How does adding salt or raising the temperature change the surface tension of water? Surface tension is the invisible film on water that lets small objects float. You float a tiny square of aluminum foil on water and add rice grains one at a time until the foil sinks. You repeat this at five temperatures from 15°C to 55°C. Then you do it again using salt water at the same temperatures. Counting the rice grains each time shows you how salt and heat weaken that invisible film.