Compounds & Mixtures

Elements, Compounds, and Mixtures

All matter can be classified as an element, compound, or mixture. Understanding the differences is fundamental to chemistry.

Elements

An element is a pure substance made of only one type of atom. Elements cannot be broken down into simpler substances by chemical reactions.

• There are 118 known elements, all listed in the periodic table
• Each element has a unique symbol (e.g., Fe for iron, O for oxygen, Au for gold)
• Examples: iron (Fe), oxygen (O2), sulfur (S8), copper (Cu), hydrogen (H2)

Compounds

A compound is a pure substance formed when two or more elements are chemically bonded together in a fixed ratio.

Key properties of compounds:

• Fixed composition — always the same ratio of atoms (water is always H2O, never H3O or H2O2 under normal conditions)
• New properties — the compound has different properties from the elements it contains (e.g., hydrogen and oxygen are both flammable gases, but water is a liquid that extinguishes flames)
• Separated only by chemical reactions — you cannot separate water into hydrogen and oxygen by filtering or distilling; you need electrolysis (a chemical method)
• Energy change when formed — energy is usually released or absorbed when elements bond

Examples:

• Water (H2O) — hydrogen and oxygen
• Salt (NaCl) — sodium and chlorine
• Carbon dioxide (CO2) — carbon and oxygen
• Ammonia (NH3) — nitrogen and hydrogen
• Glucose (C6H12O6) — carbon, hydrogen, and oxygen

Mixtures

A mixture contains two or more substances that are physically combined but not chemically bonded.

Key properties of mixtures:

• No fixed ratio — proportions can vary (e.g., sea water can contain varying amounts of salt)
• Retain original properties — each substance keeps its own properties (salt is still salty, water is still wet)
• No energy change when mixed (or very little)
• Separated by physical methods — filtration, distillation, chromatography, etc.

Examples:

• Air (mainly nitrogen, oxygen, and other gases)
• Sea water (water, salts, dissolved gases)
• Sand and salt
• Brass (copper and zinc — an alloy)
• Blood (plasma, cells, proteins)

Comparing Elements, Compounds, and Mixtures

Feature	Element	Compound	Mixture
Made of	One type of atom	Two or more elements, bonded	Two or more substances, not bonded
Fixed composition	Yes	Yes	No
Separation	Cannot be separated chemically	Chemical methods only	Physical methods
New properties formed	N/A	Yes	No
Example	Iron (Fe)	Iron sulfide (FeS)	Iron + sulfur mixed

Separation Techniques

Mixtures can be separated using physical methods. The choice of technique depends on the properties of the substances.

Filtration

Separates: insoluble solid from a liquid (or gas)

How it works: The mixture is poured through filter paper in a funnel. The liquid (filtrate) passes through; the solid (residue) is trapped.

Example: Separating sand from water; separating a precipitate from solution.

Distillation

Separates: dissolved solid from a liquid (simple distillation), or two liquids with different boiling points (fractional distillation)

How it works: The mixture is heated. The liquid with the lower boiling point evaporates first, travels through a condenser (cooled tube), and is collected as a pure liquid (distillate). The solid or higher-boiling liquid remains.

Example: Getting pure water from salt water; separating ethanol from water (boiling points: ethanol 78°C, water 100°C).

Fractional distillation — used to separate crude oil into fractions (petrol, diesel, kerosene, etc.) based on boiling point differences.

Evaporation

Separates: dissolved solid from a liquid when only the solid is needed

How it works: The solution is heated; the liquid evaporates, leaving the solid behind.

Example: Getting salt from salt water (though this gives impure salt).

Crystallisation

Separates: dissolved solid from a liquid as pure crystals

How it works: The solution is heated gently until some liquid evaporates, then allowed to cool slowly. The solid crystallises out in a pure form.

Example: Purifying copper sulfate crystals from solution.

Chromatography

Separates: mixtures of soluble substances (e.g., dyes, inks, amino acids)

How it works: The mixture is spotted onto chromatography paper, which is dipped in a solvent. As the solvent rises, different substances travel different distances (depending on their solubility and how strongly they stick to the paper).

Rf value = distance moved by substance / distance moved by solvent front

This allows substances to be identified by comparing Rf values to known standards.

Example: Separating the dyes in black ink; identifying amino acids in a protein digest.

Magnetic Separation

Separates: magnetic materials from non-magnetic materials

How it works: A magnet is used to attract magnetic substances (e.g., iron) while non-magnetic substances remain.

Example: Separating iron filings from sand or sulfur powder.

Pure Substances

In everyday life, "pure" means clean or unadulterated. In chemistry, a pure substance is one that contains only one type of particle — either elements or compounds, with no other substances mixed in.

A pure substance has:

• A sharp, specific melting point
• A sharp, specific boiling point

An impure substance (a mixture) has:

• A lower melting point than the pure substance
• Melts and boils over a range of temperatures

This is why we can test purity — salt melts at exactly 801°C; impure salt melts at a lower temperature over a range.

Key Terms

Term	Definition
Element	Pure substance made of only one type of atom
Compound	Pure substance made of two or more elements chemically bonded in a fixed ratio
Mixture	Two or more substances physically combined, not chemically bonded
Filtration	Separating insoluble solid from liquid using filter paper
Distillation	Separating liquid(s) by evaporation and condensation
Chromatography	Separating soluble substances based on how far they travel in a solvent
Rf value	Ratio of distance moved by substance to distance moved by solvent front
Pure substance	Contains only one type of particle; has a fixed melting and boiling point

Worked Example

Q: A student has a mixture of sand (insoluble), salt (soluble), and iron filings. Describe how they could separate all three substances.

Step 1: Use a magnet to remove the iron filings from the mixture. Iron is magnetic; sand and salt are not.

Step 2: Add water to dissolve the salt. The sand does not dissolve. Use filtration — the sand is trapped in the filter paper (residue); the salt solution (filtrate) passes through.

Step 3: Heat the salt solution to evaporate the water. Use evaporation or crystallisation to recover the pure salt.

Result: Three pure substances — iron filings, sand, and salt — are separated from the mixture.

Common Mistakes

• Saying a compound is the same as a mixture — in a compound, elements are chemically bonded in a fixed ratio; in a mixture, substances are only physically combined.
• Confusing filtration and distillation — filtration separates insoluble solids from liquids; distillation separates dissolved solids or liquids with different boiling points.
• Saying all mixtures are impure — technically true in chemistry, but remember that useful substances like air and alloys are mixtures and are not "impure" in a harmful sense.
• Thinking you can separate a compound by physical methods — you need a chemical reaction (e.g., electrolysis to split water into hydrogen and oxygen).

Tips and Tricks

• Soluble + insoluble in water: add water, filter (insoluble stays), then evaporate (soluble stays as solid).
• The key word for compound vs mixture: bond — a compound has chemical bonds between atoms; a mixture does not.
• Chromatography Rf value is always between 0 and 1 — a value of 1 means the substance moved as far as the solvent.
• Melting point test for purity: a pure substance melts at a sharp, specific temperature. An impure substance melts over a range and at a lower temperature.