Elements, Compounds and Mixtures

Element. A substance made of only ONE type of atom. There are about 118 known elements, all listed on the periodic table. Examples: iron (Fe), oxygen (O₂), sulfur (S₈), helium (He), gold (Au).

Compound. A substance made of two or more elements chemically bonded together in a fixed ratio. The fixed ratio is set by the chemical formula:

• Water is H₂O — always 2 hydrogen atoms to 1 oxygen atom.
• Carbon dioxide is CO₂ — always 1 carbon to 2 oxygen.
• Sodium chloride is NaCl — always 1 sodium ion to 1 chloride ion.

The chemical bonds in a compound must be broken by a chemical reaction to recover the original elements (e.g. electrolysis of water gives H₂ and O₂; thermal decomposition of CaCO₃ gives CaO and CO₂).

Crucially, a compound has DIFFERENT properties from the elements it contains:

• Sodium is a soft, silvery, violently reactive metal.
• Chlorine is a yellow-green poisonous gas.
• Sodium chloride (the compound) is a safe, white, edible solid — totally different from either parent element.

Mixture. Two or more substances that are NOT chemically bonded, present in any proportion. Each substance keeps its own physical and chemical properties. Examples:

• Air = mixture of N₂ (78%), O₂ (21%), Ar (0.9%), CO₂ (0.04%) plus traces of others.
• Sea water = water + dissolved salts (NaCl, MgCl₂, etc.).
• Brass = mixture (alloy) of copper and zinc atoms.
• Iron filings + sulfur powder = mixture (the iron is still magnetic, the sulfur is still yellow) UNTIL you heat them strongly — then they react chemically to form iron(II) sulfide (FeS), which is a compound with completely different properties.

Particle-diagram cues (popular MCQ format):

• Element = single coloured atoms, all the same.
• Compound = atoms of different elements bonded in a fixed pattern (e.g. always paired 2:1).
• Mixture = different atoms / molecules side by side in no fixed ratio.

Pure substances have characteristic, fixed melting and boiling points:

• Pure water melts sharply at 0 °C and boils sharply at 100 °C at 1 atm.
• Pure sodium chloride melts at 801 °C.
• Pure ethanol boils at 78 °C.

If a substance is impure (i.e. a mixture), the melting point is:

1. LOWER than the pure value, and
2. SPREAD over a range rather than a single sharp temperature.

For example, sea water freezes at about -2 °C (not 0 °C) and over a small temperature range as the increasingly concentrated salt water keeps freezing slightly lower. This is why salt is used to clear icy roads in winter — salty water freezes only well below 0 °C.

For boiling, the rule reverses sign:

1. Impurities RAISE the boiling point.
2. The boiling temperature rises during boiling (as water boils off, the remaining solution becomes more concentrated and so boils at an even higher temperature). There is no single fixed bp.

Why purity matters in real life. In pharmaceuticals, a drug must be highly pure — a sharp melting point in a melting-point tube is one of the first tests of purity in a chemistry lab. In food science, the melting / boiling profile tells you about composition (chocolate, butter, alloys).

Filtration. Separates an INSOLUBLE solid from a liquid or a solution.

• Apparatus: filter paper folded into a filter funnel above a clean conical flask.
• The mixture is poured through. The insoluble solid is trapped on the filter paper (the residue); the liquid passes through (the filtrate).
• Examples: sand + water (sand is the residue; pure water is the filtrate); muddy river water (mud is the residue, water + dissolved salts is the filtrate).

Crystallisation. Recovers a soluble SOLID from a solution. The solvent is evaporated until the solution is saturated; further cooling forces solid crystals to grow out of solution.

Procedure for getting pure salt from sea water:

1. Pour the salt solution into an evaporating basin.
2. Heat gently over a Bunsen flame (or, for better results, over a water bath to prevent spitting).
3. Stop heating when the solution is just saturated — test with a glass rod: if drips form crystals on a cool slide, it's ready.
4. Leave to cool. Crystals form as the solute comes out of solution.
5. Filter or pour off the remaining liquid; pat the crystals dry between two pieces of filter paper, or dry in a warm oven.

Why not boil dry? If you heat to dryness on a Bunsen flame, the crystals can scorch / decompose. Stop at the point of crystallisation and let the rest happen by cooling.

Combined application. To separate salt from sand, you exploit the fact that one dissolves and one does not:

1. Add water → salt dissolves, sand does not.
2. Filter → sand is the residue, salt solution is the filtrate.
3. Crystallise the filtrate → recover pure dry salt.

Simple distillation. Recovers the pure LIQUID solvent from a solution where the dissolved solute has a much higher boiling point.

• Apparatus: round-bottomed flask containing the solution + anti-bumping granules; Bunsen below; delivery tube leading to a horizontal Liebig condenser (cold water IN at the bottom, OUT at the top — countercurrent flow); collection beaker.
• Mechanism: the solvent boils, vapour passes into the condenser, condenses back to liquid and is collected. The non-volatile solute stays in the flask.
• Example: distilling pure water from salt water — the water boils at 100 °C and is collected; the salt stays behind in the flask.

Fractional distillation. Separates a mixture of MISCIBLE LIQUIDS with different boiling points (liquids that mix completely).

• Apparatus: as for simple distillation BUT with a fractionating column (packed with glass beads or plates) between the flask and the condenser, and a thermometer at the top of the column.
• Mechanism: the column has a temperature gradient — hot at the bottom, cool at the top. Both liquids evaporate; the higher-bp liquid keeps condensing back onto the cool glass beads in the column and trickles back to the flask, while the lower-bp liquid passes up the column to the condenser and is collected.
• Example: ethanol (bp 78 °C) from water (bp 100 °C). When the thermometer reads 78 °C, ethanol comes over. Once the thermometer climbs above 78 °C, water is starting to come too — change the receiver.

Industrial use. Fractional distillation of crude oil in industry separates a complex mixture of hydrocarbons into fractions (petrol, kerosene, diesel, etc.) using a tall, hot column with collection trays at different heights and temperatures. (Covered in detail in 4CH1 Topic 4 — Organic Chemistry.)

Paper chromatography separates soluble substances (e.g. dyes in a felt-tip pen, amino acids in a protein hydrolysate, plant pigments). Each substance has its own balance between solubility in the solvent and attraction to the paper — substances that are more soluble in the solvent travel further.

Procedure.

1. Draw a baseline 1-2 cm from the bottom of the chromatography paper in pencil (graphite is insoluble in the solvent).
2. Place small concentrated spots of the test mixture (and any reference samples) on the baseline. Let each spot dry; build up the spot with multiple drops if needed.
3. Place the paper in a tank with a small depth of solvent — the solvent level MUST be below the baseline.
4. Cover the tank to prevent solvent evaporation and to allow the solvent to rise steadily up the paper.
5. When the solvent has nearly reached the top, take the paper out and mark the solvent front in pencil. Let the paper dry.
6. If the spots are colourless (e.g. amino acids), spray with a locating agent (e.g. ninhydrin) and warm gently — the spots appear as purple / pink patches.

Rf calculation.

$R_f = \dfrac{\text{distance moved by substance}}{\text{distance moved by solvent}}$

Measure both distances from the SAME baseline. Distance to substance = baseline → centre of spot. Distance to solvent = baseline → solvent front. Rf is always between 0 and 1 and has no units.

Interpretation. Each substance has a characteristic Rf at a given solvent and temperature, similar to a "fingerprint". To identify an unknown spot:

• Run reference samples alongside the unknown on the same paper.
• A spot that travels the same distance as a reference is the same substance.
• Alternatively, compare the Rf with database values (but only valid if the solvent and temperature match the database).