The Ocean and Sea Water: Treasure in Every Drop!
Hong Kong is surrounded by the sea, but what is inside sea water? In this chapter you will learn what is dissolved in sea water, how to get common salt and pure water from it, how to test for water and sodium chloride, how to choose a separation method, and how electricity can split water. Let's dive in!
1. Sea Water Is a Mixture
Sea water is a mixture: mostly water, with many minerals dissolved in it. On average, 1 kg of sea water contains about 35 g of dissolved salts – about 3.5% by mass.
What minerals are in sea water?
Sodium chloride (\(\text{NaCl}\)) – the main one, roughly three-quarters of all the dissolved salts. This is common salt.
Magnesium compounds – such as magnesium chloride and magnesium sulphate. They give sea water a slightly bitter taste.
Calcium and potassium compounds – such as calcium sulphate, calcium carbonate and potassium chloride. Shellfish and corals use dissolved calcium to build their shells and skeletons.
Where do the minerals come from?
Rain slowly dissolves minerals from rocks and soil, and rivers carry them into the sea. When the Sun heats the sea, water evaporates but the salts stay behind. Over millions of years the salts have built up.
Useful minerals from the sea
Common salt is used for cooking, preserving food such as salted fish, and making other chemicals. Magnesium compounds from sea water can be used to make magnesium, a light metal used in alloys for bicycles and laptops. The sea floor also holds sand, gravel and metal-rich lumps called nodules.
Did you know? Most homes in Hong Kong use sea water to flush their toilets. This saves a very large amount of fresh water every day.
Key takeaway: Sea water is a mixture of water and dissolved minerals – mainly sodium chloride, plus magnesium, calcium and potassium compounds.
2. Getting Salt and Pure Water from Sea Water
We can separate sea water by physical methods: no new substances are made. First ask: do we want the salt (the solid) or the water (the liquid)?
Evaporation: getting the solid quickly
Heat sea water in an evaporating dish. The water escapes as steam, leaving the dissolved solids. Heating until all the water is gone is called evaporation to dryness. It is quick, but the water is lost and all the dissolved substances are left together, so the salt is not pure. Hot solid may also spit, so wear goggles.
Crystallisation: getting purer crystals
Heat the solution to boil off some water until it is saturated (it cannot dissolve any more solid). Then leave it to stand. As it cools and more water slowly evaporates, crystals form. Filter them off, rinse with a little cold distilled water and dry them between filter papers.
The crystals form before all the water is gone, so many other dissolved substances stay in the leftover liquid. That is why crystallisation gives purer salt than evaporation to dryness, and slow crystallisation gives bigger, well-shaped crystals.
Salt pans: crystallisation powered by the Sun
In salt pans (shallow ponds by the sea), the Sun and wind slowly evaporate sea water. Workers rake up the salt crystals before the pond dries out, leaving many unwanted substances in the bitter leftover liquid. Solar energy is free, but this needs much land and sunny, dry weather. Yim Tin Tsai in Sai Kung, whose name includes the words "salt field" (Yim Tin), was once a salt-making village.
Distillation: getting pure water
Sea water is boiled in a flask. The steam passes into a condenser, where cold water flowing around the outside makes it condense back into liquid. This pure water, the distillate, is collected. The salts stay in the flask.
Memory aid: Distillation = Evaporation + Condensation. Boil it, cool it, collect it! It gives very pure water but uses a lot of energy.
Desalination: fresh water for cities
Desalination means removing salts from sea water to make fresh water. Many plants use reverse osmosis: sea water is pushed at high pressure through very fine membranes that let water pass but hold back the salts. Hong Kong's desalination plant at Tseung Kwan O uses reverse osmosis. Its water does not depend on rainfall, but it uses a lot of electricity.
Key takeaway: For salt, use evaporation (quick) or crystallisation (purer). For pure water, use distillation or other desalination methods.
3. Chemical Tests: Is It Water? Is It Salt?
Many liquids look like water. Chemical tests give a clear, observable change that shows what is present.
Testing for water
Anhydrous copper(II) sulphate – add a few drops of the liquid to the white powder. If water is present, it turns blue. ("Anhydrous" means "without water".)
Dry cobalt(II) chloride paper – touch the liquid with dry, blue cobalt(II) chloride paper. If water is present, it turns pink. The paper must be dry before use.
Memory aid: Copper goes White to Blue; Cobalt goes Blue to Pink.
Present does not mean pure!
These tests only show that water is present. Sea water and orange juice pass them too. To check that water is pure, measure its boiling and freezing points: at normal atmospheric pressure, pure water boils at 100 °C and freezes at 0 °C. Sea water boils above 100 °C and freezes below 0 °C.
Testing for sodium: the flame test
Dip a clean nichrome wire loop into the sample and hold it in a non-luminous (blue) Bunsen flame. Sodium gives a bright golden yellow flame. Clean the wire well, as even a trace of sodium shows up. Other metals give other colours – see Rocks and minerals.
Testing for chloride: acidified silver nitrate
To a little sample solution, add a few drops of dilute nitric acid, then silver nitrate solution. A white precipitate (a solid formed in the solution) of silver chloride shows that chloride is present. The acid stops substances such as carbonates from giving a false result. Silver nitrate stains skin, so wear gloves and goggles.
Putting the tests together
Example: One bottle holds distilled water, the other sea water.
Step 1 – Both turn anhydrous copper(II) sulphate blue: both contain water, so this cannot tell them apart.
Step 2 – Only sea water gives a golden yellow flame (sodium).
Step 3 – Only sea water gives a white precipitate with acidified silver nitrate (chloride).
Conclusion – It contains sodium chloride, so it is sea water.
Key takeaway: Water: copper(II) sulphate white to blue, cobalt(II) chloride paper blue to pink. Sodium: golden yellow flame. Chloride: white precipitate with acidified silver nitrate.
4. Choosing the Right Separation Method
You must evaluate which method fits a job, and give a reason. Remember: filtration separates an insoluble solid (the residue) from a liquid (the filtrate).
Ask yourself three questions
1. Is the substance dissolved? Undissolved bits (sand, seaweed, plastic) can be filtered off. Dissolved salt cannot.
2. Do I want the solid or the liquid? Solid: evaporation or crystallisation. Liquid: distillation.
3. How pure, fast and cheap? Crystallisation is purer but slower; distillation uses more energy.
Which method? Try these situations
A. Remove sand and plastic bits from a beach water sample: filtration, because they do not dissolve.
B. Get drinking water from sea water on a boat: distillation, because we want the water, not the salt.
C. Find the total mass of dissolved salts in 100 g of sea water: evaporation to dryness, because we want all the solids and purity does not matter.
D. Get pure, well-shaped salt crystals: crystallisation.
E. Get clean salt from muddy sea water: filtration first to remove the mud, then crystallisation of the filtrate. Real problems often need two methods!
Common mistake: "Filter sea water to make it drinkable." No! Salt is dissolved, so it passes through the filter paper with the water. Filtered sea water is still salty.
Key takeaway: Ask: dissolved or not? Solid or liquid wanted? How pure, fast and cheap? Then give your reason.
5. Electrolysis of Water
The methods above are physical. Electrolysis is different: it uses electricity to split a compound into simpler substances. Water breaks down into hydrogen and oxygen, so this is a chemical change.
Word equation: \(\text{water} \xrightarrow{\text{electricity}} \text{hydrogen} + \text{oxygen}\)
Chemical equation: \(2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2\)
Pure water conducts electricity very poorly, so a little electrolyte, such as sodium sulphate or dilute sulphuric acid, is added. Sodium chloride is not used because it can produce poisonous chlorine gas.
A microscale set-up
1. Pour sodium sulphate solution into a small Petri dish.
2. Fill two small tubes with the solution and turn them upside down over two carbon electrodes (such as pencil leads), with no air trapped.
3. Connect the electrodes to a 9 V battery.
4. Watch bubbles collect in the tubes and compare the volumes of gas.
Why microscale? Using only a few millilitres of chemicals is cheaper, safer and makes less waste.
Results: twice as much hydrogen
Negative electrode: hydrogen. Positive electrode: oxygen. The volume of hydrogen is about twice that of oxygen, \(\text{hydrogen} : \text{oxygen} \approx 2 : 1\), matching the formula \(\text{H}_2\text{O}\). Real results may be a little above 2 : 1, because oxygen dissolves in water slightly more than hydrogen.
Memory aid: "PO" – the Positive electrode gives Oxygen.
Testing the gases
Hydrogen burns with a "pop" sound when a burning splint is held at the mouth of the tube. Oxygen relights a glowing splint. More on gas tests in The carbon cycle and gases from air.
Hydrogen is a clean fuel: burning it produces only water.
Common mistake: Saying oxygen forms at the negative electrode or has twice the volume. It is hydrogen that forms at the negative electrode with twice the volume.
Key takeaway: Electrolysis splits water into hydrogen (negative electrode) and oxygen (positive electrode) in a volume ratio of about 2 : 1.
Quick Review
1. Sea water: about 3.5% dissolved salts, mostly sodium chloride, plus magnesium, calcium and potassium compounds.
2. Evaporation is quick; crystallisation (as in salt pans) gives purer salt.
3. Distillation (evaporation + condensation) and reverse osmosis give fresh water from sea water.
4. Water: copper(II) sulphate white to blue; cobalt(II) chloride paper blue to pink. Present, not necessarily pure!
5. Sodium: golden yellow flame. Chloride: white precipitate with acidified silver nitrate.
6. Filtration removes only insoluble solids. Always give reasons for your method.
7. Electrolysis of water: hydrogen at the negative electrode, oxygen at the positive, about 2 : 1 by volume.
Great job, ocean scientist!