Chemistry | Seawater composition, salt extraction, electrolysis, water isolation

Our Amazing Oceans: A Treasure Chest of Chemicals!

Hello! Ready to dive into the chemistry of the sea? The ocean isn't just a giant swimming pool; it's a huge, salty soup packed with useful chemicals. In this chapter, we'll explore what seawater is made of, how we can get valuable things like table salt and pure drinking water from it, and even how we can use electricity to unlock other important substances hidden within. It’s all about separating mixtures and identifying chemicals – skills that are super important in chemistry!

What's in Seawater? The Ultimate Mixture

First things first, seawater is a fantastic example of a mixture. More specifically, it's a solution. Remember these key terms?

- A solute is the substance that dissolves. (e.g., the salt)
- A solvent is the substance that does the dissolving. (e.g., the water)
- A solution is what you get when a solute dissolves in a solvent.

So, in seawater, water is the solvent, and all the dissolved minerals are the solutes.

The Main Ingredients of Seawater

While it contains many different dissolved minerals, the most famous one by far is common salt, which chemists call sodium chloride (NaCl). This is what makes the sea salty! But there's more in there too:

- Sodium chloride (the main one!)
- Magnesium chloride
- Calcium sulphate
- And many other minerals in smaller amounts.

Did you know?

The Dead Sea is one of the saltiest bodies of water on Earth! It's almost 10 times saltier than the average ocean, which is why you can float in it so easily.

Key Takeaway

Seawater is a mixture (a solution) containing water (the solvent) and various dissolved minerals (solutes), with sodium chloride being the most common.

Getting Useful Stuff from Seawater: Separation Techniques

Since seawater is a mixture, we can separate its parts using physical separation methods. These methods don't involve chemical reactions; they just separate the components based on their different physical properties, like boiling point or solubility. Let's look at the main techniques we use.

1. Filtration

What it is: A method to separate an insoluble solid from a liquid.
Analogy: Think of making coffee with a filter paper. The paper lets the liquid coffee through but traps the solid coffee grounds.

How it works:
1. Pour the mixture through filter paper in a filter funnel.
2. The liquid (the filtrate) passes through the tiny pores in the paper.
3. The insoluble solid particles (the residue) are too big to pass through and get left behind.

In the context of seawater, you'd use filtration to remove things like sand and shells, but NOT the dissolved salt, which would pass right through with the water.

2. Evaporation

What it is: A method to get a soluble solid (solute) from a solution by boiling away the liquid solvent.
When to use it: When you want to keep the solid and don't need to keep the liquid.

How to get salt from seawater by evaporation:
1. Pour some seawater into an evaporating dish.
2. Heat the dish gently. The water will absorb heat and turn into steam (it evaporates).
3. Continue heating until all the water is gone.
4. The solid salt crystals will be left behind in the dish.

Common Mistake Alert!

Don't heat the solution too strongly at the end! If you do, the salt might spit out of the dish. It's better to heat until most of the water is gone and then turn off the heat, letting the rest evaporate slowly.

3. Crystallisation

What it is: A slower, more controlled way to get a soluble solid from its solution, which often produces larger, purer crystals.
Analogy: This is how rock candy is made! A sugar solution is left to cool and evaporate slowly, allowing large sugar crystals to form.

How it works:
1. Heat the solution to evaporate some, but not all, of the solvent. This creates a hot, saturated solution (one that can't dissolve any more solute at that temperature).
2. To check if it's ready, dip a glass rod in the solution and let it cool. If small crystals form on the rod, the solution is ready.
3. Let the saturated solution cool down slowly. As it cools, the solid becomes less soluble, and it will start to form pure crystals.
4. Once the crystals have formed, you can separate them from the remaining liquid by filtration.

4. Distillation

What it is: A method to separate a liquid solvent from a solution. It's perfect for purifying liquids.
When to use it: When you want to keep the liquid and the solid is not needed (or you want to keep both!).
Key Concept: It works because the solvent and solute have different boiling points. Water boils at 100°C, but salt has a very high boiling point (over 1400°C!).

How to get pure water from seawater by distillation:
1. Boiling: Heat the seawater in a flask. The water turns into steam, leaving the salt behind.
2. Condensing: The steam travels into a condenser, which is a tube with cold water circulating around the outside.
3. Collecting: The cold condenser cools the steam, turning it back into pure liquid water (this is called condensation). This pure water, called the distillate, is collected in a separate beaker.

Choosing the Right Tool for the Job

This is a really important skill! You need to decide which method is best.

- If your goal is to get the solid salt from seawater, you should use evaporation (quick) or crystallisation (for better crystals).
- If your goal is to get pure drinking water from seawater, you MUST use distillation. Evaporation just releases the water as steam into the air; distillation allows you to capture that steam and turn it back into pure liquid water.

Key Takeaway

We use physical methods to separate seawater. Use filtration for insoluble solids. Use evaporation/crystallisation to get the dissolved salt. Use distillation to get pure water.

How Do We Know It's Salt and Water? Chemical Tests

Okay, so we've separated our substances. But how can we be 100% sure we have pure water and sodium chloride? We use chemical tests! A chemical test gives a specific, observable result in the presence of a particular chemical.

Test for the Presence of Water

There are two simple tests for water. Don't worry, you don't need to drink it!

Test 1: Anhydrous Copper(II) Sulphate
- What is it? A white powder that is very sensitive to water.
- Procedure: Add a few drops of the liquid you are testing to the white powder.
- Positive Result: If water is present, the powder will turn from white to blue.

Test 2: Dry Cobalt(II) Chloride Paper
- What is it? A piece of blue paper.
- Procedure: Dip the paper into the liquid, or add a drop of the liquid onto the paper.
- Positive Result: If water is present, the paper will turn from blue to pink.

Tests for Common Salt (Sodium Chloride)

Since common salt is sodium chloride, we need to test for two things: sodium ions (Na⁺) and chloride ions (Cl⁻).

Test for Sodium Ions (Na⁺) - The Flame Test

What it is: A test where different metal ions produce a characteristic colour when heated in a flame.
Procedure:
1. Take a clean wire loop (usually nichrome or platinum).
2. Dip the loop into a sample of the salt or its solution.
3. Place the loop at the edge of a hot, blue Bunsen flame.
Positive Result for Sodium: The flame will turn a bright, intense golden-yellow colour.

Test for Chloride Ions (Cl⁻)

What it is: A precipitation reaction to confirm chloride ions.
Procedure:
1. Dissolve your salt sample in some distilled water.
2. Add a few drops of dilute nitric acid. (This is important to prevent other substances, like carbonates, from interfering with the test).
3. Add a few drops of silver nitrate solution.
Positive Result for Chloride: A white precipitate (a solid that forms in the solution) will appear. This white solid is silver chloride (AgCl).

Key Takeaway

To identify our products, we use specific tests: Anhydrous copper(II) sulphate turns from white to blue for water. A flame test gives a golden-yellow flame for sodium. Acidified silver nitrate gives a white precipitate for chloride.

A Shocking Twist! Electrolysis of Seawater

So far, we've only used physical changes. But we can also cause a chemical change in seawater using electricity. This process is called electrolysis.

Electrolysis literally means "splitting with electricity". We pass a powerful electric current through concentrated seawater (often called brine) to break down the substances in it and form new, very useful products.

This is a massive industrial process, and it gives us three incredibly important chemicals:

Products of Electrolysis and Their Uses

1. Chlorine Gas (Cl₂)
- A greenish-yellow, poisonous gas.
- Uses: Sterilising drinking water and swimming pools, making bleach, and producing plastics like PVC (polyvinyl chloride).

2. Hydrogen Gas (H₂)
- A colourless, flammable gas.
- Uses: As a clean fuel (it just produces water when it burns!), making margarine, and producing ammonia for fertilisers.

3. Sodium Hydroxide Solution (NaOH)
- A corrosive and alkaline solution.
- Uses: Making soap and detergents, manufacturing paper, and producing other chemicals.

Quick Review Box

- Physical Separation (like distillation) just separates a mixture. The chemicals themselves don't change. (e.g., you start with H₂O and NaCl, you end with H₂O and NaCl, just in different places).
- Electrolysis is a chemical change. The starting chemicals are broken down and new chemicals are formed. (e.g., you start with H₂O and NaCl, you end with H₂, Cl₂, and NaOH).

Key Takeaway

Electrolysis is a chemical process that uses electricity to break down concentrated seawater, producing three vital industrial products: chlorine, hydrogen, and sodium hydroxide.