many rice grains it holds before sinking at temperatures from 15°C to 55°C.
A soap bubble is surface tension shaped into a sphere — that same thin, stretchy skin stretched around a pocket of air. What keeps it from popping right away? You prepare five solutions, each with a different additive: corn syrup, glycerin, sugar, lemon juice, and plain soap as a control. Blow a bubble from each, start a stopwatch, and record how long it lasts before it bursts.
Water molecules cling tightly together, and that pull — surface tension — keeps oil floating in a separate layer on top. The two stay apart no matter how long you wait. Add liquid soap, though, and it breaks that surface tension, letting the oil blend in with the water.
Water pulls itself together into a thin, stretchy skin on the surface. Milk contains water, so its surface has that same pull holding it steady. You use a cotton swab with dish soap, touch the food coloring, and watch the colors move.