Page 172 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

l Water has a maximum density of 1000 kg per cubic meter (m ) at 4°C:
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KEY POINT at this temperature, water is at its densest. When the temperature of water is
The effects of the either raised or lowered from 4°C, water loses density. This peculiar behaviour
changes in density of
water can benefit water is known as the ‘anomalous expansion’ of water. At 100°C, water has a density
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heating by creating heat of 958 kg/m and at 0°C, its density is 915 kg/m . This can be expressed
circulation by convection. as a percentage. When heated, water expands by 4 per cent; when cooled
We will deal with heat it expands by 10 per cent. When water is turned to steam, it expands by
transfer through water 1600 times, so 1 m of water will transform into 1600 m of steam!
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later in the chapter.
l The boiling point of water at sea level is 100°C: if the pressure is raised
from this, the boiling point increases. At 1 bar pressure, the boiling point of
water is 120°C. Similarly, if the pressure is lowered, then the boiling point
decreases. At the top of Mount Everest, the boiling point of water is 69°C.
l Water freezes at 0°C: again, pressure can affect this. If the pressure
increases then the freezing point is lower. Dissolved minerals can also affect
the freezing point.
l The relative density of water is 1: this is the measurement that all other
solids and liquids are measured against.
l The specific heat capacity of water is 4.187 kJ/kgK: the specific heat
capacity of a substance is the amount of heat required to raise the
temperature of 1 g of the substance by 1°C (or by 1 K). In the case of water,
it takes 4.187 kJ of heat to raise 1 kg of water by 1°C.
l Water itself is a poor conductor of electricity: it is the presence of
dissolved minerals that makes water a good conductor of electricity. Sea
water, for example, is a very good conductor of electricity because of the
dissolved salts and minerals it contains.
l Water is a poor conductor of heat: compared to most metals, water is a
poor conductor of heat. In fact, water is a better insulator of heat than it is
conductor. That is why it takes so much energy to raise the temperature of
water by 1°C (see specific heat capacity, above).
l Water is known as the ‘universal solvent’: almost all substances dissolve
in water to a certain extent. Because of this, it is almost impossible to get
chemically pure water on Earth.
l Water is classified as being hard or soft: the hardness and softness of water
affects its pH value (see Table 3.11).
l Water goes through several stages to be turned into steam: at
atmospheric pressure, the boiling point of water is 100°C. To raise the
temperature of the water from 0°C to 100°C takes 419 kJ/kg of energy (hf).
To turn the boiling water at 100°C to steam at 100°C takes a further 2257
kJ/kg of energy (hfg). At this point, the steam is said to be saturated steam.
In other words, it is saturated with heat. The total heat, therefore, to turn
water at 0°C to steam at 100°C takes 2676 kJ/kg of heat energy. Any further
heat added after this does not increase the temperature of the steam; it
remains at 100°C and the steam is known as ‘superheated’ steam because of
the extra heat energy. To increase the temperature of the steam, the initial
pressure of the water will have to be increased.

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