The water cycle, salinity and dissolved gases

Particle theory: solids, liquids and gases

Everything in this topic comes back to one model. Matter is made of very small particles - atoms, molecules or ions - with empty space between them, and those particles are moving all the time. The three states of matter differ only in how closely the particles are packed, how free they are to move and how strongly they attract one another. The particles themselves do not change when water freezes or boils; only their arrangement and their energy change.

In a solid, the particles are packed closely together in a regular, repeating arrangement and are held in fixed positions by strong forces of attraction. They cannot move from place to place, but they vibrate about those fixed positions, and the hotter the solid the faster they vibrate. A solid therefore has a fixed shape and a fixed volume, it does not flow, and it can hardly be compressed, because there is no empty space to push the particles into. Ice is water in this state.

In a liquid, the particles are still close together and still touching, but they are arranged at random and the forces between them are weaker, so they can slide past one another. A liquid therefore has a fixed volume but no fixed shape: it takes the shape of its container and flows, and it is almost impossible to compress because the particles are still in contact. Liquid water is the state in which nearly all the chemistry of the ocean happens, because the particles can move and mix while staying close together.

In a gas, the particles are far apart, arranged at random, and moving quickly in straight lines until they collide with each other or with the walls of the container. The forces between them are negligible. A gas therefore fills whatever container it is in, spreads out through the whole volume, flows, and is easily compressed, because most of the space between the particles is empty. Water vapour is water in this state, and it is invisible.

Temperature is a measure of the average kinetic energy of the particles in a substance, and it is measured in degrees Celsius (°C). Heating transfers energy to the particles, so they vibrate or travel faster and their average kinetic energy rises, which is what the thermometer records. Temperature is not a measure of the total energy stored in a substance: a bath of warm water contains far more energy than a cup of boiling water, even though its temperature is lower.

Diffusion and changes of state

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, and it happens because the particles are moving randomly all the time. No particle is travelling purposefully down the gradient; each one moves at random. But because there are more particles in the crowded region, more of them cross into the emptier region than come back, so the net effect is a spread outwards until the concentration is even. A drop of ink spreading through a glass of water is diffusion, and so is the smell of a gas crossing a room.

Diffusion is faster when the particles have more energy and further to travel between collisions. It is therefore faster at a higher temperature, because the particles have more kinetic energy and move faster, and it is faster in a gas than in a liquid, because the particles are much further apart and collide far less often. Diffusion is also why dissolved oxygen and carbon dioxide spread through sea water without anything stirring it.

A change of state happens when the particles gain or lose enough energy to overcome, or to re-form, the forces of attraction between them. Melting is a solid changing to a liquid; freezing is a liquid changing to a solid; evaporation is a liquid changing to a gas at its surface, at any temperature below its boiling point; and condensation is a gas changing back to a liquid. Ice melting in a drink and water vapour condensing on a cold window are the two you will use most in this topic.

The energy changes are the part students lose marks on. Melting and evaporation absorb energy from the surroundings: the energy is used to pull the particles apart against the forces holding them together, not to make them move faster, so the temperature does not rise while the change of state is going on. Freezing and condensation release energy to the surroundings, for the same reason in reverse: the particles come together and give out the energy that was needed to separate them. This is why steam at 100 °C causes a much worse burn than water at 100 °C - the steam releases a large amount of energy as it condenses on the skin.

The water cycle

The water cycle is the continuous circulation of water between the ocean, the atmosphere and the land, and the Sun supplies all of its energy. Water evaporates from the surface of the ocean, turning from liquid to water vapour. The vapour rises, expands and cools, and condenses onto tiny particles in the air, such as sea salt and dust, to form droplets so small that they stay suspended as cloud. When the droplets join together and become heavy enough, they fall as precipitation - rain, snow, sleet or hail. Water that falls on land runs across the surface into streams and rivers as surface run-off, and the rivers carry it back to the sea.

The ocean is by far the largest store in the cycle, holding about 97 per cent of the Earth's water, and it supplies most of the water that evaporates into the atmosphere. That is why the cycle is best described as starting and ending at the ocean: the ocean is the reservoir, the atmosphere is the fast-moving middle, and the land is a smaller store that water passes through on its way back.

To interpret the cycle you have to say what drives each step and what it leaves behind. Evaporation is driven by energy from the Sun and is fastest where the water is warm, the air is dry and the wind is strong. Condensation happens because the rising air cools. Precipitation happens when the droplets grow too heavy to stay up. Surface run-off happens because gravity carries the water downhill once the ground can absorb no more.

The most important consequence for marine science is that evaporation removes water and leaves the dissolved salts behind. Water vapour is pure water; the salts stay in the sea. Every drop that evaporates therefore makes the remaining sea water slightly more concentrated, which is the starting point for the whole of the salinity section of this topic. Run over millions of years, the same two processes explain why the sea is salty at all: rivers have carried dissolved ions into it, and evaporation has been taking the water away ever since.

• Evaporation: liquid water at the ocean surface gains energy from the Sun and becomes water vapour.
• Condensation: the vapour rises, cools and forms droplets on condensation nuclei such as sea salt and dust, making cloud.
• Precipitation: droplets grow and fall as rain, snow, sleet or hail.
• Surface run-off: water falling on land flows downhill in streams and rivers back to the sea.
• The ocean holds about 97% of the Earth's water and is the source of most evaporation.
• Evaporation leaves the salts behind, so it concentrates the sea water that remains.

What controls the rate of evaporation from the oceans

Evaporation happens at the surface of a liquid, and only particles in the surface layer can escape. Within any liquid the particles have a range of energies; the fastest ones at the surface can break free of the attraction of their neighbours and enter the air as vapour, while the slower ones are left behind. Because the fastest particles leave, the average kinetic energy of the water that remains falls, which is why evaporation cools the water - the same reason sweating cools you.

Temperature. At a higher temperature a larger proportion of the particles have enough energy to escape, so evaporation is faster. Surface area. A larger area of water exposed to the air means more particles are at the surface and able to escape, so a shallow, wind-blown sea evaporates faster than a deep, sheltered one. Wind speed. Moving air carries the vapour away, so the air just above the surface stays dry and the concentration gradient is maintained; in still air the layer just above the water becomes humid and evaporation slows. Humidity. Dry air can accept more vapour than humid air, so evaporation is faster when the air is dry.

These four factors explain the pattern you can see on a map. Evaporation is fastest in the subtropical seas at about 20 to 30 degrees north and south of the equator, where the Sun is strong, the skies are clear and the trade winds blow steadily, which is why the saltiest open-ocean water is found in those belts. It is slower near the equator, where heavy rain and humid air work against it, and slower again at high latitudes, where the water is cold and the air is often already close to saturation.

Melting ice: floating ice compared with land-based ice

This comparison turns on one fact: a floating object displaces its own weight of water. An iceberg, an ice shelf or a sheet of sea ice floats because ice is slightly less dense than the sea water around it, and the volume of water it pushes aside weighs exactly the same as the whole lump of ice.

When that floating ice melts, the meltwater fills exactly the space the ice was already displacing, so the water level does not change. You can test it: mark the level in a beaker of water, float an ice cube in it, wait for the cube to melt completely, and the level is exactly where it was. The same applies to the Arctic sea ice and to ice shelves that are already floating - their melting does not on its own raise sea level.

Land-based ice is different. A glacier or an ice sheet rests on land, so the water frozen in it is not in the ocean and is not displacing any sea water. When it melts, that water runs into the sea and adds water that was not there before, so the level rises. To show the difference, support a second ice cube of the same mass on a wire grid above the water so that the meltwater drips in, and compare the rise in the two beakers.

For a fair comparison both beakers must start with the same volume of water at the same temperature, and the two pieces of ice must have the same mass. The independent variable is whether the ice is floating or supported on land, the dependent variable is the water level, and everything else is controlled. The meltwater is fresh, so in the real ocean it also dilutes the surface water and lowers its salinity, which is one of the environmental factors in the next section.

• Floating ice displaces its own weight of water, so when it melts the level stays the same.
• Land-based ice was not in the ocean, so its meltwater adds new water and the level rises.
• Control the mass of ice, the volume of water, the starting temperature and the container.
• Meltwater is fresh water, so it lowers the salinity of the surface layer where it enters.

Elements, compounds and mixtures, and the pH of natural waters

An element is a substance made of only one type of atom, and it cannot be split into anything simpler by chemical means. Oxygen is an element: its particles are molecules made of two oxygen atoms joined together, written O2, and no chemical reaction breaks oxygen down into a simpler substance. About 21% of the atmosphere is oxygen.

A compound is two or more elements chemically combined in fixed proportions, and its properties are different from those of the elements it contains. Calcium carbonate is a compound of calcium, carbon and oxygen with the formula CaCO3 - always one calcium atom to one carbon to three oxygens, never any other ratio. It is a white solid, it is the main mineral in limestone and chalk and in the shells and skeletons of corals and many molluscs, and it is insoluble in pure water.

A mixture contains two or more substances that are not chemically combined. They can be present in any proportions, each substance keeps its own properties, and they can be separated by physical means such as filtration, evaporation or distillation. Sea water is a mixture: water with dissolved salts and dissolved gases in it, in proportions that vary from place to place. Nothing new is formed when salt dissolves in water - the salt is still salt and the water is still water, which is why you can get the salt back by evaporating the water.

pH is a scale from 0 to 14 that measures how acidic or alkaline a solution is. A solution of pH 7 is neutral; below 7 it is acidic, and the lower the number the more acidic it is; above 7 it is alkaline, and the higher the number the more alkaline it is. Universal indicator shows the pH as a colour: red at pH 1 to 3, orange and yellow around 4 to 6, green at 7, blue at 8 to 10 and purple at 11 to 14.

The three natural waters in the syllabus have three different pH values. Sea water is slightly alkaline at about pH 8.1 to 8.2, because of the carbonate and hydrogencarbonate ions dissolved in it. Fresh water in rivers and lakes is usually close to neutral but varies with the rocks it has flowed over. Rain water is slightly acidic, around pH 5.5 to 6, because carbon dioxide from the air dissolves in it to form a weak acid, carbonic acid.

That last point is the link to dissolved gases. If you bubble carbon dioxide through sea water and test it with universal indicator, the colour moves towards yellow-green and the pH falls: the carbon dioxide dissolves, reacts with the water to form carbonic acid and makes the solution more acidic. The same reaction happens at the surface of the ocean, which is why sea water is slightly less alkaline now than it was before humans added carbon dioxide to the atmosphere.

Sea water as a solution, and salinity

A solution is formed when a solute dissolves in a solvent. The solvent is the substance that does the dissolving and is present in the larger amount - in sea water it is the water - and the solute is the substance that dissolves, which in sea water means the salts and the dissolved gases. To dissolve means that the particles of the solute separate and mix with the particles of the solvent, so that the result is a single, transparent phase. A substance that can dissolve is soluble; one that cannot is insoluble.

Sea water is about 96.5% water and about 3.5% dissolved salts by mass, with a small amount of dissolved gas. Of the dissolved salts, sodium chloride is by far the most abundant - roughly three-quarters of the total - and it is the salt you taste. Magnesium sulfate is the second most abundant. Calcium carbonate is present in much smaller amounts, along with the hydrogencarbonate and carbonate ions that the pH depends on; it is only slightly soluble, which is why it precipitates to form the skeletons of corals and the muds on shallow tropical sea floors.

Salinity is the concentration of dissolved salts in the water. It is measured in parts per thousand (ppt), which means grams of dissolved salts per 1 000 g (one kilogram) of sea water. If you evaporate exactly 1 000 g of sea water to dryness and 35 g of solid salts are left behind, the salinity is 35 ppt. A 500 g sample that leaves 17.5 g of salts gives the same answer: 17.5 / 500 x 1 000 = 35 ppt.

The average salinity of the open ocean is about 35 ppt, which is the same as 3.5%. Parts per thousand is used rather than per cent because the differences that matter between one sea and another are small - a change from 35 to 40 ppt is five parts in a thousand, which would be 0.5% and would disappear if you rounded it. Writing 35 per cent instead of 35 ppt is out by a factor of ten, and it is one of the commonest errors in this topic.

Most salts become more soluble as the water gets warmer, which is the opposite of what happens to gases. To investigate it, add a known mass of salt to a fixed volume of water at a series of temperatures, stir, and record the mass that dissolves before no more will. Temperature is the independent variable and the mass dissolved is the dependent variable, and the volume of water, the salt used and the amount of stirring must all be kept the same.

• Solvent: the liquid that does the dissolving - water in sea water.
• Solute: the substance that dissolves - the salts and the dissolved gases.
• Solution: the mixture that results, with the solute particles evenly spread through the solvent.
• Soluble means able to dissolve; insoluble means not able to dissolve.
• Salinity is the concentration of dissolved salts, measured in parts per thousand (ppt) = grams of salt per kilogram of sea water.
• Average ocean salinity is about 35 ppt = 3.5%.

Why salinities differ, and estuaries

Three bodies of water show the range, and you should know the approximate value and the reason for each. The Pacific Ocean has a typical salinity of about 34.5 to 35 ppt, because it is vast, deep and well mixed, and the water added by rain, rivers and melting ice is roughly balanced by the water removed by evaporation. The Baltic Sea has a low salinity of roughly 5 to 10 ppt, and lower still in its northern gulfs. The Red Sea has a high salinity of about 40 to 42 ppt.

The Baltic is low because fresh water is added faster than it is removed, and because the sea is almost closed. Many large rivers drain into it, precipitation is high, evaporation is low because the summers are short and cool, snow and ice melt into it each spring, and its only connection to the ocean is the narrow, shallow Danish straits, so very little salty ocean water gets in. The result is brackish water, with a gradient from nearly fresh in the north and east to more saline near the straits.

The Red Sea is high for the opposite reasons. It lies in a desert region, so almost no rivers reach it and there is very little rain; the Sun is intense all year and the air is dry, so evaporation is very high; and it connects to the Gulf of Aden only through the narrow strait of Bab el Mandeb, so the salty water it produces is not flushed out quickly. Evaporation removes water and leaves the salts behind, so the salinity climbs.

The syllabus lists five environmental factors that control salinity, and each works by adding or removing water or salts. Erosion of rocks on land releases ions from the rocks, which are then carried towards the sea. Run-off in rivers carries those dissolved ions into the sea, and also adds fresh water that dilutes it. Precipitation and evaporation work against each other: rain adds fresh water and lowers salinity, evaporation removes water and raises it. Melting of ice sheets and glaciers adds fresh water and lowers salinity, especially in polar seas in summer. Temperature acts mainly through evaporation, since warm water evaporates faster, and it also raises the solubility of the salts themselves.

An estuary is a partly enclosed body of water where a river flows into the sea. Estuaries are tidal, so they are flushed and refilled twice a day, and fresh water from the river mixes with sea water pushed in by the tide. That mixing means the salinity changes during the tidal cycle: on a rising tide, sea water moves upstream and the salinity at any fixed point rises; on a falling tide, the river flow dominates and the salinity falls. Salinity also falls as you move upstream, so an estuary has a gradient from nearly fresh at its head to fully saline at its mouth, and the water in between is brackish.

Dissolved gases: oxygen and carbon dioxide

Oxygen and carbon dioxide dissolve into sea water from the atmosphere at the surface, where water and air are in contact. The surface layer is the part of the ocean that is in equilibrium with the air above it, and the gases enter by diffusion across the boundary - the same random movement of particles that spreads a smell across a room. How much dissolves depends on the gas, on the temperature of the water and on how much of the gas there is in the air above.

Oxygen has a low solubility in water. Only about 9 mg of oxygen dissolves in a litre of fresh water at 20 °C, and rather less in sea water, so dissolved oxygen at the surface is usually between about 5 and 10 mg per litre. Carbon dioxide is far more soluble, roughly a hundred times more at about 1.5 g per litre at 20 °C, and much of what dissolves reacts with the water to form carbonic acid, which then breaks up into hydrogencarbonate and carbonate ions. That is why the sea holds a very large reservoir of dissolved carbon.

The comparison with the atmosphere is the point of the second syllabus statement. In the atmosphere, oxygen makes up about 21% and carbon dioxide about 0.04%, so there is roughly 500 times more oxygen than carbon dioxide. In sea water the position is reversed in relative terms: because oxygen is so poorly soluble and carbon dioxide so soluble, the total mass of dissolved carbon dioxide, mostly as hydrogencarbonate, is several times greater than the mass of dissolved oxygen.

An increase in temperature reduces the solubility of both gases. The dissolved gas particles have more kinetic energy at a higher temperature, so more of them have enough energy to escape from the surface back into the air and the equilibrium shifts towards the gas phase; the water simply cannot hold as much. This is why a warm bottle of drink goes flat faster than a cold one, and why cold polar water holds much more dissolved oxygen than warm tropical water.

You can investigate it by bubbling a gas through a fixed volume of water held at a series of temperatures in a water bath, or by sealing samples and measuring the dissolved oxygen with a meter. Keep the volume of water, the gas, the flow rate and the time the same, change only the temperature, and measure the dissolved gas. A graph of the results curves downwards as the temperature rises.