6 · Suspensions, and telling everything apart

Suspensions

Definition A suspension is a mixture that contains insoluble substances in a solvent.

Dirty water from a drain is the textbook example — it contains substances that do not dissolve in water. Wastewater, calamine lotion and some soups are suspensions too. Because the substances are insoluble, three things follow, and each one is a test you can do:

Solution or suspension?

Sugar solution

Muddy water

The insoluble substances in a suspension can be removed by filtration — Chapter 4. You cannot filter a solution, because the dissolved particles are as small as the water particles and go straight through the paper.

Solution	Suspension
Particles of the solute cannot be seen.	Insoluble substances can be seen.
The solute does not settle to form solid deposits.	Insoluble substances can settle to form solid deposits.
Light can pass through fully.	Light cannot pass through fully.

Element, compound or mixture? Three questions

This is Table 3.3, and it is the most examinable thing in the chapter. An element is made of only one type of substance, so the three questions do not apply to it. For everything else, ask:

Question	Compound	Mixture
Does it have a fixed proportion of constituents by mass?	yes	no
Does it have the same properties as its constituents?	no	yes
Can its constituents be separated easily?	no	yes

Notice the pattern The compound column reads yes · no · no and the mixture column reads no · yes · yes. They are exact opposites on every line. If you remember either column, you have the other for free.

And the full comparison including how each is formed (Table 3.1):

	Element	Compound	Mixture
Composition	one element only	two or more elements	elements only, compounds only, or both
Formation	mostly a natural process	a chemical reaction	a physical change
Proportion	—	fixed	not fixed
Properties of constituents	—	not retained	retained
Separation	—	not easy	easy

Is smoke an element, a compound or a mixture? A mixture. Evidence: it contains several gases (carbon dioxide, water vapour, others) plus tiny solid particles; its composition varies with what is burning; the solid particles can be separated from the gases by filtering. Three properties, three pieces of evidence — the same answer shape as ice kacang.

Suspensions

Solution or suspension?

Element, compound or mixture? Three questions

Check yourself