Ice floats in water because the solid form of water is lighter; it means the ice is less dense than the liquid form. This is unusual because in other substances it is just the other way round. Initially, water behaves like other substances. When it is heated, it expands and evaporates. When it is cooled, its volume becomes smaller. After a certain temperature, however, water becomes ‘wayward’. At 4°C, it reaches its densest state. On further cooling, it expands again! Thus, water behaves abnormally. This is also known as anomaly of water. On freezing, i.e., at 0°C, its volume increases again. This is the reason why water bottles full of water burst if they are kept in deep freezer for a long time.
On a warm skin water evaporates; i.e., it is converted from liquid to gaseous state. In this process, the skin loses some of its heat; as a result, the surface of the skin is cooled. This process is known as ‘evaporative cooling’. Our body also uses this effect to regulate the body temperature when it is hot. It secretes sweat, which evaporates, and thus cools down the skin. We can also cool the cold drinks in this way while camping by wrapping the bottle in a wet cloth and placing it in wind.
Water striders are very light-weight insects. Their legs are covered with millions of tiny hairs that trap air bubbles. These air bubbles along with surface tension of the water keep water striders afloat. Water molecules attract one another very strongly; this is known as ‘cohesion’. The resultant force acts downwards on the surface of water, above which there is only air and no water, so a flimsy solid layer is formed. This effect is known as surface tension. Because of surface tension water always ‘tries’ to keep the smallest possible surface area. Thus, raindrops on the window pane are spherical.