Water on Earth

Water on Earth10.1 The distribution of water on Earth

The presence of water on Earth in the solid, liquid and gaseous states is unique within our solar system.

The fact that the Earth has retained this water is a result of the planets gravitational force, determined by its mass, and the Earths temperature, which is determined by its distance from the sun. Water has played a central role in the evolution of life on Earth

and remains critical to the maintenance of life on the planet.Water is the only pure substance on Earth commonly found in all

three states.

Water has some remarkable properties that are crucial to the maintenance of all living systems.

Liquid water covers nearly 75% of the Earths surface in the form of oceans, seas, lakes, ground water and rivers, which are collectively referred to as the hydrosphere.

The polar ice caps cover another portion of our planet, dominating the regions north of the Arctic Circle and south of the Antarctic Circle.

Our atmosphere contains varying amounts of gaseous water,

water vapour, in the range 05%.

Water in the lithosphere can occur as moisture or permafrost within soils and rocks, and as water of crystallisation within minerals. Water of crystallisation or water of hydration refers

to water molecules that form part of the crystal structure of many ionic substances for example hydrated copper(II) sulfate, CuSO4.5H2O,

Water is the major constituent of living matter, representing approximately 70% of the biosphere. It is a component of all cells, the transport system for nutrients and waste products in living things, a reaction medium for many biochemical processes,

and, together with carbon dioxide, is a reactant in photosynthesis.

10.2 SolutionsThe oceans and seas dominate our planet. They are not pure substances, but mixtures containing large quantities of dissolved substances, called solutions.

Solutions are defined as homogeneous mixtures. In a solution, one substance, known as the solute, is dissolved in another substance, the solvent.

Aqueous solutionsSolutions in which the solute is dissolved in water are called aqueous solutions and are indicated in chemical equations by the use of the symbol (aq) to the right of the solute. For example, a solution of glucose in water is indicated as C6H12O6(aq) and an aqueous solution of sodium chloride as NaCl(aq). Common naturally occurring aqueous solutions include sea water, blood plasma, stomach acid, sap in plants and drinks like coffee and lemonade.Saturated and unsaturated solutionsIf sugar is continually added to water, there comes a point at which no more will dissolve. At this stage the solution is said to be saturated. A saturated solution is one in which no more solute will dissolve under the existing conditions of temperature and pressure. An unsaturated solution is a solution that contains less than the quantity of solute needed to saturate it under the existing conditions.SolubilityDifferent substances can exhibit very different solubilities in a particular solvent. For example, salt is an example of a substance that is readily soluble in water, while oxygen is less soluble and sand is virtually insoluble.

10.3 The Importance of water in Earth processes

The movement of water through the atmosphere, lithosphere, hydrosphere and biosphere is referred to as the water cycle.

Water makes up approximately 70% of the biosphere and has a critical role in the maintenance of life on Earth. is an essential reactant in the production of glucose by the process of photosynthesis.

6CO2(g) + 6H2O(l) + light energy C6H12O6(aq) + 6O2(g) glucose is the basic currency of energy in the biosphere.

Cellular respiration is essentially the reverse and provides the energy required to sustain life.

C6H12O6(aq) + 6O2(g) 6CO2(aq) + 6H2O(l) + energy

acts as a solvent for many solutes. Water dissolves oxygen, various salts and nutrients, and is therefore important in supporting plant and animal life. many waste products such as carbon dioxide, ammonia and urea are soluble in water, providing an important mechanism by which plant and animal wastes can be removed. The human body is over two-thirds water, with blood, being over 80% water. is also a major component of our lymph system, and the moisture lining our lungs allows the gases oxygen and carbon dioxide to be transferred between the air we breathe and our respiratory system. transfers heat energy around our body from cells performing respiration to our skin, where it can escape. has a relatively high heat capacity, which means it can absorb (and release) large quantities of heat energy with relatively small temperature changes. Affects the air temperatures experienced in regions close to the coast.

performs a vital role in both the formation of landforms and the production of soil by weathering and erosion. is critical to human existence in other ways reliable supply of good quality water is a central requirement of modern societies. used for drinking, washing, growing crops and in

many industrial processes. constitute a transport system, important in moving great quantities of food, raw materials and consumer products around the planet. serves as a source of great entertainment and enjoyment to many people.

10.4 Water density

The density of liquid water, like all other matter, is defined as follows:

Density = mass

volume

The units for density are usually expressed as kg m3 or g cm3.

As the temperature increases, it usually expands, and its volume increases. This is caused by the increased kinetic energy of the particles. As a result, the density of the substance decreases. Most substances increase in density when they freeze, again because a decrease in volume occurs during the freezing process.Waters density is unusual in two ways. First, water expands when it changes from the liquid to the solid stateThe lower density of ice in relation to liquid water allows ice to float on top of water, a situation that is critical to life forms in polar and other regions that experience extremely cold weather conditions. Second, as liquid water is cooled, its density increases until it reaches 4C, but below this temperature its density decreases. This occurs because water initially contracts with cooling but at temperatures below 4C it expands due to the intermolecular forces in water causing the water molecules to arrange themselves in a particular way.

Water on Earth

Review exercise 10.1

1 Identify the most common state in which water is found in:

a the biosphere

b the hydrosphere

c the atmosphere.

2 List three natural reservoirs of liquid water that occur on the Earth.

3 Explain the term water of crystallisation, using an example to support your answerr. Review exercise 10.2

1 Outline the differences between compounds, mixtures and solutions.

2 The solubility of sodium chloride in water is 36 g per 100 g of water at 25C. Identify the following solutions as saturated or unsaturated at 25C:

a 30 g of NaCl in 100 g of water

b 38 g of NaCl in 100 g of water

c 18 g of NaCl in 50 g of water.

Review exercise 10.3

1 Account for the importance of water to living things.

2 Explain the effect that waters capacity to store heat energy has on the climate of a coastal area.

3 Identify and compare the processes by which water brings about the physical and chemical weathering of rocks.

Review exercise 10.4

1 Explain, in terms of the particle theory of matter, why the density of substances usually decreases when temperature increases.

2 The density of ice at 2C is 0.917 g cm3. Calculate the mass of 1 litre (1 dm3) of ice at this temperature.

3 a Explain the unusual variation in the density of water as a function of temperature.

b Outline how this density variation is important for life on Earth.

c Use Table 10.4 to identify the temperature at which water is most dense.