back to index ofchaptersSuggested answers to questions from the text page 83
1. Name five unique properties of sea water.
Ans: A study of sea water reveals that it has many unique properties:
1. it accounts for over 85 per cent of the mass of marine organisms andis the medium in which the chemical reactions that support life in the seatake place;
2. it provides support in the form of buoyancy for some of the organismsthat live in the sea; and
3. it is less dense in its solid form than its liquid form, so it formsicebergs which float in the Arctic and Antarctic waters.
Sea water also shares the unique properties of all water:
4. water is the only substance on the Earth's surface that can existnaturally in all three states as a solid, liquid and gas; and
5. water can dissolve more substances than any other liquid.
2. Name four salts found in sea water.
Ans.
Sodium chloride
Magnesium chloride
Potassium chloride
Magnesium sulphate
Calcium sulphate
3. Draw a diagram of the Hoffman's voltameter and show how it can beused to demonstrate the composition of water.
Ans. See Figure 81.3
4. What is a salt-water wedge?
Ans. Salt water is heavier than fresh water so in an estuary you canfind a salt-water wedge which is fresh water on the top and salt water onthe bottom as shown in Figure 82.2.
See Figure 82.2
5. What is the difference between fresh, brackish and salt water?
Ans. Salinity is the amount of dissolved salts in the water. The salinityof sea water is shown in Figure 82.1.
Sea water has 3035 g salt per L. As you head up a river away fromthe sea the salinity will drop due to the influence of fresh water fromrain entering rivers and creeks.
Brackish water (water that has some salt in it) is defined as water witha salt content of 0.517 g/L. Fresh water is defined as water with a saltcontent less than 0.5 g/L.
6. How are ocean currents formed?
Ans. Water is very good at absorbing heat energy from the Sun. Comparingsurface temperatures of oceans in the Southern and Northern Hemispheresshows that the oceans have similar temperatures at similar latitudes. Thesetemperatures vary from 30oC at the equator to 0oC at the poles.
These variations of temperature cause the ocean currents of the world.The warm water of the tropics flows towards the poles and the icy waterof the poles flows towards the equator (see Chapter 12).
The cold water, being more dense, sinks beneath the warmer water. Warmerwater is found only at the surface because the energy from the Sun doesnot penetrate to the depths.
7. What is a thermocline and how does it affect life in the sea?
Ans. Thermoclines
Because only the surface of the ocean is heated by sunlight, the depthsare cold and there is minimal mixing because warmer water cannot displacethe deeper, colder water. The narrow band of water between the two layerswhere water temperature decreases rapidly with depth is called the thermocline(Figure 83.2).
The thermocline stops many minerals from mixing and can affect oceanproductivity.
Suggested answers to questions from the text page 85
1. Why does cold water sink below warm water?
The relative density of a substance is the ratio of the density of thatsubstance to the density of pure water (see Figure 87.1).
The relative density of sea water is determined by salinity and watertemperature. Dense water sinks and is found at the bottom of the oceans.Less dense water is found floating at the surface as shown in Figure 87.2(see 'Aspects of salinity').
Objects that are more dense than water have a relative density greaterthan 1. These substances sink in pure water.
2. Define these terms:
Answers.
a. photic zone
Light The photic zone is the zone or depth to which lightpenetrates in the sea and is the zone where the plants of the sea live.
b. buoyancy
Defining buoyancy
Buoyancy is the ability or tendency to float. Marine organisms that havethe same density as sea water tend to be neutrally buoyant; but what isthe force that keeps these animals from sinking?
c. upthrust
Upthrust
If a diver is put into a full tank of water, the water will overflow.If we weigh that water, we find that it is equal to the volume of the diver.The mass of the water that was displaced is called the upthrust.
If the diver wants to get as deep as possible, the weight needs to becontrolled. This can be done by weight belts as shown in Figure 84.1.
3. What instrument is used to measure the clarity of water?
Ans. A simple instrument used to measure the clarity of water is calledthe secchi disc (shown in Figure 85.2).
4. What colour frequency of light is absorbed most quickly by water?
Ans. Salt water is transparent. This means it allows light to pass throughit. The amount of light that penetrates the water depends on the dissolvedminerals, silt and detritus material contained in it.
White light is made up of the full spectrum of colours of the rainbow.Red light is absorbed first by the water and does not penetrate as far asthe blue or violet light. This is because the shorter the wavelength oflight, the greater the depth it penetrates.
5. Discuss the two major factors that determine the colour of the seas.
Ans. The colour of our seas and oceans depends on a number of factors.The two major factors are the amount of ions and the amount of plant lifein the water. Blue water indicates that the surface water contains veryfew ions and so has little plant life, so it can be considered a 'desert'area.
Green sea water contains large concentrations of ions, which in turncause an abundance of plant growth, which in turn attracts large numbersof animals.
The colour of the water is caused by the scattering and absorption oflight by the water. Water of low density causes blue light to be more readilyscattered, thus giving it a blue colour because blue light arrives backat the surface without being absorbed.
6. How does a hydrometer determine if the water is salty or fresh?
Ans. Sea water has a greater upthrust than fresh water. This is becausesalt water has a greater weight than the same volume of distilled water.This can be demonstrated by using an hydrometer. A hydrometer is an instrumentmade of a hollow glass tube weighted at one end (Figure 84.2) to float upright.The hydrometer sinks deeper in less dense liquids.
7. Explain how a diver may attain negative buoyancy.
Ans. If the diver wants to get as deep as possible, the weight needsto be controlled. This can be done by weight belts as shown in Figure 84.1.
8. How would temperature variations in a rock pool compare with thosein a lake?
Ans. The range of temperatures a marine organism has to cope with alsodepends on the volume of water in which it lives. The smaller the volume,the larger the range of temperatures (see page 73).
From page 73: Temperatures near the fringe of the sea vary greatly comparedto temperatures underwater (see Chapter 8). The range of temperatures marineorganisms have to cope with depends also on the volume of water and tidalrange in which they live. The smaller the volume of water, the larger therange of temperatures. Only a small amount of solar energy is needed toraise the temperature of the pool in Figure .1 by 5oC, but the ocean inthe background requires a large amount of energy to cause the same temperaturefluctuation.
Suggested answers to questions from the text page 86
8. How do animals adapt to living in hydrostatic pressure?
Ans. Many fish have air bladders which are used to change densities.This is not a good question.
9. What is the difference between atmospheric pressure and hydrostaticpressure?
Ans. The pressure found in the air around the Earth is called atmosphericpressure. The pressure in the waters of the ocean is called hydrostaticpressure.
1. What are some adaptations animals have which allow them to overcomethe buoyancy of the water?
Suggestions:
Read Chapter 15 , pages 197 - 198. Then see page 200.
2. How is the eye of a fish different to the human eye? What is peripheralvision?
Suggestions:
Read Chapter 15 of the Senior Marine Studies Text - see Page 433
3. Discuss some of the adaptations plants may have that allow them tofloat near the surface in the photic zone.
Suggestions:
See Chapter 17, page 175. Also see the Senior Marine Studies Text - seePage 414, 465
4. Suggest why the colour of algae vary with the depth at which theyare found.
Suggestions:
See page 85 of this book on colour of the seas.
5. Suggest how rust bubbles may form under paint.
See Chapter 14, pages 141 and 142. Look up the word corrosion in a Chemistrytext.
6. Design an experiment to compare the rate at which the following metalscorrode in sea water: zinc, iron, aluminium, stainless steel and copper.
Suggestions.
Weight the samples of each metal and place them in salt water. Weighthem again after a number of days.
7. What is so special about the salinity in the Red Sea?
Suggestions
Go to the library, find a book on Oceanography.
1. hydrogen
2. oxygen
3. H2O
4. Salinity
5. Isohalines
6. salinity
7. band of water
8. thermocline
9. Buoyancy
10. overflow
11. volume
12. upthrust
13. hydrometer
14. weighted
15. photic
16. penetrates
17. atmospheric
18. hydrostatic
19. Viscosity
20. respiration
21. photosynthesis
22. carbon dioxide
23. oxygen