Planet Earth: The Oceans
FoundationSeawater holds about 3.5% dissolved salts, mostly sodium chloride. Evaporation yields the salt; simple distillation yields pure water; flame, chloride and water tests identify the sample.
- Seawater is about 3.5% dissolved salts (mostly sodium chloride) and 96.5% water
- A solution is solvent plus solute; when no more solute dissolves, it is saturated
- Salt by evaporation and crystallisation (cheap); pure water by distillation (vapour condensed)
文體小知識：科學知識說明文——講解海水的成分、蒸發結晶與蒸餾兩種物理分離法的取捨，以及鑑定鈉、氯化物和水的試驗。
§Text Passage
1The oceans cover about seven-tenths of the Earth's surface and are the largest store of chemical resources. Seawater contains about 3.5% dissolved salts, mainly sodium chloride (common salt), together with salts of magnesium, calcium and potassium; the remaining 96.5% is water. The salt dissolves in the water to form a solution: water is the solvent, salt the solute, and when no more salt can dissolve, the solution is saturated.
2Salt is obtained from seawater by evaporation and crystallisation: seawater is led into shallow pans, where sunshine and wind slowly evaporate the water. The solution grows more concentrated and, once saturated, the salt crystallises out. In the laboratory, seawater is heated gently in an evaporating dish until the water leaves and crystals form. The method is cheap and suits mass production, but the salt may carry impurities such as sand and mud.
3To obtain pure water from the sea, distillation is used: the seawater is boiled, the water vapour escapes, passes through a condenser and is collected as liquid water, while the salt stays behind. Distilled water contains virtually no impurities and is the main physical way of obtaining pure water. Besides importing water from Guangdong, Hong Kong also draws on desalination — based on distillation or reverse osmosis.
4Chemists identify the components of seawater with several tests: in a flame test sodium gives a golden-yellow flame, showing the sample contains sodium; adding silver nitrate solution gives a white precipitate, showing chlorides; anhydrous copper(II) sulfate turning from white to blue shows the sample contains water. Electrolysis of brine yields chlorine and other products used to make bleach and disinfectants — ocean resources are closely tied to daily life.
1Summary 重點摘要
Seawater holds about 3.5% dissolved salts, mostly sodium chloride. Evaporation yields the salt; simple distillation yields pure water; flame, chloride and water tests identify the sample.
2Vocabulary cards Vocabulary
The substance that dissolves in a solvent, such as salt in seawater.
The liquid that dissolves a solute, such as water.
A solution that cannot dissolve any more solute at that temperature.
The slow change of a liquid into gas at its surface; heating speeds it up, and it is used to obtain salt from seawater.
The process in which solute separates from a saturated solution as crystals.
Boiling a liquid and condensing the vapour to collect the pure liquid.
The apparatus that cools hot vapour back into liquid; cooling water flows in at the bottom and out at the top.
Identifying a metal element from the colour it gives a flame: sodium is golden yellow, calcium brick red.
Adding silver nitrate solution; a white precipitate shows that chloride is present.
A white powder that turns blue when it meets water, used to test for water.
3Step by step Steps
- Step 1: Seawater and the language of solutionsSeawater is about 3.5% dissolved salts — mostly sodium chloride, with salts of magnesium, calcium and potassium — and 96.5% water. The salt dissolves in the water: water is the solvent, salt the solute, and together they form a solution. For example, stir a spoonful of salt into a glass of water: the grains vanish and the water tastes salty — that is a solution. Keep adding salt until some grains remain undissolved, and you have a saturated solution at that temperature.
- Step 2: Why physical separation worksSalt and water are merely mixed; they have not combined into a new substance. So evaporation, distillation, crystallisation and filtration can separate them, with no new substance forming. For example, the salt left behind after seawater dries up is still sodium chloride, with the same properties as table salt — separation does not change the substances themselves. Which method to choose depends on whether you want the solute (salt) or the solvent (pure water) — exactly the trade-off DSE data questions love to ask.
- Step 3: Getting salt by evaporation and crystallisationTo obtain salt, evaporate seawater in shallow pans or an evaporating dish. As water evaporates, the solution becomes more and more concentrated; once saturated, further evaporation makes the salt crystallise out. For example, coastal salt fields channel seawater into shallow ponds, let sun and wind evaporate the water, then collect the white salt — cheap and ideal for mass production. The salt, however, may carry sand, mud and traces of impurities, so it is less pure than recrystallised laboratory products.
- Step 4: Getting pure water by distillationTo obtain pure water, use simple distillation: boil the seawater in a distillation flask; water vapour rises, enters the condenser, cools and drips into the collecting vessel as liquid water, while the salt, whose boiling point is extremely high, never vaporises and stays in the flask. For example, in the laboratory cooling water flows into the condenser at the bottom and out at the top so the vapour is fully chilled; the distilled water collected is almost free of impurities — the basic principle of desalination.
- Step 5: Judging between the two methodsEvaporation and crystallisation are cheap and save energy, ideal for obtaining large amounts of salt — but the product is salt, not pure water, and it may carry impurities. Distillation gives almost pure water but consumes much heat energy and costs more. For example, salt fields choose evaporation because sunshine and wind are free, while desalination plants in water-scarce regions choose distillation or reverse osmosis because their target is pure water. Weigh up the desired product and the cost — the curriculum's way of evaluating whether a separation method is appropriate.
- Step 6: The flame test and the chloride testTo show a salt sample contains sodium, use the flame test: dip a clean platinum wire into the sample solution and hold it in a Bunsen flame — sodium colours the flame golden yellow. To show chlorides, add silver nitrate solution; a white precipitate (silver chloride) forms. For example, a seawater sample gives a golden-yellow flame and a white precipitate with silver nitrate — together the two tests prove the seawater contains sodium chloride, the combination the curriculum calls 'showing sodium and chloride in a salt sample'.
- Step 7: The water test and electrolysis of brineTo test whether a sample contains water, use anhydrous copper(II) sulfate: the white powder turns blue with water; alternatively cobalt(II) chloride paper turns from blue to pink. Electrolysing brine yields chlorine, hydrogen and sodium hydroxide; chlorine is used to make bleach and disinfectants. For example, the chemicals in household bleach and water treatment are tied to the products of electrolysis. Water is precious: besides importing water from Guangdong, Hong Kong opens new sources by desalination — the sea and society are closely linked.
4Diagrams Diagrams
5Worked example Example
Choosing separation methods: you are given a beaker of seawater and asked to obtain, first, a sample of salt and, then, some pure water. Which physical separation methods would you use? Name each method and explain the principle and your reasons.
- Read the question: two products are wanted — salt (the solute) and pure water (the solvent) — so match evaporation-crystallisation with salt and distillation with water; one method cannot answer both parts
- For salt: heat the seawater gently in an evaporating dish (or evaporate it in salt pans); as water is lost the solution saturates and salt crystals form
- For pure water: boil the seawater in a distillation flask; the vapour condenses in the condenser and is collected as liquid water, while the salt stays behind because it never vaporises
- Evaluate: use evaporation-crystallisation for salt because it is cheap; pure water requires distillation, since only condensed vapour gives water free of salt
- Answer: Salt is obtained by evaporation and crystallisation: heat the seawater gently in an evaporating dish (or evaporate it under the sun); as water keeps leaving, the solution reaches saturation and sodium chloride crystallises out — the crystals are the salt sample. Pure water is obtained by simple distillation: boil the seawater, let the vapour pass into a condenser, and collect the condensed liquid; the salt never vaporises and stays in the flask. Both are physical separations with no new substance formed. Evaporation is cheaper, distillation gives pure water at a higher energy cost — the choice depends on which product you want.
6Common mistakes Mistakes
7Quick check Quick check
Q1What percentage of seawater is salt, and what is the main salt?
Q2Which physical methods give salt and pure water from seawater?
Q3What colour does sodium give in a flame test, and what does it show?
Q4How do you test a sample for chloride?
Q5About how many grams of salt are in 1 kg of seawater?
8Recap Recap
- Seawater is about 3.5% dissolved salts (mostly sodium chloride) and 96.5% water
- A solution is solvent plus solute; when no more solute dissolves, it is saturated
- Salt by evaporation and crystallisation (cheap); pure water by distillation (vapour condensed)
- Three key tests: flame test for sodium (golden yellow), silver nitrate for chloride (white precipitate), anhydrous copper(II) sulfate for water (white to blue)
- Evaporation, distillation, crystallisation and filtration are all physical separations — no new substance forms