C2: Elements, Compounds and Mixtures

What you'll learn

This topic covers the fundamental distinctions between elements, compounds and mixtures—the building blocks of all chemical understanding. You'll learn how to classify different types of matter, understand what happens during chemical reactions versus physical changes, and master the separation techniques used in laboratories and industry worldwide.

Key terms and definitions

Element — a pure substance made of only one type of atom that cannot be broken down by chemical means

Compound — a substance formed when two or more elements are chemically bonded together in fixed proportions

Mixture — two or more elements or compounds that are not chemically bonded together and can be separated by physical methods

Atom — the smallest particle of an element that can exist

Molecule — two or more atoms chemically bonded together

Chemical formula — a representation showing the elements in a compound and the ratio of atoms present (e.g. H₂O, CO₂)

Filtration — a separation technique that separates an insoluble solid from a liquid using filter paper

Distillation — a separation technique used to separate a liquid from a solution by evaporation followed by condensation

Core concepts

Elements

Elements are the simplest form of matter. Each element consists of only one type of atom with a unique atomic number. There are approximately 100 naturally occurring elements, all organised in the periodic table.

Key characteristics:

• Cannot be broken down into simpler substances by chemical reactions
• Each element has a unique symbol (e.g. C for carbon, Na for sodium, Cl for chlorine)
• Elements can exist as individual atoms (e.g. noble gases like helium, He) or as molecules (e.g. oxygen, O₂)
• Some elements are diatomic, meaning they naturally exist as molecules of two atoms: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂

Examples you should know:

• Metals: iron (Fe), copper (Cu), magnesium (Mg), calcium (Ca)
• Non-metals: oxygen (O₂), nitrogen (N₂), sulfur (S), carbon (C)

Compounds

Compounds form when atoms of different elements chemically bond together in fixed proportions. The properties of a compound are completely different from the properties of the elements it contains.

Key characteristics:

• Atoms are held together by chemical bonds (ionic or covalent)
• Can only be separated into elements by chemical reactions, not physical methods
• Always contain the same elements in the same proportions by mass
• Represented by chemical formulae showing the ratio of atoms

Common examples:

• Water (H₂O): 2 hydrogen atoms bonded to 1 oxygen atom
• Carbon dioxide (CO₂): 1 carbon atom bonded to 2 oxygen atoms
• Sodium chloride (NaCl): 1 sodium atom bonded to 1 chlorine atom
• Methane (CH₄): 1 carbon atom bonded to 4 hydrogen atoms

Important distinction: Sodium metal (Na) is a soft, highly reactive metal that explodes in water. Chlorine gas (Cl₂) is a toxic yellow-green gas. Yet sodium chloride (NaCl) is common table salt—safe to eat. This demonstrates how compounds have completely different properties from their constituent elements.

Mixtures

Mixtures contain two or more substances (elements or compounds) that are not chemically bonded together. The components retain their individual properties.

Key characteristics:

• No chemical bonds between the different substances
• Components can be separated by physical methods
• No fixed composition—the proportions can vary
• Properties are a combination of the components' properties

Types of mixtures:

• Homogeneous mixtures: uniform composition throughout (e.g. salt dissolved in water, air)
• Heterogeneous mixtures: composition is not uniform (e.g. sand and water, oil and water)

Examples in everyday life:

• Air (mainly nitrogen, oxygen, argon and carbon dioxide)
• Seawater (water with dissolved salts)
• Crude oil (mixture of different hydrocarbons)
• Blood (cells suspended in plasma)
• Concrete (cement, sand, gravel and water)

Distinguishing between elements, compounds and mixtures

Use this decision tree approach in exams:

1. Does it contain only one type of atom? → Element
2. Does it contain different types of atoms chemically bonded? → Compound
3. Does it contain substances not chemically bonded? → Mixture

Visual representations:

• Element diagrams show identical atoms (same colour/letter)
• Compound diagrams show different atoms bonded together in fixed ratios
• Mixture diagrams show separate groups of atoms/molecules not bonded to each other

Separation techniques for mixtures

You must know when and how to use each separation method for the OCR specification.

Filtration

Used to separate an insoluble solid from a liquid.

Method:

1. Place filter paper in a funnel
2. Pour the mixture through the filter paper
3. The liquid (filtrate) passes through; the solid (residue) remains

Examples: separating sand from water, removing tea leaves from tea

Crystallisation

Used to obtain a soluble solid from a solution.

Method:

1. Heat the solution to evaporate some water
2. Stop heating when crystals start to form
3. Leave to cool—more crystals will form
4. Filter to collect the crystals
5. Dry the crystals

Examples: obtaining salt from seawater, purifying copper sulfate

Note: Simple evaporation to dryness can also be used but may decompose some solids (crystallisation is gentler).

Simple distillation

Used to separate a liquid from a solution, or to separate liquids with very different boiling points.

Method:

1. Heat the solution in a flask
2. The liquid with the lower boiling point evaporates first
3. Vapour passes into a condenser
4. Cold water in the condenser's outer jacket cools the vapour
5. The vapour condenses back to liquid
6. Pure liquid (distillate) is collected

Examples: obtaining pure water from seawater, separating ethanol from water

Key point: The thermometer bulb must be positioned at the top of the flask where vapour exits to accurately measure the boiling point of what's distilling over.

Fractional distillation

Used to separate mixtures of liquids with similar boiling points.

Method:

1. Similar to simple distillation but includes a fractionating column
2. The column contains glass beads or has a large surface area
3. Vapours repeatedly condense and evaporate as they rise
4. Liquids with lower boiling points reach the top and distil over first
5. Temperature increases gradually, allowing separation of different fractions

Examples: separating crude oil into fractions, separating liquid air into oxygen and nitrogen

Industrial application: Fractional distillation of crude oil occurs in tall fractionating towers where different hydrocarbons are separated based on chain length and boiling point.

Chromatography

Used to separate mixtures of dissolved substances (often used to identify components).

Paper chromatography method:

1. Draw a pencil line (baseline) near the bottom of chromatography paper
2. Place spots of the mixture on the baseline
3. Place the paper in a beaker with a shallow layer of solvent (mobile phase)
4. The solvent must be below the baseline
5. Solvent rises up the paper, carrying dissolved substances
6. Different substances travel different distances
7. Remove before solvent reaches the top; mark the solvent front

Key terms:

• Stationary phase: the paper (doesn't move)
• Mobile phase: the solvent (moves up the paper)
• Rf value = distance moved by substance ÷ distance moved by solvent

Rf values are constant for a given substance in a particular solvent system, allowing identification of unknown substances.

Examples: separating food colourings, identifying compounds in forensic analysis, testing the purity of pharmaceuticals

Worked examples

Example 1: Identifying substances

Question: The diagram shows particles in three different substances, A, B and C.

A: ●●●●●● (all identical atoms) B: ●○●○●○ (pairs of different atoms bonded together, all identical pairs) C: ●●○○●●● (mixture of different types of atoms/molecules)

Identify whether each substance is an element, compound or mixture. [3 marks]

Answer:

• A is an element [1 mark] — contains only one type of atom
• B is a compound [1 mark] — contains two different types of atoms chemically bonded together in a fixed ratio
• C is a mixture [1 mark] — contains different substances not chemically bonded together

Example 2: Separation techniques

Question: A student has a mixture of sand, salt and water.

(a) Describe how the student could obtain pure sand from the mixture. [2 marks]

(b) Describe how the student could then obtain pure salt from the remaining solution. [3 marks]

Answer:

(a) Use filtration [1 mark]. Pour the mixture through filter paper in a funnel; the sand will remain on the paper as residue [1 mark].

(b) Heat the salt solution in an evaporating basin [1 mark] until crystals start to form, then stop heating [1 mark]. Allow to cool, then filter and dry the salt crystals [1 mark].

(Alternative for 3rd mark: Continue heating until all water evaporates and only salt remains.)

Example 3: Chromatography calculations

Question: A student performed paper chromatography on food colouring X. The substance travelled 3.6 cm from the baseline. The solvent front travelled 4.5 cm from the baseline.

Calculate the Rf value for substance X. [2 marks]

Answer:

Rf = distance moved by substance ÷ distance moved by solvent [1 mark]

Rf = 3.6 ÷ 4.5 = 0.8 [1 mark]

(Note: Rf values have no units and are always less than 1)

Common mistakes and how to avoid them

• Confusing mixtures and compounds: Remember that in compounds, atoms are chemically bonded and cannot be separated by physical methods. In mixtures, substances are not bonded and retain their properties.

• Incorrect use of separation techniques: Match the technique to the mixture type. Don't try to filter a solution—both components are dissolved. Don't use simple distillation for liquids with similar boiling points—use fractional distillation instead.

• Poor chromatography technique: Always use pencil for the baseline (pen ink would dissolve and interfere). Ensure the solvent level starts below the baseline. Spots should be small and concentrated for best separation.

• Misunderstanding element representation: Remember that many elements exist as diatomic molecules (H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂), not single atoms. When writing about oxygen gas, use O₂, not O.

• Confusing evaporation and distillation: Evaporation obtains the solid from a solution; distillation obtains the liquid (solvent) from a solution. Know which product you're trying to collect.

• Forgetting Rf value rules: Rf values are always between 0 and 1, have no units, and must be calculated to 2 decimal places or as given in the question. Show your working: formula first, then substitution.

Exam technique for "C2: Elements, Compounds and Mixtures"

• Command words matter: "Describe" requires a method with linked steps. "Explain" needs reasons why something happens. "State" needs a simple fact without explanation. When describing separation techniques, always give the apparatus and what happens at each stage.

• Drawing particle diagrams: Use consistent symbols or shading for different atoms. Show compounds with atoms touching or bonded. Show mixtures with clear spaces between different substances. For full marks, the ratio of atoms in compounds must be correct.

• Chromatography questions: Always show the formula before substituting values for Rf calculations. Measure from the baseline, not from the bottom of the paper. Remember that substances with higher Rf values are more soluble in the mobile phase.

• Mark allocation guides detail: 1 mark usually = 1 distinct point. A 3-mark question on separation needs three clear steps or pieces of apparatus mentioned. Don't repeat the same point in different words—add new information instead.

Quick revision summary

Elements contain only one type of atom; compounds contain different atoms chemically bonded in fixed ratios; mixtures contain substances not chemically bonded. Separation techniques depend on physical properties: filtration for insoluble solids, crystallisation for dissolved solids, simple distillation for liquids from solutions, fractional distillation for liquid mixtures, and chromatography for identifying dissolved substances. Rf values identify substances in chromatography. Chemical bonds differentiate compounds from mixtures—only chemical reactions can separate compounds into elements.