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Lesson 4 of 6 · Textbook 3.3

Mixtures

The most important mixture is the one you are breathing. Nothing in it is joined to anything else — which is exactly why its amounts can change.

What a mixture is

Definition A mixture is made up of two or more elements and/or compounds that are not chemically combined.

“Not chemically combined” means the substances keep their own particles and their own properties. They are just sharing space.

Air

Air is a mixture of nitrogen, oxygen, carbon dioxide, water vapour and small amounts of other gases. The proportions are worth memorising:

Composition of clean, dry air

nitrogen 78% oxygen 21%
Usually constantnitrogen · oxygen
Varies by time and placecarbon dioxide · water vapour · dust
The other 1%CO₂, water vapour, argon, others
Because air is a mixture, its composition can differ from place to place. A humid day has more water vapour; a busy road has more carbon dioxide and dust. The nitrogen and oxygen barely change.
Two facts examiners like

High altitude: the composition of air is the same as at sea level — still 21% oxygen. There is simply less air, so less oxygen is available per breath. That is why climbers carry oxygen tanks.

Air pollution: Singapore measures it with the PSI (Pollutant Standards Index), based on six pollutants. Under 100 is fine for normal outdoor activity; over 100, people with heart or lung disease should avoid strenuous exercise outdoors. A single agreed measurement system is what makes the advice comparable from day to day.

Three kinds of mixture

What a mixture is made of gives you three categories. Build one yourself and see which it is.

Mixture builder

Contents—
Type—
Looks like—

Try iron + carbon, then tin + copper, then sodium chloride + water, then all four gases in air.
KindExamples from the textbook
Elements onlySteel — iron, carbon and other elements; stronger than pure iron.
Pewter — tin with copper and silver; jewellery and kitchen utensils.
Compounds onlyConcrete — calcium carbonate, silicon dioxide and water.
Salt solution — sodium chloride and water.
Coffee — coffee powder (many compounds) and water.
Elements and compoundsFuel — carbon-containing compounds plus the element sulfur (removed as a contaminant).
Seawater — sodium chloride, magnesium chloride, and a little of the element iodine.
Milk — water, fats and sugars, plus small amounts of iron, phosphorus and calcium.

Three properties every mixture has

These are the mirror image of the compound properties from Lesson 3, and the exam will test you on the contrast.

  1. A mixture keeps the properties of its constituents. Salt tastes salty; salt solution tastes salty too. Nothing new has been made.
  2. The constituents are not in a fixed proportion. A pinch of salt in water is salt solution. A spoonful in the same water is also salt solution. Both are the same mixture, in different proportions.
  3. The constituents can be separated easily — by physical means such as filtering, evaporating or using a magnet. No chemical reaction is needed, because none was needed to make the mixture in the first place.
Ice kacang is a mixture — say why properly Shaved ice, syrup, jelly, red beans, attap chee, corn. Each ingredient keeps its own taste and appearance (properties retained); every stall uses different amounts (no fixed proportion); and you can pick out the beans with a spoon (easily separated). Three properties, three pieces of evidence — that is a full answer.

Check yourself