Almost All the Water on Earth Is Water You Cannot Drink
Learn how Earth's water is distributed and why so little of it is usable, trace the four stages of the water cycle including transpiration, list the physical properties of pure water, and see why water is densest at four degrees.
If the Earth is covered in water, why is fresh water scarce?
Because about 97 per cent of it is salty ocean water, which no land plant or animal can use.
That leaves about per cent as fresh water. But most of even that is locked up in polar ice caps and glaciers, leaving roughly one part in a hundred of the planet's water available as groundwater, rivers and lakes.
So the water a city can actually pump is a thin slice of a small slice. This page covers the first part of the ICSE Class 8 Chemistry chapter on water: where water is, how it moves, what pure water is like, and the one way in which it behaves backwards.
That leaves about per cent as fresh water. But most of even that is locked up in polar ice caps and glaciers, leaving roughly one part in a hundred of the planet's water available as groundwater, rivers and lakes.
So the water a city can actually pump is a thin slice of a small slice. This page covers the first part of the ICSE Class 8 Chemistry chapter on water: where water is, how it moves, what pure water is like, and the one way in which it behaves backwards.
Where is water found, and why does life depend on it?
Water covers roughly three-quarters of the Earth's surface, which is why the planet looks blue from space.
Its distribution:
- Oceans and seas — about of all water, and saline
- Polar ice caps and glaciers — about , frozen and largely inaccessible
- Groundwater, rivers, lakes and ponds — about , the usable fresh water
- Atmosphere — a small amount as water vapour, clouds and humidity
- Living organisms — a human body is roughly two-thirds water by mass, and some fruits and vegetables far more
Why living things need it. Water is not one convenience but several at once:
- It is the solvent in which nutrients, minerals and gases are transported — sap in plants, blood and urine in animals.
- It is the medium in which every reaction inside a cell takes place; a dry cell simply stops working.
- It is a raw material for photosynthesis, which makes the food and the oxygen the rest of life depends on.
- It regulates temperature — sweating in animals and transpiration in plants both cool by evaporation.
- It provides habitat for an enormous number of aquatic organisms.
The property behind most of that. Water's usefulness comes chiefly from being an excellent solvent, which is why it is called the universal solvent. But this cuts both ways: because it dissolves so much, natural water is never pure. Rain picks up gases from the air; river water dissolves salts from the rocks it flows over; sea water is loaded with sodium chloride.
So pure water is a laboratory substance, prepared by distillation, and the properties in the next-but-one section belong to that and not to anything you could scoop from a river.
Its distribution:
- Oceans and seas — about of all water, and saline
- Polar ice caps and glaciers — about , frozen and largely inaccessible
- Groundwater, rivers, lakes and ponds — about , the usable fresh water
- Atmosphere — a small amount as water vapour, clouds and humidity
- Living organisms — a human body is roughly two-thirds water by mass, and some fruits and vegetables far more
Why living things need it. Water is not one convenience but several at once:
- It is the solvent in which nutrients, minerals and gases are transported — sap in plants, blood and urine in animals.
- It is the medium in which every reaction inside a cell takes place; a dry cell simply stops working.
- It is a raw material for photosynthesis, which makes the food and the oxygen the rest of life depends on.
- It regulates temperature — sweating in animals and transpiration in plants both cool by evaporation.
- It provides habitat for an enormous number of aquatic organisms.
The property behind most of that. Water's usefulness comes chiefly from being an excellent solvent, which is why it is called the universal solvent. But this cuts both ways: because it dissolves so much, natural water is never pure. Rain picks up gases from the air; river water dissolves salts from the rocks it flows over; sea water is loaded with sodium chloride.
So pure water is a laboratory substance, prepared by distillation, and the properties in the next-but-one section belong to that and not to anything you could scoop from a river.
What are the four stages of the water cycle?
Evaporation, transpiration, condensation and precipitation — the continuous circulation of water between the Earth's surface and the atmosphere.
Evaporation. Heat from the Sun turns surface water from oceans, rivers, lakes and wet soil into water vapour, which rises with the warm air. This happens at all temperatures, not only at boiling point, and it is the process that lifts water into the atmosphere in the first place.
Transpiration. Plants draw water up from the soil through their roots and lose it as vapour through the stomata of their leaves. On a large scale this adds a great deal of vapour to the air — a forest returns far more water to the atmosphere than the same area of bare ground.
The two together are sometimes called evapotranspiration, and transpiration is the stage students most often leave out.
Condensation. Rising air cools with height. Cooler air holds less vapour, so the vapour condenses onto tiny dust particles to form the water droplets that make clouds. The same process on a cold surface gives dew and mist.
Precipitation. Droplets in a cloud collide and merge until they are too heavy for the rising air to support, and they fall as rain, or as snow and hail where it is cold enough.
Completing the loop. Water reaching the ground either runs off into streams and rivers and back to the sea, soaks in to become groundwater, or is taken up by plants and transpired again. No stage is a dead end, which is why the cycle has continued indefinitely.
The point worth keeping. The total quantity of water on Earth does not change — the cycle redistributes it and purifies it. Evaporation leaves dissolved salts behind, so the vapour that rises is pure and the rain that falls is the cleanest natural water there is. Sea water becoming rain is distillation on a planetary scale, driven by nothing but sunshine.
Evaporation. Heat from the Sun turns surface water from oceans, rivers, lakes and wet soil into water vapour, which rises with the warm air. This happens at all temperatures, not only at boiling point, and it is the process that lifts water into the atmosphere in the first place.
Transpiration. Plants draw water up from the soil through their roots and lose it as vapour through the stomata of their leaves. On a large scale this adds a great deal of vapour to the air — a forest returns far more water to the atmosphere than the same area of bare ground.
The two together are sometimes called evapotranspiration, and transpiration is the stage students most often leave out.
Condensation. Rising air cools with height. Cooler air holds less vapour, so the vapour condenses onto tiny dust particles to form the water droplets that make clouds. The same process on a cold surface gives dew and mist.
Precipitation. Droplets in a cloud collide and merge until they are too heavy for the rising air to support, and they fall as rain, or as snow and hail where it is cold enough.
Completing the loop. Water reaching the ground either runs off into streams and rivers and back to the sea, soaks in to become groundwater, or is taken up by plants and transpired again. No stage is a dead end, which is why the cycle has continued indefinitely.
The point worth keeping. The total quantity of water on Earth does not change — the cycle redistributes it and purifies it. Evaporation leaves dissolved salts behind, so the vapour that rises is pure and the rain that falls is the cleanest natural water there is. Sea water becoming rain is distillation on a planetary scale, driven by nothing but sunshine.
What are the physical properties of pure water?
- Colour, odour and taste — colourless, odourless and tasteless. Water that tastes of anything contains something dissolved in it.
- Melting point — , so ice melts and water freezes at that temperature.
- Boiling point — at normal atmospheric pressure. That last phrase matters, since water boils below on a mountain and above it in a pressure cooker.
- Density — at its **maximum of at , which is why the gram was originally tied to a cubic centimetre of water.
- Effect on litmus — neutral, changing neither blue nor red litmus.
- Conductivity — pure water is a poor conductor of electricity. Ordinary water conducts because of the salts dissolved in it, which is the basis of every wet-hands warning.
- Specific heat capacity — unusually high, so water warms and cools slowly. This is why coastal places have milder weather than inland ones, and why water is used as a coolant in vehicle radiators.
- Latent heat — also unusually high, which is why evaporation cools so effectively and why steam scalds so badly.
Why so many of these are unusual. A substance with such small, light molecules would be expected to be a gas at room temperature, to have a low boiling point and to hold little heat. Water is liquid, boils high and stores heat well because its molecules attract one another strongly — the same intermolecular force met in the chapter on matter, unusually powerful in water's case.
And the two boiling and melting points are not arbitrary numbers. They are the fixed points** used to define the Celsius scale itself, which is why they come out as exactly and . That is a fact about the scale, not a coincidence about water.
A boundary case. Dissolved impurities shift both points — salt water freezes below and boils above . So sharp melting and boiling points are themselves a test of purity: water that boils at exactly at normal pressure is pure, and water that does not, is not.
- Melting point — , so ice melts and water freezes at that temperature.
- Boiling point — at normal atmospheric pressure. That last phrase matters, since water boils below on a mountain and above it in a pressure cooker.
- Density — at its **maximum of at , which is why the gram was originally tied to a cubic centimetre of water.
- Effect on litmus — neutral, changing neither blue nor red litmus.
- Conductivity — pure water is a poor conductor of electricity. Ordinary water conducts because of the salts dissolved in it, which is the basis of every wet-hands warning.
- Specific heat capacity — unusually high, so water warms and cools slowly. This is why coastal places have milder weather than inland ones, and why water is used as a coolant in vehicle radiators.
- Latent heat — also unusually high, which is why evaporation cools so effectively and why steam scalds so badly.
Why so many of these are unusual. A substance with such small, light molecules would be expected to be a gas at room temperature, to have a low boiling point and to hold little heat. Water is liquid, boils high and stores heat well because its molecules attract one another strongly — the same intermolecular force met in the chapter on matter, unusually powerful in water's case.
And the two boiling and melting points are not arbitrary numbers. They are the fixed points** used to define the Celsius scale itself, which is why they come out as exactly and . That is a fact about the scale, not a coincidence about water.
A boundary case. Dissolved impurities shift both points — salt water freezes below and boils above . So sharp melting and boiling points are themselves a test of purity: water that boils at exactly at normal pressure is pure, and water that does not, is not.
Why is water densest at four degrees rather than at freezing point?
Because between and water contracts on heating and expands on cooling — the reverse of every ordinary substance. This is the anomalous expansion of water.
Tracing it. Warm water from upwards:
- From to it contracts, so its density rises.
- At the volume is least and the density is greatest, at about .
- Above it behaves normally — expanding, with density falling.
So the densest water is at and not at its freezing point, which is where every other liquid is densest.
What this does to a pond in winter. As the air cools, surface water cools, becomes denser and sinks, with warmer water rising to replace it. This churning continues until the whole pond reaches .
Then the rule flips. Cooling the surface below makes it less dense, so it stops sinking and stays on top. It cools further in place, reaches and freezes into a sheet of ice.
Beneath that sheet sits the densest water, at about — warm enough for life. Ice is also a poor conductor of heat, so the sheet insulates the water below it, and the pond stays liquid underneath all winter.
Its significance for aquatic life. Fish, plants and other organisms survive in that water. Had water behaved normally, the coldest water would have kept sinking and the pond would have frozen solid from the bottom up, killing everything in it.
Two separate oddities, easily confused. Water also expands when it actually freezes, by about nine per cent, which makes ice less dense than water so it floats — and bursts unprotected pipes in cold weather. The anomaly described here is different: it concerns the liquid between and , and it is what decides where in the pond the coldest water sits, and therefore that freezing begins at the top.
Tracing it. Warm water from upwards:
- From to it contracts, so its density rises.
- At the volume is least and the density is greatest, at about .
- Above it behaves normally — expanding, with density falling.
So the densest water is at and not at its freezing point, which is where every other liquid is densest.
What this does to a pond in winter. As the air cools, surface water cools, becomes denser and sinks, with warmer water rising to replace it. This churning continues until the whole pond reaches .
Then the rule flips. Cooling the surface below makes it less dense, so it stops sinking and stays on top. It cools further in place, reaches and freezes into a sheet of ice.
Beneath that sheet sits the densest water, at about — warm enough for life. Ice is also a poor conductor of heat, so the sheet insulates the water below it, and the pond stays liquid underneath all winter.
Its significance for aquatic life. Fish, plants and other organisms survive in that water. Had water behaved normally, the coldest water would have kept sinking and the pond would have frozen solid from the bottom up, killing everything in it.
Two separate oddities, easily confused. Water also expands when it actually freezes, by about nine per cent, which makes ice less dense than water so it floats — and bursts unprotected pipes in cold weather. The anomaly described here is different: it concerns the liquid between and , and it is what decides where in the pond the coldest water sits, and therefore that freezing begins at the top.
Exam tip
Exam tip: say normal atmospheric pressure with every boiling point
Quote the boiling point as ** at normal atmospheric pressure. The condition is part of the fact, and questions about mountains and pressure cookers are built on it.
Name all four stages of the water cycle and do not omit transpiration — it is the one most often forgotten, and it is worth its own mark.
Say the cycle redistributes and purifies water rather than creating it, and add that evaporation leaves dissolved salts behind**.
For the distribution, give the figures as approximations with their nature: about saline ocean, about frozen, about usable fresh water.
State that water is **densest at , and that the anomaly operates only between and **. Above water is entirely normal.
For the frozen-pond question, give the full chain: surface water cools and sinks until the pond is at , then cooler surface water stops sinking, freezes on top, and the ice insulates the water below.
Keep pure water is a poor conductor ready, and note that ordinary water conducts because of dissolved salts.
And remember sharp melting and boiling points are a test of purity.
Name all four stages of the water cycle and do not omit transpiration — it is the one most often forgotten, and it is worth its own mark.
Say the cycle redistributes and purifies water rather than creating it, and add that evaporation leaves dissolved salts behind**.
For the distribution, give the figures as approximations with their nature: about saline ocean, about frozen, about usable fresh water.
State that water is **densest at , and that the anomaly operates only between and **. Above water is entirely normal.
For the frozen-pond question, give the full chain: surface water cools and sinks until the pond is at , then cooler surface water stops sinking, freezes on top, and the ice insulates the water below.
Keep pure water is a poor conductor ready, and note that ordinary water conducts because of dissolved salts.
And remember sharp melting and boiling points are a test of purity.
Did you know
Why does the sea take so much longer to warm up than the land?
Walk on sand at midday and it burns your feet; step into the sea a metre away and it is cool. Both received the same sunlight.
The difference is water's unusually high specific heat capacity — it takes far more heat to raise a kilogram of water through one degree than to do the same to a kilogram of sand or rock. The sea absorbs the same energy and its temperature barely shifts.
And the effect runs both ways. Having warmed slowly, water also cools slowly, holding its heat long after the land has given its up. That is why the sand is cold by night while the sea stays mild.
This single property explains a great deal of geography. Coastal towns have smaller differences between day and night, and between summer and winter, than inland places at the same latitude — because a vast body of slow-changing water sits next door, steadying the temperature. It is also why water, and not something cheaper, is pumped through a vehicle's radiator.
The difference is water's unusually high specific heat capacity — it takes far more heat to raise a kilogram of water through one degree than to do the same to a kilogram of sand or rock. The sea absorbs the same energy and its temperature barely shifts.
And the effect runs both ways. Having warmed slowly, water also cools slowly, holding its heat long after the land has given its up. That is why the sand is cold by night while the sea stays mild.
This single property explains a great deal of geography. Coastal towns have smaller differences between day and night, and between summer and winter, than inland places at the same latitude — because a vast body of slow-changing water sits next door, steadying the temperature. It is also why water, and not something cheaper, is pumped through a vehicle's radiator.
Key takeaways
Water, its cycle and its properties: quick revision
- Water covers about three-quarters of the Earth's surface: about ** saline ocean, about frozen in ice caps and glaciers, and only about usable fresh water in groundwater, rivers and lakes.
- Life needs water as a solvent for transport, a medium for cell reactions, a raw material for photosynthesis, a temperature regulator, and a habitat.
- Because water dissolves so much, natural water is never pure; pure water is obtained by distillation.
- Water cycle — evaporation from surfaces, transpiration from plant stomata, condensation as rising air cools to form clouds, and precipitation as rain, snow or hail.
- The cycle redistributes and purifies; the total amount of water does not change, and evaporation leaves salts behind.
- Pure water**: colourless, odourless, tasteless; melting point ; boiling point at normal atmospheric pressure; maximum density at ; neutral to litmus; a poor conductor; and unusually high specific heat and latent heat.
- Dissolved impurities lower the freezing point and raise the boiling point, so sharp fixed points are a test of purity.
- Anomalous expansion: between and water contracts on heating and expands on cooling, so it is **densest at **.
- A pond therefore churns until all of it is at , then freezes from the top, and the insulating ice leaves water below for aquatic life.
- Separately, water expands about nine per cent on freezing, so ice floats and unprotected pipes burst.
Try writing out the frozen-pond explanation from memory in full — it is the longest chain of reasoning in this chapter and the one most often asked.
- Life needs water as a solvent for transport, a medium for cell reactions, a raw material for photosynthesis, a temperature regulator, and a habitat.
- Because water dissolves so much, natural water is never pure; pure water is obtained by distillation.
- Water cycle — evaporation from surfaces, transpiration from plant stomata, condensation as rising air cools to form clouds, and precipitation as rain, snow or hail.
- The cycle redistributes and purifies; the total amount of water does not change, and evaporation leaves salts behind.
- Pure water**: colourless, odourless, tasteless; melting point ; boiling point at normal atmospheric pressure; maximum density at ; neutral to litmus; a poor conductor; and unusually high specific heat and latent heat.
- Dissolved impurities lower the freezing point and raise the boiling point, so sharp fixed points are a test of purity.
- Anomalous expansion: between and water contracts on heating and expands on cooling, so it is **densest at **.
- A pond therefore churns until all of it is at , then freezes from the top, and the insulating ice leaves water below for aquatic life.
- Separately, water expands about nine per cent on freezing, so ice floats and unprotected pipes burst.
Try writing out the frozen-pond explanation from memory in full — it is the longest chain of reasoning in this chapter and the one most often asked.