Chapter map — Diversity of Matter

The three definitions

ElementThe basic building blocks of living and non-living matter. Cannot be broken down into simpler substances. One kind of atom. 118 known; arranged by properties in the Periodic Table; metals (most) and non-metals.

CompoundTwo or more elements chemically combined. Formed by a chemical reaction. Fixed proportion by mass. Properties different from its elements. Not easily separated — needs a lot of energy.

MixtureTwo or more elements and/or compounds not chemically combined. Formed by a physical change. Proportion not fixed. Properties of constituents retained. Easily separated by physical means.

Table 3.3 — the three questions

Question	Compound	Mixture
Fixed proportion of constituents by mass?	yes	no
Same properties as its constituents?	no	yes
Constituents separated easily?	no	yes

An element is one substance only, so these questions do not apply to it. The two columns are exact opposites.

Examples to have ready

Type	Examples
Elements	copper (coins), carbon (pencil lead), nitrogen (liquid, ice cream), neon (signs), aluminium (pots), oxygen, iron, sodium, chlorine, technetium (first artificial)
Compounds	water (H, O) · carbon dioxide (C, O) · sodium chloride / salt (Na, Cl) · sodium hydrogencarbonate / baking soda (Na, H, C, O) · ammonia (N, H) · calcium carbonate / marble (Ca, C, O) · glass · plastics
Mixtures — elements only	steel (iron + carbon) · pewter (tin + copper + silver)
Mixtures — compounds only	concrete (calcium carbonate + silicon dioxide + water) · salt solution · coffee
Mixtures — both	fuel (carbon compounds + sulfur) · seawater (sodium chloride + magnesium chloride + iodine) · milk (water, fats, sugars + iron, phosphorus, calcium) · air

Air

Nitrogen 78% · oxygen 21% · carbon dioxide, water vapour and other gases 1%.

Nitrogen and oxygen: usually constant over time and place.
Carbon dioxide, water vapour and dust: vary with time and place.
High altitude: same composition, less air — so less oxygen per breath.
Singapore measures air quality with the PSI (six pollutants). Under 100: normal activity. Over 100: heart/lung patients avoid strenuous outdoor exercise.

Solutions

Solution — a mixture in which one substance dissolves completely in another. Solute dissolves; solvent does the dissolving. Water: the universal solvent.
Dilute — low percentage of solute in a fixed volume. Concentrated — high percentage.
Soluble — dissolves completely. Insoluble — does not dissolve.

The two factor lists — do not mix them up

Rate of dissolving — how fast	Solubility — how much
Size of solute particles — smaller, faster Temperature of solvent — hotter, faster Rate of stirring — faster, faster	Type of solute — sugar > salt > chalk (0) in water Type of solvent — salt dissolves in water, not in oil

Solubility — definitionThe largest amount of solute that can dissolve in a fixed volume of a particular solvent at a specific temperature.

Table 3.2 — solution vs suspension

Solution	Suspension
Solute particles cannot be seen	Insoluble substances can be seen
Solute does not settle	Insoluble substances can settle to form solid deposits
Light passes through fully	Light cannot pass through fully

A suspension is a mixture containing insoluble substances in a solvent — dirty water, wastewater, calamine lotion, some soups. Insoluble substances are removed by filtration.

3.1 — why it matters

Melamine: useful in plastics; harmful in food. Mixed into milk powder in 2008 because its composition resembles milk protein, fooling protein tests. Babies suffered kidney damage.
Recycling needs you to know what an item is made of. E-waste contains hazardous metals and other substances — separate bin, never general rubbish.