Is Matter Around Us Pure?

1. Pure Substances vs Mixtures

All matter around us is either a pure substance or a mixture. The key question is: is its composition fixed or variable?

Remember: A pure substance is not necessarily a single element. Water (H2O) is a pure substance (compound). What makes it pure is its fixed composition, not its simplicity.

2. Homogeneous vs Heterogeneous Mixtures

Mixtures are classified by whether their composition is uniform throughout.

Homogeneous mixtures are also called solutions. The three broad types of mixtures based on particle size are solutions, colloids, and suspensions, covered in the next three sections.

3. Solutions — Solute, Solvent, Concentration

A solution is a homogeneous mixture of two or more substances. The component present in smaller quantity is the solute; the component present in larger quantity (that dissolves the solute) is the solvent.

Examples:

• Salt water: solute = NaCl, solvent = water.
• Lemonade: solute = sugar and lemon juice, solvent = water.
• Alloys (e.g. brass = Cu + Zn): solid solutions — both components are solids.
• Air: solute = CO2, O2, noble gases, etc.; solvent = N2 (major component).
• Soda water: solute = CO2 (gas), solvent = water (liquid) — gas dissolved in liquid.

Properties of a solution:

• It is a homogeneous mixture.
• Particles of solute are smaller than 1 nm (1 nanometre = 10^-9 m) — invisible even under a microscope.
• Solute particles do not scatter a beam of light (no Tyndall effect).
• Solute particles do not settle on standing.
• The solute cannot be separated from the solution by filtration.

Concentration of a Solution

Concentration tells us how much solute is dissolved in a given amount of solution or solvent.

Saturated, Unsaturated and Supersaturated Solutions

• Unsaturated solution: more solute can still be dissolved at the same temperature.
• Saturated solution: no more solute can be dissolved at that temperature. Any extra solute remains undissolved at the bottom.
• Supersaturated solution: contains more dissolved solute than a saturated solution at the same temperature — possible only under special conditions (e.g. slow cooling). It is unstable; slight disturbance causes excess solute to crystallise out.

Effect of temperature: solubility of most solids in water increases with temperature. So a saturated solution at a lower temperature can become unsaturated if heated. For gases, solubility decreases with temperature and increases with pressure (Henry's Law).

4. Colloids — the Tyndall Effect

A colloid (or colloidal solution) is a mixture in which tiny particles (called the dispersed phase) are distributed through a medium (called the dispersion medium). Particle size: 1 nm to 100 nm — intermediate between a true solution and a suspension.

Properties of Colloids

• Particles are too small to be seen with the naked eye but large enough to scatter light.
• They do not settle on standing.
• They cannot be separated by ordinary filtration, but can be separated by centrifugation.
• They show the Tyndall effect.

Tyndall Effect

When a beam of light is passed through a colloid, the colloidal particles scatter the light, making the beam visible from the side. This scattering of light by colloid particles is called the Tyndall effect.

Common Examples of Colloids

Soap solution is a colloid (soap micelles form colloidal particles in water). This is why soap solution shows Tyndall effect but a salt solution does not.

5. Suspensions

A suspension is a heterogeneous mixture in which the solute particles are large enough to be seen with the naked eye (particle size > 100 nm) and do NOT dissolve in the solvent but remain dispersed in it temporarily.

Properties of Suspensions

• It is a heterogeneous mixture.
• Particles are visible to the naked eye or under a microscope.
• Particles settle down on standing (due to gravity).
• Particles can be separated by filtration through filter paper.
• Particles scatter a beam of light (visible beam path), similar to colloids.

Comparing Solution, Colloid and Suspension

6. Separation Methods

Mixtures are separated by physical methods that exploit differences in physical properties of the components. Choosing the right method depends on the nature of the mixture.

A. Evaporation

Principle: the solvent evaporates on heating and leaves behind the dissolved solid solute.
Used for: separating a dissolved solid from a liquid when the solid is not damaged by heat, e.g. common salt from seawater, copper sulphate from its solution.
Limitation: the solvent is lost; cannot recover it. Also gives impure product if other soluble salts are present.

B. Filtration

Principle: solid particles larger than the pores of the filter paper are trapped; the liquid (filtrate) passes through.
Used for: separating an insoluble solid from a liquid, e.g. chalk in water, sand from water, tea leaves from tea.
Note: cannot separate dissolved solids — for that, use evaporation or crystallisation.

C. Crystallisation

Principle: a hot saturated solution is cooled slowly; the solid crystallises out in a pure form while impurities remain in the solution (mother liquor).
Used for: purifying solids that are soluble in water, e.g. salt, sugar, alum, copper sulphate.
Advantage over evaporation: gives a purer product and can recover the solvent.

D. Simple Distillation

Principle: a liquid is heated to its boiling point, the vapour is collected and condensed back to a liquid in a different vessel.
Used for: separating a volatile liquid from a non-volatile dissolved solid, or separating two miscible liquids with a large difference in boiling points (> 25 degC).
Example: separating acetone (b.p. 56 degC) from water (b.p. 100 degC); getting distilled water from salt water.

E. Fractional Distillation

Principle: same as distillation but a fractionating column (long glass tube packed with glass beads or other material) is used. Liquids with close boiling points condense and re-evaporate many times inside the column, so they separate more effectively.
Used for: separating two miscible liquids with boiling points close together.
Major industrial uses:

• Separation of air: liquid air is fractionated to get N2 (b.p. -196 degC), Ar (b.p. -186 degC), and O2 (b.p. -183 degC).
• Refining of crude petroleum: different fractions (petrol, kerosene, diesel, fuel oil) collected at different temperatures in a fractionating column.

F. Chromatography

Principle: components of a mixture travel at different speeds along a stationary medium (chromatography paper) when carried by a moving solvent. Components with greater affinity for the paper travel more slowly; those with less affinity travel faster. The ratio of distance travelled by a component to the distance travelled by the solvent is called the Rf value and is characteristic for each substance.
Used for:

• Separating dyes in black ink or food colours.
• Separating pigments from natural colours (e.g. leaf extract).
• Detecting drugs in urine samples (forensics).
• Separating amino acids from a mixture.

G. Centrifugation

Principle: the mixture is spun at high speed in a centrifuge; denser particles experience greater centrifugal force and move to the bottom of the tube, while the lighter liquid stays on top.
Used for:

• Separating cream from milk (cream = fat, less dense than aqueous milk).
• Separating colloidal particles from their medium.
• Blood testing in pathology labs — separating red blood cells from plasma.
• Washing machines use centrifugation to spin water out of wet clothes.

H. Magnetic Separation

Principle: one component is magnetic (attracted to a magnet) while the other is not.
Used for:

• Separating iron filings from sulphur powder.
• Picking up iron ore from a mixture of sand in mining and metallurgy.
• Removing iron from grain or wheat in flour mills.

7. Elements — Metals, Non-metals and Metalloids

An element is a pure substance that cannot be broken down into simpler substances by any chemical method. Elements are the fundamental building blocks of all matter. There are 118 known elements.

Metals

• Shiny (lustrous), malleable (can be beaten into sheets), ductile (can be drawn into wires).
• Good conductors of heat and electricity.
• Mostly solid at room temperature (exception: mercury is liquid).
• High melting and boiling points (exception: gallium melts at ~30 degC; sodium and potassium have low melting points and are soft).
• Examples: iron (Fe), copper (Cu), gold (Au), silver (Ag), aluminium (Al), sodium (Na), calcium (Ca).

Non-metals

• Dull, brittle (when solid), non-lustrous.
• Poor conductors of heat and electricity (exception: graphite, a form of carbon, is a good conductor of electricity).
• Can be solid, liquid, or gas at room temperature.
• Examples: carbon (C), oxygen (O2), nitrogen (N2), sulphur (S), phosphorus (P), chlorine (Cl2), bromine (Br2 — liquid), hydrogen (H2), iodine (I2).

Metalloids (Semi-metals)

• Show properties of both metals and non-metals — intermediate behaviour.
• Semiconductors — used in electronics (transistors, solar cells).
• Examples: boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb).

Noble gases: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) are non-metals that exist as single atoms (monoatomic). They are extremely unreactive and form almost no compounds under ordinary conditions.

8. Compounds vs Mixtures

A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio by mass. This produces a new substance with properties entirely different from its constituent elements.

9. Physical and Chemical Changes

Understanding physical vs chemical changes is essential because mixtures are separated by physical methods but compounds are split by chemical means.

Crystallisation, distillation, and all separation methods involve physical changes only — the identity of each component is preserved. That is why mixtures can be separated this way but compounds cannot.

Signs that a chemical change has occurred: change in colour, change in smell, production of gas (effervescence), change in temperature, formation of a precipitate (insoluble solid), or emission of light.