1 of 17 © Boardworks Ltd 2012

Precipitates

2 of 17 © Boardworks Ltd 2012

3 of 17 © Boardworks Ltd 2012

Solutions contain dissociated ions, which are free to move.

When mixed, ions can react to form insoluble solid products.

For example, mixing solutions of silver nitrate and sodium

chloride produces a white precipitate of silver chloride.

What is a precipitation reaction?

An insoluble solid product is called a precipitate.

You can spot a precipitate because the mixture goes cloudy.

silver

nitrate

AgNO3 (aq)

sodium

chloride

NaCl (aq)

sliver

chloride

AgCl (s)

sodium

nitrate

NaNO3 (aq)

+

+

+

+

4 of 17 © Boardworks Ltd 2012

Formation of precipitates

5 of 17 © Boardworks Ltd 2012

Speed of precipitation reactions

For chemical reactions to occur, the

reactant particles must collide.

the reactants are in solution and

therefore are free to move around

the reactants are ions, so the

opposite charges on the particles

attract one another.

This is because:

Most precipitation reactions occur

almost instantaneously.

6 of 17 © Boardworks Ltd 2012

Isolating the precipitate

The precipitate from a precipitation

reaction can be separated from the

reaction mixture by filtration.

Buchner

funnel

filter paper

vacuum

pump

Buchner

flask

A Buchner funnel and

flask can be used to

accelerate the process.

This apparatus uses a

vacuum pump to draw the

mixture through the filter.

The filtrate is finally

washed and dried.

7 of 17 © Boardworks Ltd 2012

8 of 17 © Boardworks Ltd 2012

Common ions

This table shows some common positive and negative ions.

Can you predict the formulae of the compounds they form?

+Li Ca Na Mg Al K

F

O

N

Br

S

Cl

LiF CaF2 NaF MgF2 AlF3 KF

Li2O CaO Na2O MgO Al2O3 K2O

Li3N Ca3N2 Na3N Mg3N2 AlN K3N

LiBr CaBr2 NaBr MgBr2 AlBr3 KBr

Li2S CaS Na2S MgS Al2S3 K2S

LiCl CaCl2 NaCl MgCl2 AlCl3 KCl

–

9 of 17 © Boardworks Ltd 2012

What is the ionic formula?

10 of 17 © Boardworks Ltd 2012

State symbols

11 of 17 © Boardworks Ltd 2012

Ionic equations

What is the word equation for the precipitation reaction

between barium chloride and sodium sulfate?

sodium

sulfate

barium

sulfate

barium

chloride+ +

sodium

chloride

Na2SO4(aq) BaSO4(s)BaCl2(aq) + + 2NaCl(aq)

What is the symbol equation?

Ionic equations show the disassociated ions in aqueous

ionic reactants. What is the ionic equation for this reaction?

SO42– BaSO4(s)Ba2+ + 2Na+2Cl– ++ + 2Na+2Cl– +

12 of 17 © Boardworks Ltd 2012

Constructing ionic equations

13 of 17 © Boardworks Ltd 2012

Spectator ions

In ionic precipitation reactions, there are often ions that are not

involved in the reaction. These are known as spectator ions.

The spectator ions are easily identified using the ionic equation.

This equation shows that the silver and the iodine ions have

reacted, joining together to make the precipitate.

The sodium (Na+) and nitrate (NO3–) ions are spectator ions.

This means the ionic equation can be simplified to:

NaI (aq) AgNO3 (aq) AgI (s) NaNO3 (aq)+ +

Na+ (aq)

AgI (s)

+I– (aq)+Ag+ (aq)+NO3– (aq)

+ Na+ (aq) + NO3– (aq)

I– (aq) + Ag+ (aq) AgI (s)

14 of 17 © Boardworks Ltd 2012

Which ions are spectators?

15 of 17 © Boardworks Ltd 2012

16 of 17 © Boardworks Ltd 2012

Glossary

17 of 17 © Boardworks Ltd 2012

Multiple-choice quiz