Pure Substances, Mixtures & Separation Methods

20min Part 1 / Ch1 / Lesson 1

Prerequisites: 0-0-1

Objectives

Distinguish between pure substances and mixtures
Differentiate between elements, simple substances, and compounds
Explain separation/purification methods (filtration, distillation, recrystallization, extraction, chromatography) by their principles

Classification of Matter

All matter is classified as either a pure substance or a mixture

Pure substance = consists of only one type of matter (has fixed melting/boiling points). Mixture = two or more pure substances mixed together (no fixed melting/boiling points).

How to Distinguish Pure Substances from Mixtures

Property	Pure Substance	Mixture
Melting/boiling point	Fixed	Not fixed
Density	Fixed	Varies with composition
Chemical formula	Can be written	Cannot

Common Misconception

“Air is a pure substance” → It’s a mixture. Air is mainly nitrogen $\mathrm{N_2}$ (~78%) and oxygen $\mathrm{O_2}$ (~21%).

Allotropes

Allotropes: Different structural forms of the same element.

Element	Allotropes
Oxygen O	$\mathrm{O_2}$ (oxygen), $\mathrm{O_3}$ (ozone)
Carbon C	Diamond, graphite, fullerene
Sulfur S	Rhombic, monoclinic, plastic sulfur
Phosphorus P	White phosphorus, red phosphorus

Separation & Purification Methods

Method	Principle	Example
Filtration	Particle size difference	Separating sand from saltwater
Distillation	Boiling point difference	Obtaining pure water from seawater
Fractional distillation	Boiling point (multiple components)	Refining petroleum
Recrystallization	Solubility temperature dependence	Purifying potassium nitrate
Extraction	Solubility in different solvents	Extracting iodine with hexane
Chromatography	Adsorption affinity difference	Separating ink pigments
Sublimation	Ability to sublime	Purifying iodine

Summary of separation methods

Separation methods are easiest to remember by what difference they exploit: boiling point → distillation, solubility → recrystallization, solvent affinity → extraction.

Worked Example

Classifying Substances

Classify each as “element,” “compound,” or “mixture”:

(a) Diamond　(b) Saltwater　(c) Sodium chloride　(d) Air　(e) Iron

(a) Diamond → Element (allotrope of carbon C; one element only)
(b) Saltwater → Mixture (NaCl + H₂O)
(c) Sodium chloride → Compound (Na + Cl; two elements, one substance)
(d) Air → Mixture (N₂, O₂, Ar, etc.)
(e) Iron → Element (Fe only)

Check Your Understanding

Q1 Which is a characteristic of pure substances?

Q2 Which of the following is a mixture?

Q3 Which separation method uses differences in boiling point?

Exercises

Q1. What separation method would you use to obtain pure water from seawater? Explain why.

Solution

Distillation is appropriate.

Water (bp 100°C) and dissolved salts like NaCl (bp 1413°C) have vastly different boiling points. Heat the seawater to evaporate water, then condense the steam to collect pure liquid water.

Q2. How would you purify KNO₃ contaminated with a small amount of NaCl?

Solution

Recrystallization is appropriate.

KNO₃ has a large change in solubility with temperature, while NaCl’s solubility barely changes. Dissolve in hot water, then cool — KNO₃ crystallizes out while NaCl remains dissolved.