# Elements, compounds and mixtures

> Edexcel International GCSE Chemistry · 4CH1 2017
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-elements-compounds-and-mixtures/

This guide covers core classification of elements, compounds and mixtures, purity rules, key separation techniques, paper chromatography, and Rf value calculations as required for Edexcel IGCSE Chemistry 4CH1 sub-topic 1(b).

**Prerequisites:** [Basic atomic structure](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-atomic-structure/); [Safe practical working in chemistry lab](https://www.owlsprep.com/study/edexcel-igcse-chemistry-practical-skills/)

## Learning objectives

- Classify substances as element, compound or mixture
- Recall the difference in melting/boiling points between pure substances and mixtures
- Select and describe appropriate separation techniques for different mixtures
- Interpret paper chromatograms and calculate Rf values to identify mixture components
- Carry out paper chromatography practical investigations safely

## Classifying Elements, Compounds and Mixtures

**Core Substance Classifications** — 1. **Element**: Pure substance made of only one type of atom, cannot be broken down by chemical means. 2. **Compound**: Pure substance made of two or more elements chemically bonded in fixed proportions. 3. **Mixture**: Combination of two or more substances not chemically bonded, no fixed proportions, can be separated physically.

*Example:* Element = iron (Fe), Compound = sodium chloride (NaCl), Mixture = salt water

**Worked example:** Classify each of the following as element, compound or mixture: a) Magnesium ribbon, b) Copper sulfate, c) Air

1. Step 1: Recall the definition of each category
2. Step 2a: Magnesium is made of only magnesium atoms, so it is an element
3. Step 2b: Copper sulfate is made of copper, sulfur and oxygen atoms chemically bonded in fixed proportions, so it is a compound
4. Step 2c: Air is a mix of nitrogen, oxygen, carbon dioxide and other gases with no fixed proportions, not chemically bonded, so it is a mixture

## Purity and Melting/Boiling Point Characteristics

**Pure Substance** — Single element or compound with no other substances mixed in, has a fixed, sharp melting and boiling point. Mixtures (impure substances) melt or boil over a range of temperatures.

**Worked example:** A sample of ethanol melts at -117°C to -114°C, while pure ethanol melts at exactly -114°C. Is the sample pure? Explain your answer.

1. Step 1: Recall pure substances have a fixed melting point
2. Step 2: The sample melts over a range of 3°C, not at a single fixed temperature
3. Step 3: Conclusion: The sample is impure (a mixture of ethanol and other substances)

> **tip**
>
> If a question gives a melting point range, the substance is always a mixture/impure. If it gives a single exact temperature matching the known value for the substance, it is pure.

## Separation Techniques for Mixtures

You must be able to select the correct separation technique for any given mixture, and describe how each works for the exam. The table below summarises the 5 core techniques you need to recall:

| Technique | Use case | How it works |
| --- | --- | --- |
| Filtration | Separate insoluble solid from liquid | Mixture poured through filter paper; solid residue stays on paper, liquid filtrate passes through |
| Crystallisation | Separate soluble solid from solution | Solution heated to evaporate some solvent, left to cool; pure solid crystals form, can be filtered and dried |
| Simple distillation | Separate solvent from dissolved solute in solution | Solution heated, solvent boils, vapour condenses in cool condenser, collected as pure liquid distillate |
| Fractional distillation | Separate miscible liquids with different boiling points | Mixture heated, vapours rise up fractionating column, liquid with lowest boiling point condenses first and is collected |
| Paper chromatography | Separate mixtures of soluble coloured substances (e.g. inks, food colourings) | Mixture spotted onto chromatography paper, solvent moves up paper, components separate as they move different distances |

**Worked example:** State the correct separation technique for each mixture: a) Sand from water, b) Pure water from salt water, c) Red and blue food colourings in a mixture, d) Ethanol (boiling point 78°C) from water (boiling point 100°C)

1. Step 1: Match each mixture to the correct technique use case
2. Step 2a: Sand is insoluble in water → Filtration
3. Step 2b: Separate solvent (water) from dissolved solute (salt) → Simple distillation
4. Step 2c: Soluble coloured substances → Paper chromatography
5. Step 2d: Miscible liquids with different boiling points → Fractional distillation

> **warning**
>
> Do not confuse simple and fractional distillation. If the mixture is two or more liquids, use fractional; if it is a solid dissolved in a liquid and you want the liquid, use simple. If you just want the solid, use crystallisation.

## Paper Chromatography and Rf Calculations

Paper chromatography separates components of a mixture based on how soluble they are in the solvent. More soluble components move further up the paper. A chromatogram can show if a substance is pure (only one spot) or a mixture (multiple spots). You can identify components by comparing their Rf values to known reference values.

**Rf Value** — Ratio of the distance moved by a solute spot (measured to the centre of the spot) to the distance moved by the solvent front (furthest point the solvent reaches on the paper). Rf has no units and is always between 0 and 1.

*Notation:* $R_f$

$$R_f = \frac{\text{distance moved by spot}}{\text{distance moved by solvent front}}$$

**Worked example:** A chromatogram of a food colouring mixture has a yellow spot that moves 4.2 cm. The solvent front moves 12.0 cm from the origin line. Calculate the Rf value of the yellow dye, giving your answer to 2 significant figures.

1. Step 1: Write down the Rf formula
2. $$R_f = \frac{\text{distance moved by spot}}{\text{distance moved by solvent front}}$$
3. Step 2: Substitute the values given
4. $$R_f = \frac{4.2}{12.0} = 0.35$$
5. Step 3: Check value is between 0 and 1 with no units. Final answer = 0.35

> **tip**
>
> If a spot stays on the origin line (distance moved = 0), its Rf value is 0, meaning it is insoluble in the solvent used.

*Calculator:* allowed

## Paper Chromatography Practical Investigation

1. Draw a pencil origin line 1 cm from the bottom of the chromatography paper (pencil does not dissolve in solvent, so it won't move)
2. Spot the mixture and known reference substances onto the origin line, leave to dry
3. Pour a small volume of solvent into a beaker, make sure solvent level is below the origin line (so spots don't dissolve into the solvent in the beaker)
4. Place the chromatography paper into the beaker, cover with a lid to saturate the atmosphere with solvent vapour and prevent solvent evaporating too quickly
5. Remove the paper when the solvent front is near the top of the paper, mark the solvent front with a pencil immediately before it dries
6. Leave the paper to dry, then analyse the spots to identify components by matching Rf values or position to reference substances

**Check your understanding**

1. Why do you use a pencil to draw the origin line?

   - Pencil marks are darker than pen
   - Pencil graphite does not dissolve in solvent
   - Pencil is easier to erase if you make a mistake

   *Answer:* Pencil graphite does not dissolve in solvent

   *Why:* Pen ink would dissolve in the solvent and move up the paper, ruining the chromatogram. Pencil graphite is insoluble so it stays in place.

2. Why must the solvent level be below the origin line?

   - To stop the spots dissolving into the solvent in the beaker
   - To make the solvent move faster up the paper
   - To prevent the solvent from evaporating

   *Answer:* To stop the spots dissolving into the solvent in the beaker

   *Why:* If the solvent covers the origin line, the spotted mixture will wash off the paper into the beaker, and no separation will occur.

## Common pitfalls

- **Wrong:** Classifying a substance with a fixed melting point as a mixture
  - Why it fails: Pure elements and compounds have fixed melting/boiling points, only mixtures have ranges
  - Correct: Only classify substances with a melting/boiling range as impure/mixtures
- **Wrong:** Using simple distillation to separate ethanol and water
  - Why it fails: Simple distillation is for separating solvent from dissolved solute, not two miscible liquids
  - Correct: Use fractional distillation for mixtures of miscible liquids with different boiling points
- **Wrong:** Calculating Rf as distance of solvent front divided by distance of spot
  - Why it fails: Reverse calculation gives values >1 which are invalid, Rf formula requires spot distance first
  - Correct: Always divide the distance the spot moved by the distance the solvent front moved, measure to the centre of the spot
- **Wrong:** Using pen to draw the origin line on chromatography paper
  - Why it fails: Pen ink dissolves in the solvent and moves up the paper, contaminating the chromatogram
  - Correct: Always use a pencil to draw the origin line and mark the solvent front
- **Wrong:** Stating that Rf values have units of cm or m
  - Why it fails: Rf is a ratio of two distances, so units cancel out
  - Correct: Always give Rf values as a decimal between 0 and 1 with no units

## Cheatsheet

| Key Concept | Key Fact / Formula |
| --- | --- |
| Element | Only one type of atom, pure, fixed melting point |
| Compound | Two+ elements chemically bonded, pure, fixed melting point |
| Mixture | Two+ substances not bonded, impure, melting/boiling range |
| Filtration | Insoluble solid + liquid separation |
| Crystallisation | Soluble solid + solution separation |
| Simple distillation | Solvent + dissolved solute separation |
| Fractional distillation | Miscible liquids with different boiling points separation |
| Paper chromatography | Soluble coloured substances separation |
| Rf Formula | $R_f = \frac{\text{spot distance}}{\text{solvent front distance}}$, no units, 0 ≤ Rf ≤ 1 |

## What's next

Now that you have mastered elements, compounds and mixtures, you can move on to more core principles of chemistry content. Next, you will learn about atomic structure, which explains how elements form compounds via chemical bonding, and chemical formulae and equations, which use the fixed proportions of compounds to describe reactions. You will also apply separation techniques later when you study crude oil fractional distillation in the organic chemistry unit, and water purification in the inorganic chemistry section.

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