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Chapter 10: Water

Distribution

Earth's Surface Water Percentage
Saltwater oceans 97
Glaciers and polar ice caps 2.4
Below-ground aquifers 1.6
Other land surface water such as rivers and lakes 0.025

Water covers 71% of Earth's surface and totals about 1,460 teratonnes (Tt). That's 1,460,000,000,000,000,000 litres.

Properties

Water is by far the most common liquid on Earth, and is essential to all known life. However in several ways it is also one of the more unusual liquids around us. Some of the properties of water include:

  • Odourless.

  • Tasteless. (If left to stand it absorbs CO2 from the air, forming carbonic acid which gives the water a sour taste.)

  • Colourless in small quantities. See the Colour section below.

  • High surface tension. See the Surface Tension section below.

  • Expands when freezes. Ice is less dense than water and so floats on water, which helps insulate the water underneath, keeping it from freezing. This is important for helping protect any life in the water (such as fish). See the Expands When Freezes section below.

  • Solvent. A lot of things dissolve in pure water quite easily. See the Solvent section below.

  • High specific heat capacity – the amount of energy it takes to raise the temperature of a certain volume of water.

  • High latent heats – the amount of energy it takes to turn ice into water at 0°C, or water at boiling point into steam. See the Heat Capacity section below.

Colour

In small quantities water is colourless because it lets all colours through well enough that we can't see any colour tinge.

In large quantities (like in a swimming pool or the ocean) water has a pale blue colour. This is because water absorbs more red light that blue light – about 100 times as much. It still lets through most of the light, but over a few metres it adds up to a very noticeable blue colour. It's not just red light that water absorbs. In the visible spectrum the longer the wavelength the more the colour is absorbed (see the graph below), so red is most absorbed, then orange, and so on. Violet light gets through most easily.

The more water there is the more light is absorbed. This means the more water you look through the bluer it will seem as the non-blue colours get more absorbed than the blue. Bright red swimming togs or fish will look dark a few metres under the surface, or when viewed a few metres to one side. Since blue light is not absorbed as much, blue swimming togs or fish look the same just under the surface as they do several metres away (or down).

Near UV light is also not absorbed very much so wetsuits or fish or even coral with fluorescent colours will still look brightly coloured quite deep or some distance away sideways, and they stand out even more because non-fluorescent objects at the same depth or distance are dull and dark. Sunburn can also be a problem.

The blue colour of water can also be seen in deep holes in fresh snow and in ice caves in glaciers.

Water strongly absorbs microwave energy at 2.45GHz, or a wavelength of 12.25cm (not shown on the graph below). This is basically why microwave ovens work.

Impurities often give water interesting colours. For example, copper ions (Cu2+) can give a very intense blue colour. In the South Island I once saw a green cloud above a lake that had a very green colour thanks to glacier silt and/or chlorophyll and it was reflecting light up onto the cloud.

Note that the sky looks blue mainly because of light scattering, not light absorption.

Polar molecule

The water molecule has a bent shape a little bit like a Mickey Mouse silhouette, which means that the centre of positive charge (midway between the two hydrogen atoms) is not in the same place as the negative charge from the oxygen atom. That means its molecule has a positive charge at one end and a negative charge at the other. We say that water has a polar molecule.

Water's polar molecule is responsible for:


Melting and boiling temperatures

Water has a higher melting and boiling temperatures than methane (CH4). Methane molecules weigh about the same as water molecules but methane has the centre of charge of its four H atoms in the same place as the negative charge from the C atom (they're arranged in a tetrahedral shape around it). The charge in different parts of the water molecules mean that they are attracted to each other and do not want to head off in different directions (as a gas) like methane molecules do. This attraction between water molecules is called hydrogen bonding.

Advanced - heat capacity

Specific heat capacity is the amount of energy needed to raise the temperature of a certain amount of a substance. It takes more energy to raise a gram of water by 1 °C than any other common liquid except ammonia (which is a gas at room temperature). This means heating up a kettle of water takes a lot of energy, but it also means that water acts as a temperature buffer, making life much easier on Earth – there's so much water that to change the temperature of it would take a lot of energy or take a long time (or both).

The latent heat of fusion (standard enthalpy change of fusion) and the latent heat of vaporisation (standard enthalpy change of vaporisation) are the extra energy needed for a substance to melt or evaporate, respectively.

Wikipedia explains the latent heat of fusion quite well:

The heat of fusion can be observed if you measure the temperature of water as it freezes. If you plunge a closed container of room temperature water into a very cold environment (say -20 °C), you will see the temperature fall steadily until it drops just below the freezing point (0 °C). The temperature then rebounds and holds steady while the water crystallises. Once completely frozen, the temperature will fall steadily again.

The temperature stops falling at (or just below) the freezing point due to the heat of fusion. The energy of the heat of fusion must be withdrawn (the liquid must turn to solid) before the temperature can continue to fall.

The energy needed for water to vaporise is why we feel cold sitting around wet on the edge of a swimming pool on a windy day. The wind is causing the water (or even just perspiration) to evaporate, and the water sucks heat out of us as it does that.

Because water has such a high latent heat of vaporisation, it absorbs a lot of energy when it changes from a liquid into a gas (or releases a lot of energy when going the other way). This has important consequences for keeping our planet a nice place to live because it helps stabilise the temperature of our atmosphere.


Graph from hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase.html, which explains
the energy changes of the phase changes in some detail.

Microwave ovens can superheat water above its normal boiling point. From the Physics Department of the University of New South Wales:

The latent heat of vapourisation [sic] of water is L = 2.23 MJ/kg. This means that it takes 2,230,000 Joules of heat to evaporate 1 kg of water at 100 °C and at normal atmospheric pressure. (One kilogramme of water is about one litre.)

The specific heat capacity of water is c = 4.2 kJ/kg. This means that it takes 4,200 Joules of heat to raise the temperature of 1 kg of water by 1 °C.

Suppose that we heat one kilogram of water from 100 °C (its normal boiling temperature) to 101 °C, i.e. it is now superheated by 1 °C. When it begins to boil, it will very quickly cool to 100 °C, and the heat liberated turns water into steam. Cooling this kg of water by 1 °C gives 4.2 kJ, which is enough to evaporate c/L = 4200/2230000 kg of water. This is only 1.9 millilitres of water, which does not sound very much, but it turns into 3 litres of steam. Those three litres of steam are created inside the hot water, quite suddenly, so the water is ejected violently from the container.

The opposite can also occur – water can be supercooled. YouTube has videos of water at its triple point. When the water is given a shake, freezing is initiated, and the whole tube of water suddenly freezes.

Surface tension

Water has a very high surface tension, which enables insects and one type of lizard to walk on water.

Surface tension also allows us to do weird things like float paper clips on water, even though iron is 7.86 times as dense as water.

Another demonstration is adding pins to a full cup of water – surface tension means the water level rises above the rim of the cup but doesn't overflow until possibly hundreds of pins have been added.

Expands when freezes

When substances turn from a liquid to a solid they normally contract. When water freezes it expands and therefore gets less dense, which means it floats on water. This is vitally important in lakes and streams as it acts as insulation to keep the water underneath it from freezing.

In ice the water molecules are lined up so that the positive end of one water molecule is next to the negative end of the next water molecule, and this arrangement takes up slightly more room than the random arrangement water molecules are in while a liquid.

The expansion of water can be used to break a pencil taped over the top of an open container of water when the water freezes.

Wilson Bentley was famous for taking photographs of snowflakes and sadly died of pneumonia just before Christmas 1931 – a danger of his work, perhaps. During his life he captured over 5,000 photographs of snowflakes. Click the photo at right for a larger version.

One of the reasons snowflakes are so hard to photograph is that even in subzero conditions they evaporate. Evaporation of water under freezing point also means washing can be hung on a clothesline wet and it will still dry – although when I saw this in Romania in December 2002 the dry washing had icicles hanging off the bottom.

Solvent

Water dissolves salts, starches and sugars. It dissolves so many things it's sometimes called the universal solvent. Water can do this because it has a polar molecule and because a very small amount of it dissociates into H+ and OH- ions. (This dissociation is not enough to explain why water conducts electricity. It's actually impurities that contribute ions that do that.)

Atmosphere and pressure under water

The air around us contains about 3% water. The measure of how much water the air can contain is called humidity. Relative humidity is what is normally referred to when people talk of humidity, which is basically the amount of water the air contains relative to the maximum amount it could contain. When relative humidity reaches 100% the water condenses and it starts to rain. The hotter the air the more water it can hold, so the lower the relative humidity will be for the same amount of water. This also means that to make it rain you can lower the temperature of the air past the point where the relative humidity reaches 100%.

Inside during the lesson the relative humidity was 53%. When we went outside after the lesson the relative humidity was as high as 75%.

Absolute humidity is the amount of water vapour in kilograms per cubic metre of dry air.

Interestingly, humid air weighs less than dry air. This is because a water molecule weighs less than either nitrogen or oxygen molecules which make up most of the air.

Atmospheric pressure can lift water about 10.3 metres. In other words, if you have a long tube sealed at the top end, you can lift the top end 10.3 metres up before the water will "cavitate" or create a vacuum at the top end of the pipe.

It also means if you dive under water to 10.3m down you will experience two times atmospheric pressure, or 2 atmospheres. (It's sometimes described as 1 atmosphere above atmospheric pressure.)

It also means that you could work out very roughly how much pressure the water main is at, or what pressure your water gun has holding by shooting either straight up and measuring the height the water goes to. Note that using this to calculate the water mains pressure is more accurate than using it for a water gun because the water gun is small and when the trigger is pulled the pressure drops considerably as the water flows out.

For those interested, various pages on the Internet state the CPS2500 (the large gun we used) is 26 psi to 35 psi, which is 1.77 to 2.38 atmospheres, the same as car tyres are set to. (That's above atmospheric pressure, BTW.) An earlier gun version, the CPS2000, was said to be 44 psi – a whole 3 atm – and was supposedly taken off the market for safety reasons.

For extended water fights I highly recommend the Super Shower Backpack, although it can be tricky balancing that much water while running around.