The best time to water is in the early morning or at night, when less water will evaporate into the cool air. During the day, the heat of the sun speeds up evaporation.

Water (relative molecular mass = 18) is a liquid at room temperature, in contrast with other substances with small molecules such as methane (r.m.m. = 16), ammonia (17), hydrogen sulphide (34) and carbon dioxide (44) which are gases. In gases the molecules are widely spaced and free to move about independently of each other, whereas in liquids the molecules are close together. In water, the molecules are held close together by hydrogen bonding. It is the hydrogen bonds that cause water to exist as a liquid at the temperatures and pressures that normally prevail on the Earth's surface. Life as we know it depends on this property.

Water has a high heat capacity; that is, a great deal of heat energy is required to raise the temperature of water. This is because much of the energy is used to break the hydrogen bonds which restrict the mobility of the molecules. As a result, water is relatively slow to heat up or cool down. In fact, the specific heat capacity of liquid water is the highest of any known substance.

The latent heat (enthalpy) of fusion of water (the heat energy needed to melt ice) is unusually high. By the same token, relatively large amounts of heat energy must be extracted from liquid water before it freezes. The latent heat of vaporisation of water (the heat energy required to vaporise liquid water) is also unusually high. Thus the evaporation of water requires a great deal of energy and has a remarkable cooling effect.

Most liquids contract on cooling, reaching their maximum density at their freezing point. Water is unusual in reaching its maximum density above its freezing point - at 4°C. So when water freezes the ice formed is less dense than water and floats on top. Ice on the surface effectively insulates the water below, thereby making it less likely that the bulk of water (sea, pond or lake) will freeze up even if the air above is very cold.

At the surface of water the molecules are orientated so that most hydrogen bonds point inwards towards other water molecules. This give water a very high surface tension, higher than any other liquid except mercury. Despite this, water molecules slide past each other relatively easily, and water has a remarkably low viscosity.

Compared with other liquids, water has extremely strong adhesive and cohesive properties that prevent it breaking under tension. Water adheres strongly to most surfaces, and can be drawn up through narrow tubes without the water column breaking.

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