Testing Water Purity and Summary of Separation Techniques

This final lesson in the topic brings together everything you have learned about states of matter and separation techniques, as required by the Edexcel GCSE Chemistry specification (1CH0, Topic 1). You will learn how to test for dissolved solids in water, review all separation techniques, and practise choosing the right technique for a given situation. This lesson also covers common exam mistakes and how to write high-quality answers for describe-the-method questions.

---

Testing for Dissolved Solids in Water

The Evaporation Test

To determine whether a sample of water contains dissolved solids:

1. Measure a known volume of the water sample (e.g. 50 cm\u00b3) using a measuring cylinder.
2. Weigh a clean, dry evaporating basin and record its mass.
3. Pour the water sample into the evaporating basin.
4. Heat the evaporating basin gently on a tripod and gauze (or water bath) to evaporate all the water.
5. Allow the basin to cool, then re-weigh it.
6. Calculate the mass of the residue: mass of residue = final mass of basin − initial mass of basin.

If the mass increases after evaporation, the water contained dissolved solids. The mass of the residue tells you how much dissolved material was present in the sample.

Comparing Water Samples

You can compare the purity of different water samples:

Sample	Volume (cm\u00b3)	Residue Mass (g)	Conclusion
Distilled water	50	0.00	No dissolved solids (pure)
Tap water	50	0.03	Small amount of dissolved solids
Seawater	50	1.75	Large amount of dissolved solids

Exam Tip: If asked how to determine whether a water sample contains dissolved solids, describe the evaporation method. If the exam asks "how can you tell if water is pure?", you could also measure its boiling point — pure water boils at exactly 100 \u00b0C. If it boils above 100 \u00b0C, it contains dissolved impurities.

---

Flame Tests: A Brief Introduction

While a full treatment of flame tests belongs to the qualitative analysis topic, you should be aware that flame tests can help identify the metal ions present in dissolved solids:

Metal Ion	Flame Colour
Lithium (Li\u207a)	Crimson (red)
Sodium (Na\u207a)	Yellow
Potassium (K\u207a)	Lilac
Calcium (Ca\u00b2\u207a)	Orange-red
Copper (Cu\u00b2\u207a)	Green/blue-green
Barium (Ba\u00b2\u207a)	Green

Method for Flame Tests

1. Clean a nichrome wire loop by dipping it in concentrated hydrochloric acid and holding it in a Bunsen flame. Repeat until the wire produces no colour in the flame.
2. Dip the clean wire into the sample (solid or solution).
3. Hold the wire in the blue flame of a Bunsen burner.
4. Observe and record the colour of the flame.

Exam Tip: Flame tests are a quick way to identify metal ions, but they only work for certain metals. Sodium gives a very strong yellow flame that can mask other colours. If you suspect a mixture, you may need to use other tests alongside flame tests.

---

Summary Table: All Separation Techniques

This table brings together every separation technique covered in this topic:

Technique	What It Separates	Principle	When to Use
Filtration	Insoluble solid from a liquid	Solid particles are too large to pass through filter paper	Sand from water; precipitate from solution
Evaporation	Dissolved solid from a solution	Solvent evaporates, leaving solid behind	Quick recovery of salt from solution (small crystals)
Crystallisation	Dissolved solid from a solution	Controlled evaporation and slow cooling	Large, pure crystals (e.g. copper sulfate)
Simple distillation	Solvent from a solution	Solvent evaporates and is condensed	Pure water from salt water
Fractional distillation	Miscible liquids with different boiling points	Temperature gradient in fractionating column	Ethanol from water; crude oil fractions
Paper chromatography	Dissolved substances (e.g. dyes)	Different solubilities in the mobile phase	Identifying dyes in ink or food colouring

---

Decision Flowchart: Which Separation Technique Should I Use?

flowchart TD
    A["What type of mixture do you have?"] --> B{"Does it contain an<br/>insoluble solid?"}
    B -->|Yes| C["Use FILTRATION"]
    B -->|No| D{"Is a dissolved solid<br/>in a solution?"}
    D -->|Yes| E{"Do you want the<br/>solid or the liquid?"}
    E -->|"Solid (quick)"| F["Use EVAPORATION"]
    E -->|"Solid (large, pure crystals)"| G["Use CRYSTALLISATION"]
    E -->|"Liquid (the solvent)"| H["Use SIMPLE DISTILLATION"]
    D -->|No| I{"Is it a mixture of<br/>miscible liquids?"}
    I -->|Yes| J["Use FRACTIONAL DISTILLATION"]
    I -->|No| K{"Is it a mixture of<br/>dissolved coloured<br/>substances?"}
    K -->|Yes| L["Use PAPER CHROMATOGRAPHY"]
    K -->|No| M["Consider other techniques<br/>or combinations"]

    style A fill:#3498db,color:#fff
    style C fill:#27ae60,color:#fff
    style F fill:#27ae60,color:#fff
    style G fill:#27ae60,color:#fff
    style H fill:#27ae60,color:#fff
    style J fill:#27ae60,color:#fff
    style L fill:#27ae60,color:#fff
    style M fill:#e67e22,color:#fff

Exam Tip: This decision flowchart is extremely useful for exam questions that ask you to choose an appropriate separation technique. Always identify what type of mixture you have (insoluble solid + liquid? dissolved solid + liquid? miscible liquids? dissolved coloured substances?) and then select the matching technique.

---

Common Exam Mistakes

Mistake 1: Confusing Filtration and Evaporation/Crystallisation

• Filtration separates an insoluble solid from a liquid.
• Evaporation/crystallisation obtains a dissolved (soluble) solid from a solution.
• You cannot use filtration to get salt from salt water (the salt is dissolved and passes through the filter paper).

Mistake 2: Saying "Evaporate All the Water" for Crystallisation

• For crystallisation, you evaporate some of the water and then cool slowly.
• Evaporating all the water gives small, imperfect crystals and may decompose the solid.

Mistake 3: Forgetting the Fractionating Column

• Simple distillation does not use a fractionating column.
• Fractional distillation requires a fractionating column. Without it, you cannot separate miscible liquids effectively.

Mistake 4: Using a Pen for the Chromatography Baseline

• The baseline must be drawn in pencil. Pen ink dissolves in the solvent and interferes with the results.

Mistake 5: Solvent Touching the Spots

• In chromatography, the solvent level must be below the baseline. If the spots are submerged, the substances dissolve directly into the solvent instead of separating.

Mistake 6: Confusing Pure and Potable

• Pure water contains only H\u2082O.
• Potable water is safe to drink but is NOT pure — it contains low levels of dissolved minerals.

---

Exam Technique: Describe-the-Method Questions

Many exam questions ask you to "describe how you would..." separate a particular mixture. Here is a framework for answering these questions:

Step 1: Identify the Technique

Read the question carefully and determine which separation technique is needed. Use the decision flowchart.

Step 2: State the Apparatus

Name each piece of apparatus and its role.

Step 3: Describe the Method Step by Step

Write the steps in chronological order. Use numbered steps or clear linking words (first, then, next, finally).

Step 4: Include Key Details

Include important details such as:

• Why you do a particular step (e.g. "add excess copper oxide to ensure all acid has reacted").
• How you know when a step is complete (e.g. "dip a glass rod into the solution and allow to cool; if crystals form, the solution is saturated").
• Safety precautions where relevant.

Example: Describe How to Obtain Pure Water from Seawater

Answer:

1. Set up a simple distillation apparatus: place the seawater in a distillation flask connected to a condenser leading to a collection vessel.
2. Heat the seawater using a Bunsen burner or heating mantle.
3. When the water boils, water vapour rises from the flask and enters the condenser.
4. Cold water flowing through the jacket of the condenser cools the vapour, which condenses back into liquid water.
5. The pure water (distillate) is collected in the collection vessel.
6. Check the thermometer reads 100 \u00b0C to confirm you are collecting water.
7. The dissolved salts remain behind in the distillation flask.

Example: Describe How to Separate Sand from Salt Water

This requires two techniques in combination:

1. Filtration first: Pour the mixture through filter paper in a funnel. The sand (insoluble) is trapped on the filter paper as the residue. The salt water passes through as the filtrate.
2. Evaporation/crystallisation second: Pour the filtrate (salt water) into an evaporating basin. Heat gently to evaporate some water. Allow to cool slowly for crystallisation. Filter the salt crystals, wash with cold distilled water, and dry.