Chapter 10: Water
distribution | properties
| colour | polar molecule |
melting/boiling points | heat
capacity | surface tension | expands
when freezes | solvent | atmosphere
and pressure
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:
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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.
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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.
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Solvent. A lot of things dissolve in pure water quite easily. See the Solvent section below.
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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.
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.
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