Elements, compounds and mixtures

Q: How do I tell if a substance is pure from its melting point?

A pure substance melts at a single fixed temperature, while a mixture melts over a range of temperatures. For example, pure water melts at exactly 0°C, but salt water melts between -5°C and 0°C depending on salt concentration.

Q: Do I have to remember the Rf formula for the exam?

Yes, you must recall the Rf formula: . Rf values have no units, and are always between 0 and 1.

Q: What's the difference between simple and fractional distillation?

Simple distillation separates a solvent from a dissolved solute (e.g. water from salt water). Fractional distillation separates miscible liquids with different boiling points (e.g. ethanol and water) using a fractionating column.

Edexcel International GCSE Chemistry· 1.8–1.13 (sub-topic 1(b))· 15 min read

1. Classifying Elements, Compounds and Mixtures★☆☆☆☆⏱ 3 min

📘 Definition

Core Substance Classifications

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

2. Purity and Melting/Boiling Point Characteristics★★☆☆☆⏱ 2 min

📘 Definition

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)

3. Separation Techniques for Mixtures★★★☆☆⏱ 4 min

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

4. Paper Chromatography and Rf Calculations★★★☆☆⏱ 4 min

✓ Calculator OK

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.

📘 Definition

Rf Value

$R_{f}$

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.

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

5. Paper Chromatography Practical Investigation★★☆☆☆⏱ 2 min

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)
Spot the mixture and known reference substances onto the origin line, leave to dry
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)
Place the chromatography paper into the beaker, cover with a lid to saturate the atmosphere with solvent vapour and prevent solvent evaporating too quickly
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
Leave the paper to dry, then analyse the spots to identify components by matching Rf values or position to reference substances

✓ Quick check

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
Reveal answer
1 —
Pen ink would dissolve in the solvent and move up the paper, ruining the chromatogram. Pencil graphite is insoluble so it stays in place.
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

6. Common Pitfalls

Wrong move:

Classifying a substance with a fixed melting point as a mixture

Why:

Pure elements and compounds have fixed melting/boiling points, only mixtures have ranges

Correct move:

Only classify substances with a melting/boiling range as impure/mixtures

Wrong move:

Using simple distillation to separate ethanol and water

Why:

Simple distillation is for separating solvent from dissolved solute, not two miscible liquids

Correct move:

Use fractional distillation for mixtures of miscible liquids with different boiling points

Wrong move:

Calculating Rf as distance of solvent front divided by distance of spot

Why:

Reverse calculation gives values >1 which are invalid, Rf formula requires spot distance first

Correct move:

Always divide the distance the spot moved by the distance the solvent front moved, measure to the centre of the spot

Wrong move:

Using pen to draw the origin line on chromatography paper

Why:

Pen ink dissolves in the solvent and moves up the paper, contaminating the chromatogram

Correct move:

Always use a pencil to draw the origin line and mark the solvent front

Wrong move:

Stating that Rf values have units of cm or m

Why:

Rf is a ratio of two distances, so units cancel out

Correct move:

Always give Rf values as a decimal between 0 and 1 with no units

7. Quick Reference 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	$R_{f} = \frac{spot distance}{solvent front distance}$ , no units, 0 ≤ Rf ≤ 1

8. Frequently Asked

How do I tell if a substance is pure from its melting point?

A pure substance melts at a single fixed temperature, while a mixture melts over a range of temperatures. For example, pure water melts at exactly 0°C, but salt water melts between -5°C and 0°C depending on salt concentration.

Do I have to remember the Rf formula for the exam?

Yes, you must recall the Rf formula: $R_{f} = \frac{distance moved by spot}{distance moved by solvent front}$ . Rf values have no units, and are always between 0 and 1.

What's the difference between simple and fractional distillation?

Simple distillation separates a solvent from a dissolved solute (e.g. water from salt water). Fractional distillation separates miscible liquids with different boiling points (e.g. ethanol and water) using a fractionating column.

Going deeper

syllabusEdexcel IGCSE Chemistry 4CH1 2017 SpecOfficial specification for this sub-topic

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.