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What are the general rules for predicting the solubility of ionic compounds in water?
Salt | Solubility | Exceptions |
|---|---|---|
sodium (Na+), potassium (K+) ammonium (NH4+) | soluble | none |
nitrates (NO3-) | soluble | none |
chlorides (Cl-) | soluble | silver chloride (AgCl) lead (II) chloride (PbCl2) |
sulfates (SO42-) | soluble | barium sulfate (BaSO4) calcium sulfate (CaSO4) lead (II) sulfate (PbSO4) |
carbonates (CO32-) | insoluble | sodium carbonate (Na2CO3) potassium carbonate (K2CO3) ammonium carbonate ((NH4)2CO3) |
hydroxides (OH-) | insoluble | sodium hydroxide (NaOH) potassium hydroxide (KOH) calcium hydroxide (Ca(OH)2) (calcium hydroxide is slightly soluble) |
What are the roles of acids and bases in terms of proton transfer?
The earlier definition of an acid and a base can be extended
In terms of proton transfer, we can further define each substance in how they interact with protons
Acids are proton donors as they ionize in solution producing protons, H+ ions
These H+ ions make the aqueous solution acidic
Bases (alkalis) are proton acceptors as they ionize in solution producing OH- ions which can accept protons
These OH- ions make the aqueous solution alkaline
Diagram showing the role of acids and bases in the transfer of protons
What are the reactions of hydrochloric acid, sulfuric acid and nitric acid with metals, bases and metal carbonate?
Acid reactions summary
alkali + acid → water + salt
base + acid → water + salt
carbonate + acid → water + salt + carbon dioxide
metal + acid → salt + hydrogen
Acid + Alkali and Acid + Base
A base is a substance that can neutralise an acid, forming a salt and water only.
Alkalis are soluble bases. When they react with acids, a salt and water is formed. The salt formed is often as a colourless solution. Alkalis are a source of hydroxide ions (OH⁻) when in solution.
alkali + acid → water + salt
base + acid → water + salt
Examples of acid + alkali reactions:
sodium hydroxide + hydrochloric acid → sodium chloride + water
NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l)
potassium hydroxide + sulfuric acid → potassium sulfate + water
2KOH (aq) + H₂SO₄ (aq) → K₂SO₄ (aq) + 2H₂O (l)
Example of an acid + base reaction:
CuO (s) + H₂SO₄ (aq) → CuSO₄ (aq) + H₂O (l)
Acid + Carbonate
carbonate + acid → water + salt + carbon dioxide
A carbonate is a compound made up of metal ions and carbonate ions. Examples of metal carbonates are sodium carbonate, copper carbonate and magnesium carbonate.
When carbonates react with acids, bubbling is observed which is the carbon dioxide being produced. If the acid is in excess the carbonate will disappear.
Examples of acid + carbonate reactions:
calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)
potassium carbonate + hydrochloric acid → potassium chloride + water + carbon dioxide
K₂CO₃ (aq) + 2HCl (aq) → 2KCl (aq) + H₂O (l) + CO₂ (g)
Acid + Metal
metal + acid → salt + hydrogen
Metals will react with an acid if the metal is above hydrogen in the reactivity series.
When metals react with acids, bubbling is observed which is the hydrogen being produced. If the acid is in excess the metal will disappear.
Examples of acid + metal reactions:
magnesium + sulfuric acid → magnesium sulfate + hydrogen
Mg (s) + H₂SO₄ (aq) → MgSO₄ (aq) + H₂ (g)
aluminium + hydrochloric acid → aluminium chloride + hydrogen
2Al (s) + 6HCl (aq) → 2AlCl₃ (aq) + 3H₂ (g)
copper + hydrochloric acid → no reaction (since copper is below hydrogen in the reactivity series)
What are bases and alkalis?
A base is a substance that neutralises an acid by combining with the hydrogen ions in them to produce water.
A base usually means a metal oxide, a metal hydroxide or ammonia.
Alkalis are bases which are soluble in water.
Some metal oxides are soluble in water and react with it to form solutions of metal hydroxides. For example:
Na₂O (s) + H₂O (l) → 2NaOH (aq)
Apart from this and other group 1 oxides (such as potassium oxide) most other metal oxides are not soluble in water.
One exception is calcium oxide which does dissolve slightly in water to form calcium hydroxide (known as limewater):
CaO (s) + H₂O (l) → Ca(OH)₂ (aq)
Ammonia is another base. Ammonia reacts with water to form ammonium ions and hydroxide ions:
NH₃ (aq) + H₂O (l) ⇋ NH₄⁺ (aq) + OH⁻ (aq)
All the solutions produced here contain hydroxide ions (OH⁻) so they are all alkalis.
Bases are substances which can neutralise an acid, forming a salt and water
The term base and alkali are not the same
A base which is water-soluble is referred to as an alkali
So, all alkalis are bases, but not all bases are alkalis
Alkalis have pH values of above 7
In basic (alkaline) conditions red litmus paper turns blue
Bases are usually oxides, hydroxides or carbonates of metals
The presence of the OH- ions is what makes the aqueous solution an alkali
One unusual base is ammonia solution
When ammonia reacts with water it produces hydroxide ions
Some Common Alkalis and the Ions They Contain
How do you prepare a pure, dry sample of a soluble salt, starting from an insoluble reactant?
Step | Explanation |
|---|---|
Heat acid (H2SO4) in a beaker | Speeds up the rate of reaction |
Add base (CuO) until in excess (no more copper oxide dissolves) and stir with glass rod | Neutralises all the acid |
Filter the mixture using filter paper and funnel | Removes any excess copper oxide |
Gently heat the filtered solution (CuSO4) | To evaporate some of the water |
until crystals form on a glass rod | Shows a hot saturated solution formed |
Allow the solution to cool so that hydrated crystals form | Copper sulfate less soluble in cold solution |
Remove the crystals by filtration | Removes crystals |
Dry by leaving in a warm place | Evaporates the water |
How do you prepare a pure, dry sample of a soluble salt, starting from an acid and alkali?
Aim:
To prepare a sample of a dry salt starting from an acid and an alkali
Diagram:
Diagram showing the apparatus needed to prepare a salt by titration
Method:
Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (phenolphthalein or methyl orange)
Add the acid into the burette and note the starting volume
Add the acid very slowly from the burette to the conical flask until the indicator changes to appropriate colour
Note and record the final volume of acid in burette and calculate the volume of acid added (starting volume of acid - final volume of acid)
Add this same volume of acid into the same volume of alkali without the indicator
Heat to partially evaporate, leaving a saturated solution
Leave to crystallise decant excess solution and allow crystals to dry
Results:
A dry sample of a salt is obtained
How do you prepare a pure, dry sample of an insoluble salt, starting from two soluble reactants?
Insoluble salts can be prepared using a precipitation reaction
The solid salt obtained is the precipitate, thus in order to successfully use this method the solid salt being formed must be insoluble in water
The preparation of a soluble salt follows this pattern:
soluble salt 1 + soluble salt 2 ⟶ insoluble salt + soluble salt 3
AB + CD ⟶ AD + CB
The method involves measuring out a fixed volume of one solution and then adding the second salt solution until it is in a slight excess
This ensures the maximum amount of precipitate will be obtained
The precipitate is recovered by filtration and then it must be washed with distilled water remove reactants that are contaminating the residue (recovered solid)
It is then left to dry
This method is a good way to prepare silver and lead(II) salts which are often insoluble; the starting material will usually be the nitrate of silver or lead(II) since all nitrates are soluble
How do you prepare a sample of pure, dry hydrated copper(II) sulfate crystals starting from copper(II) oxide?
Aim:
To prepare a pure, dry sample of hydrated copper(II) sulfate crystals
Materials:
1.0 mol / dm3 dilute sulfuric acid
Copper(II) oxide
Spatula & glass rod
Measuring cylinder & 100 cm3 beaker
Bunsen burner
Tripod, gauze & heatproof mat
Filter funnel & paper, conical flask
Evaporating basin and dish.
The preparation of copper(II) sulfate by the insoluble base method
Practical Tip:
The base is added in excess to use up all of the acid, which would become dangerously concentrated during the evaporation and crystallisation stages
Method:
Add 50 cm3 dilute acid into a beaker and warm gently using a Bunsen burner
Add the copper(II) oxide slowly to the hot dilute acid and stir until the base is in excess (i.e. until the base stops dissolving and a suspension of the base forms in the acid)
Filter the mixture into an evaporating basin to remove the excess base
Gently heat the solution in a water bath or with an electric heater to evaporate the water and to make the solution saturated
Check the solution is saturated by dipping a cold glass rod into the solution and seeing if crystals form on the end
Leave the filtrate in a warm place to dry and crystallise
Decant excess solution and allow the crystals to dry
Results:
Hydrated copper(II) sulfate crystals should be bright blue and regularly shaped
How do you prepare a sample of pure, dry lead(II) sulfate?
Aim:
To prepare a dry sample of lead(II) sulfate
Diagram:
The preparation of lead(II)sulfate by precipitation from two soluble salts
Method:
Measure out 25 cm3 of 0.5 mol dm3 lead(II)nitrate solution and add it to a small beaker
Measure out 25 cm3 of 0.5 mol dm3 of potassium sulfate add it to the beaker and mix together using a stirring rod
Filter to remove precipitate from mixture
Wash filtrate with distilled water to remove traces of other solutions
Leave in an oven to dry
Soluble salt 1 = lead(II) nitrate Soluble salt 2 = potassium sulfateEquation for the reaction:
Pb(NO3)2 (aq) + K2SO4 (aq) → PbSO4 (s) + 2KNO3 (aq)
lead(II) nitrate + potassium sulfate → lead(II) sulfate + potassium nitrate
Note 
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