Salts
All salts are ionic compounds. A salt is formed when a metallic
ion or an ammonium ion (NH4+)
replaces one or more hydrogen ions of an acid.
HCl NaCl
HNO3 NH4NO3
replaced by
replaced by
Find out what is an acid salt!
Salts
Where have you heard of salts? Salts are essential to animal life in
small quantities, but in large excess will be very harmful.
Are all salts salty? Group I ions similar in size to sodium
tend to give salty taste. Which ions do you think give a salty taste?
Example
How do you form CaSO4? Acid + Base Salt + water
CaSO4
Comes from base
Comes from acid
One base that can be used is Calcium hydroxide.
One acid that can be used is Sulfuric acid. Ca(OH)2 + H2SO4 CaSO4 + 2H2O Can you use calcium carbonate or
calcium?
Example
How do you form NaNO3? Acid + Base Salt + Water
NaNO3
Comes from base
Comes from acid
One base that can be used is Sodium hydroxide.
One acid that can be used is Nitric acid. NaOH + HNO3 NaNO3 + H2O
Negative ion (anion)
Negative ion Acid used
SO42- (sulfate ion)
NO3- (nitrate ion)
Cl- (chloride ion)
Sulfuric acid (H2SO4)
Nitric acid (HNO3)
Hydrochloric acid (HCl)
Quickcheck
What acids do you use to get the following salts?
Sodium nitrate Potassium phosphate Ammonium ethanoate Copper(II) iodide Sodium citrate Aluminium sulfate
Water of crystallization
Many salts combine with water molecules to form crystals.
These water molecules are called water of crystallization.
Salts that contain water of crystallization are called hydrated salts.
Salts that do not contain water of crystallization are called anhydrous salts.
Water of crystallization
Name of salt Formula of anhydrous salt
Formula of hydrated salt
copper(II) sulfate
CuSO4 CuSO4.5H2O
magnesium sulfate
MgSO4 MgSO4.7H2O
sodium carbonate
Na2CO3 Na2CO3.10H2
Ozinc sulfate ZnSO4 ZnSO4.7H2OWhat does the ‘dot’ mean?
Removing water of crystallization Heating a hydrated salt removes water of
crystallization.
CuSO4.5H2OHeat
CuSO4 + 5H2O
hydrated copper(II) sulfate
anhydrous copper(II) sulfate + water
Heat
Removing water of crystallization
Cobalt(II) chloride
CoCl2.6H2OHeat
CoCl2 + 6H2O
hydrated cobalt(II) chloride
anhydrous cobalt(II) chloride + water
Heat
Find out the use of cobalt(II) chloride paper in the lab.
Soluble and insoluble saltsChlorides/ bromides/
iodides
Sulfates Carbonates Nitrates
All are soluble except:
• lead(II) chloride/bromide/iodide
• silver chloride/bromide/iodide
All are soluble except:
• barium sulfate
• calcium sulfate
• lead(II) sulfate
All are NOT soluble except:
• Sodium carbonate
• Potassium carbonate
• Ammonium carbonate
All are soluble.
Sodium salts
Potassium salts
Ammonium salts
All are soluble.
Salt Preparation
There are 3 ways of preparing salts. But to choose which way depends on:
1. Whether the salt is soluble in water?
2. Whether the starting materials are soluble in water?
Methods of preparing saltsIs the salt soluble?
No
Method 3: Precipitation
Yes
Reaction with acids
• Acid + metal• Acid + base
• Acid + carbonateAre the starting materials soluble?
YesNo
Method 1: Reaction of acids with insoluble substances
• Acid + metal• Acid + base
• Acid + carbonate
Method 2: Titration• Acid + alkali
• Acid + carbonate solution
1) Filter the mixture
2) Collect filtrate
Salt solution
Salt crystals (dry with filter paper)
1) Concentrate2) Crystallize3) Filter
Method 1
Recap: Acid + Metal oxide/hydroxide Salt +
Water Acid + Carbonate Salt + Water +
Carbon dioxide Acid + Metal Salt + Hydrogen gas In Method 1, all the substances in
red are insoluble.
Method 1
The acid is reacted with an excess of the substances (metal, carbonate or base).
Why?
AcidInsoluble base
Method 1a
Acid + Metal Salt + Hydrogen gas
ZnSO4
Comes from metal
Comes from acid
Zinc is insoluble in water and reacts with sulfuric acid. We can use zinc here.
Method 1a: Using metal
sulfuric acid zinc sulfate solution +
unreacted zinc
Keep adding zinc until no more effervescence is observed.
zinc sulfate solution
1) Filter the mixture
2) Collect filtrate
zinc sulfate crystals 1) Concentrat
e2) Crystallize3) Filter
Metals that can be used
Only moderately reactive metals like zinc, aluminum, magnesium and iron can be used.
Not suitable for 1. Very reactive metals such as sodium,
potassium and calcium. Why?2. Unreactive metals like copper and
silver. Why?
Method 1b
Acid + Metal oxide/ hydroxide Salt + Water
CuSO4
Comes from metal oxide
Comes from acid
Copper(II) oxide is insoluble in water and reacts with sulfuric acid. We can use Copper(II) oxide here. Why can’t we use copper metal?
Method 1b: Using insoluble base
sulfuric acidCopper(II) sulfate
solution + unreacted
Copper(II) oxide
Keep adding copper(II) oxide until no more effervescence is observed.
Copper(II) sulfate solution
1) Filter the mixture
2) Collect filtrate
Copper(II) sulfate crystals 1) Concentrat
e2) Crystallize3) Filter
Method 1c
Acid + Carbonate Salt + Carbon dioxide + Water
MgCl2
Comes from carbonate
Comes from acid
Magnesium carbonate is insoluble in water and reacts with hydrochloric acid.
NOTE: ALL carbonates are insoluble except potassium, sodium and ammonium carbonate!
Method 1c: Using carbonate
hydrochloric acid
Magnesium chloride solution
+ unreacted magnesium carbonate
Keep adding magnesium carbonate until no more effervescence is observed.
Magnesium chloride solution
1) Filter the mixture
2) Collect filtrate
Magnesium chloride crystals 1) Concentrat
e2) Crystallize3) Filter
Summary of method 1
Filter the crystals and dry them by squeezing them between sheets of filter paper.
Pour the filtrate into an evaporating dish and heat to evaporate most of the water. This produces a hot saturated solution of the
salt. Let the solution cool.
Remove the excess metal/metal carbonate/ base by filtering and collect the filtrate. The filtrate contains the solution of the salt you
want.)
Add the metal/metal carbonate/ base slowly with stirring to hot acid (what acid depends on what salts you want) until no more
dissolves. (This means all the acid is used up)
Method 2: Titration
Notice that all sodium, potassium and ammonium salts are SOLUBLE in water.
So you cannot use Method 1 for any of such salts! Why?
Method 2: Titration
To tell when all the acid has been completely used up, we have to use titration, by using an indicator.
What is an indicator?Indicator Colour in acidic
solutionpH range at which indicator changes colour
Colour in alkaline solution
methyl orange red 3 – 5 yellow
screened methyl orange
violet 3 – 5 green
litmus red 5 – 8 blue
bromothymol blue
yellow 6 – 8 blue
phenolphthalein colourless 8 – 10 pink
Steps to titration (Sodium nitrate)
Fill up a burette with dilute nitric acid and note down the initial burette reading (V1 cm3).
V1 cm3
Pipette 25.0 cm3 of dilute sodium hydroxide into a conical flask.
Add one or two drops of methyl orange to the NaOH solution. The solution turns yellow.
Add dilute HNO3 from the burette slowly until the solution turns orange permanently. This is the end-point. The acid is all used up.
V2 cm3
Record the final burette reading (V2 cm3). Hence, the volume of acid required for complete neutralization = (V2 – V1) cm3.
Getting Sodium nitrate
1. Pipette 25.0cm3 of NaOH into a beaker. Then add (V2 – V1) cm3 of dilute nitric acid from the burette. This time do not add indicator. Why?
2. Heat to evaporate the water till it is saturated.
3. Cool the saturated solution so that the salt crystallizes.
4. Filter to collect the crystals.5. Dry the crystals between a few sheets
of filter paper.
Titration
Simulation
Summary of method 2
Record the final burette reading (V2 cm3). Hence, the volume of acid required for complete neutralization = (V2 – V1) cm3.
Add dilute HNO3 from the burette slowly until the solution turns orange permanently. This is the end-point.
Add one or two drops of methyl orange to the alkali solution. The solution turns yellow. (Note, if you have a strong acid and weak
base, you use methyl orange, if you have a strong base and weak acid, use phenolphthalein. If both are strong, you can use either
indicator.
Pipette 25.0 cm3 of dilute alkali (depending on what salt you want) into a conical flask.
Fill up a burette with dilute acid (depending on what salt you want) and note down the initial burette reading (V1 cm3).
Summary of method 2
Dry the crystals between a few sheets of filter paper.
Filter to collect the crystals.
Cool the saturated solution so that the salt crystallizes.
Heat to evaporate the water till it is saturated.
Pipette 25.0cm3 of NaOH into a beaker. Then add (V2 – V1) cm3 of dilute nitric acid from the burette.
Insoluble salts
Easiest to prepare Just need to use precipitation Mix a solution containing the positive
ions of the salt with another solution containing the negative ions of the salt to be prepared.
What salts are insoluble?
Example: Lead(II) sulfate
Using lead(II) nitrate and dilute sulfuric acid
1. First, pour 50 cm3 of lead(II) nitrate solution into a small beaker. Add sulfuric acid (in excess) and stir until no more precipitate forms.
2. Filter and collect precipitate.3. Wash the precipitate with a small
amount of distilled water to remove impurities.
4. Allow the precipitate to dry on filter paper.
Summary of method 3
Allow the precipitate to dry on filter paper.
Wash the precipitate with a small amount of distilled water to remove impurities.
Filter and collect precipitate.
Add another reagent (again depending on what salt you want) and stir until no more precipitate forms.
First, pour 50 cm3 of one reagent (depending on what salt you want) into a small beaker.
Test yourself
How do you get the following salts:1. Magnesium sulfate2. Lead(II) chloride3. Potassium nitrate4. Sodium sulfate5. Zinc nitrate
Questions
Are these salts soluble?1. iron(III) nitrate2. potassium carbonate3. sodium ethanoate4. silver chloride5. lead(II) nitrate6. copper(II) carbonate7. ammonium iodide8. titanium(IV) chloride9. barium sulfate
YesYesYesNoYes No Yes Yes No
Test yourself
Which method will you use to get the following salts:
1. Magnesium sulfate2. Lead(II) chloride3. Potassium nitrate4. Sodium sulfate5. Copper(II) chloride6. Lead(II) carbonate7. Silver chloride8. Zinc chloride
Test Yourself
When an acid Z is added to a solution of lead(II) nitrate, an insoluble precipitate is formed.
When Z is added to a solution of silver nitrate, an insoluble precipitate is formed too.
What acid could Z be?A) hydrochloric acid B) sulfuric acidC) nitric acid