Introduction

Introduction

As far as we know, the Earth we live on is the only planet that is able to support life. Amongst other factors, Earth is just the right distance from the sun to have temperatures that are suitable for life to exist. Also, the Earth's atmosphere has exactly the right type of gases in the right amounts for life to survive. Our planet also has water on its surface, which is something very unique. In fact, Earth is often called the 'Blue Planet' because most of it is covered in water. This water is made up of freshwater in rivers and lakes, the saltwater of the oceans and estuaries, groundwater and water vapour . Together, all these water bodies are called the hydrosphere .

Interactions of the hydrosphere

It is important to realise that the hydrosphere interacts with other global systems, including the atmosphere , lithosphere and biosphere .

• Atmosphere When water is heated (e.g. by energy from the sun), it evaporates and forms water vapour. When water vapour cools again, it condenses to form liquid water which eventually returns to the surface by precipitation e.g. rain or snow. This cycle of water moving through the atmosphere and the energy changes that accompany it, is what drives weather patterns on earth.
• Lithosphere In the lithosphere (the ocean and continental crust at the Earth's surface), water is an important weathering agent, which means that it helps to break rock down into rock fragments and then soil. These fragments may then be transported by water to another place, where they are deposited. This is called erosion . These two process, i.e. weathering and erosion, help to shape the earth's surface. You can see this for example in rivers. In the upper streams, rocks are eroded and sediments are transported down the river and deposited on the wide flood plains lower down. On a bigger scale, river valleys in mountains have been carved out by the action of water, and cliffs and caves on rocky beach coastlines are also the result of weathering and erosion by water.
• Biosphere In the biosphere, land plants absorb water through their roots and then transport this through their vascular (transport) system to stems and leaves. This water is needed in photosynthesis , the food production process in plants. Transpiration (evaporation of water from the leaf surface) then returns water back to the atmosphere.

Exploring the hydrosphere

The large amount of water on our planet is something quite unique. In fact, about 71% of the earth is covered by water. Of this, almost 97% is found in the oceans as saltwater, about 2.2% occurs as a solid in ice sheets, while the remaining amount (less than 1%) is available as freshwater. So from a human perspective, despite the vast amount of water on the planet, only a very small amount is actually available for human consumption (e.g. drinking water). Before we go on to look more closely at the chemistry of the hydrosphere, we are going to spend some time exploring a part of the hydrosphere in order to start appreciating what a complex and beautiful part of the world it is.

Investigation : investigating the hydrosphere

1. Choosing a study site: For this exercise, you can choose any part of the hydrosphere that you would like to explore. This may be a rock pool, a lake, river, wetland or even just a small pond. The guidelines below will apply best to a river investigation, but you can ask similar questions and gather similar data in other areas. When choosing your study site, consider how accessible it is (how easy is it to get to?) and the problems you may experience (e.g. tides, rain).
2. Collecting data: Your teacher will provide you with the equipment you need to collect the following data. You should have at least one study site where you will collect data, but you might decide to have more if you want to compare your results in different areas. This works best in a river, where you can choose sites down its length.
   1. Chemical data Measure and record data such as temperature, pH, conductivity and dissolved oxygen at each of your sites. You may not know exactly what these measurements mean right now, but it will become clearer later in the chapter.
   2. Hydrological data Measure the water velocity of the river and observe how the volume of water in the river changes as you move down its length. You can also collect a water sample in a clear bottle, hold it to the light and see whether the water is clear or whether it has particles in it.
   3. Biological data What types of animals and plants are found in or near this part of the hydrosphere? Are they specially adapted to their environment?
   Record your data in a table like the one shown below:

   	Site 1	Site 2	Site 3
   Temperature
   pH
   Conductivity
   Dissolved oxygen
   Animals and plants

3. Interpreting the data: Once you have collected and recorded your data, think about the following questions:
   • How does the data you have collected vary at different sites?
   • Can you explain these differences?
   • What effect do you think temperature , dissolved oxygen and pH have on animals and plants that are living in the hydrosphere?
   • Water is seldom 'pure'. It usually has lots of things dissolved (e.g. Mg ²⁺ , Ca ²⁺ and NO ₃ ^$-$ ions) or suspended (e.g. soil particles, debris) in it. Where do these substances come from?
   • Are there any human activities near this part of the hydrosphere? What effect could these activities have on the hydrosphere?

The importance of the hydrosphere

It is so easy sometimes to take our hydrosphere for granted and we seldom take the time to really think about the role that this part of the planet plays in keeping us alive. Below are just some of the very important functions of water in the hydrosphere:

• Water is a part of living cells Each cell in a living organism is made up of almost 75% water, and this allows the cell to function normally. In fact, most of the chemical reactions that occur in life, involve substances that are dissolved in water. Without water, cells would not be able to carry out their normal functions and life could not exist.
• Water provides a habitat The hydrosphere provides an important place for many animals and plants to live. Many gases (e.g. CO ₂ , O ₂ ), nutrients e.g. nitrate (NO ₃ ^$-$ ), nitrite (NO ₂ ^$-$ ) and ammonium (NH ₄ ⁺ ) ions, as well as other ions (e.g. Ca ²⁺ and Mg ²⁺ ) are dissolved in water. The presence of these substances is critical for life to exist in water.
• Regulating climate One of water's unique characteristics is its high specific heat . This means that water takes a long time to heat up and also a long time to cool down. This is important in helping to regulate temperatures on earth so that they stay within a range that is acceptable for life to exist. Ocean currents also help to disperse heat.
• Human needs Humans use water in a number of ways. Drinking water is obviously very important, but water is also used domestically (e.g. washing and cleaning) and in industry. Water can also be used to generate electricity through hydropower.

These are just a few of the very important functions that water plays on our planet. Many of the functions of water relate to its chemistry and to the way in which it is able to dissolve substances in it.