Pour some salt on the ice cube to make the ice on the surface melt, then press the string down onto the ice with your finger for a few seconds. Take away your finger. The water will freeze again over the string, so you can lift up the cube.

It's easier to lift someone up in water because of water's buoyancy. Buoyancy is a force that pushes upwards in a liquid.

Cohesion and surface tension

Cohesion is the tendency of molecules of one substance to hold together by mutual attraction. The hydrogen bonding of water results in strong cohesive forces. One effect of this is that the surface of a drop of water will assume the smallest possible area, and the drop therefore forms a sphere. The water molecules at the surface are drawn in towards the body of the drop forming a skin-like layer of molecules at the surface. This force is called surface tension. Insects walking on the surface of water and the movement of water up plants are two biological processes that can occur as a result of the cohesive properties of water molecules.

Adhesion and capillarity

Adhesion is the attraction of molecules of different compounds to one another. The ability of water to cling readily to other molecules is responsible for the upward movement of water when a small-bore tube is dipped into it. This phenomenon is called capillarity. Xylem vessels of a diameter 0.02mm can, in theory, support a column of water of height 1.5m by capillarity forces. One of its main biological effects is the upward movement of water in the soil.

Thermal capacity (specific heat)

Another consequence of hydrogen bonding in water is that much heat is needed to cause increased molecular movement and hence gas (steam) formation. The heat energy must first be used to break the hydrogen bonds. For this reason the temperature of water rises only very slowly for a given amount of heat added, when compared with other substances. Similarly it cools more slowly. In all it is thermally stable and so biochemical reactions in a water medium are not subjected to large temperature fluctuations and can take place at a more constant rate. Were it not for hydrogen bonding, water would be a gas at normal environmental temperatures and life as we know it could not exist. For the same reasons much heat is needed to evaporate water and therefore even the evaporation of a small amount of water from the surface of an organism has a large cooling effect, e.g. sweating.

Density

Water has its maximum density at 4ºC. Unlike most other substances it is less dense as a solid than as a liquid. It freezes from the top downwards and the ice that forms at the surface can insulate the warmer water below this layer from the colder temperatures above it. This prevents large bodies of water from freezing solid and has contributed to the survival of aquatic organisms

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