Salts - preparing insoluble salts by precipitation

Tips: Before preparing a salt, find out if the salt is soluble or insoluble. A soluble salt is made from an acid. An insoluble salt is prepared by precipitation.

Table 1 Soluble Salts and Insoluble Salts

Soluble Salts	Insoluble Salts
All (group I) sodium, potassium, and ammonium
All nitrates
Most chlorides	Silver chloride and lead chloride
Most sulfates (sulphates)	Calcium sulfate, barium sulfate and lead sulfate
Most bromide and iodides	Silver bromide, silver iodide, lead bromide and lead iodide
Sodium, potassium(group I), and ammonium carbonates	All other carbonates

Preparing Insoluble Salts by Precipitation

Example: 

preparing insoluble barium sulfate salt.

• Barium sulfate salt is made of Ba²⁺ ions and SO₄^2- ions.
• So, we have to prepare two soluble salts solutions of which one contains Ba²⁺, its positive metal ions and the other contains the SO₄^2-, its negative non-metal ions. 
• These two ions will react and form a precipitation as barium sulfate salt. 

          Ba²⁺ (aq) + SO₄^2-  (aq) ---> BaSO₄ (s)

The precipitate of barium sulfate salt(residue) is gathered using a filter funnel, rinsed with distilled water to remove any impurities and dried. The filtrate contains the spectator ions which remained unchanged in the reaction.

Another way to separate the precipitate from the solution is using a centrifuge. This technique is used when the solid particles are very small and dispersed in the solution. The fast spinning of the tube in a centrifuge (see picture below) causing the solid gathers at the bottom of the tube due to higher density and gravity force. The liquid can be then decanted leaving the solid in the tube.

A centrifuge

More examples of preparing insoluble salts:

Table 2. Preparing Insoluble Salts 

Preparing insoluble salts	‘+’ ions	‘- ' ions	Solution used	Solution used	Salt precipitation	Spectator ions (no changes in reaction)
Barium sulfate BaSO4	Ba2+	SO42-	Barium nitrate with Ba2+	Sodium sulfate with SO42-	Barium  sulfate BaSO4	NaNO3 sodium nitrate soluble and remained unchanged as Na+ ions and NO3- ions
Reaction: Ba(NO3)2 (aq) + Na2SO4(aq) ---> BaSO4 (s) + 2 NaNO3 (aq)
Barium sulfate BaSO4	Ba2+	SO42-	Barium chloride  with Ba2+	Sodium sulfate with SO42-	Barium  sulfate BaSO4	NaCl  sodium chloride soluble and remained unchanged as Na+ ions and Cl- ions
Reaction: BaCl2 (aq) + Na2SO4(aq) ---> BaSO4 (s) + 2 NaCl2 (aq)
Barium sulfate BaSO4	Ba2+	SO42-	Barium chloride  with Ba2+	Magnesium sulfate with SO42-	Barium  sulfate BaSO4	MgCl2  magnesium chloride soluble and remained unchanged as Na+ ions and Cl- ions
Reaction: BaCl2 (aq) + MgSO4(aq) ---> BaSO4 (s) + 2 MgCl2 (aq)
Silver chloride AgCl	Ag+	Cl-	Silver nitrate with  Ag+	Sodium chloride with Cl-	Silver chloride AgCl	NaNO3  sodium nitrate soluble and remained unchanged as Na+ ions and NO3- ions
Reaction: AgNO3 (aq) + NaCl(aq) ---> AgCl (s) + NaNO3 (aq)
Lead iodide PbI2	Pb2+	I-	Lead nitrate with  Pb2+	Sodium iodide with  I-	Lead iodide PbI2	NaNO3 sodium nitrate soluble and remained unchanged as Na+ ions and NO3- ions
Reaction: Pb(NO3)2 (aq) + NaI (aq) ---> PbI2 (s) + NaNO3 (aq)

Pictures of insoluble salts:

White precipitate
silver chloride, AgCl 

Creamy precipitate
silver bromide, AgBr

Yellow precipitate
silver Iodide, AgI

White precipitate
barium sulfate, BaSO₄

Yellow precipitate
lead iodide, PbI₂

Salts - normal salts, acid salts

Salts are very important to us.

Table 1 Example of salts

Salt	Use
Ammonium chloride	Torch batteries
Ammonium nitrate	Fertilisers
Calcium carbonate	Extraction of iron, making cement, making glass
Calcium chloride	Extraction of sodium, drying agent
Calcium sulphate	Making plaster boards and casts for injured limbs
Iron(II) sulphate	Iron tablets
Magnesium sulphate	Medicines
Potassium nitrate	Fertiliser, gunpowder
Silver bromide	Photography
Sodium carbonate	Making glass, washing powders, water softener
Sodium chloride	Making hydrochloric acid, food flavouring, hospital saline, making sodium carbonate
Sodium stearate	Soaps
Tin(II) flouride	Additive to toothpaste

 

Ammonium chloride in a battery

Ammonium nitrate fertilisers

Calcium carbonate

Iron sulphate for making Iron supplement,
also named as ferrous tablet

Magnesium sulphate

Silver bromide

Sodium carbonate

Sodium chloride

Sodium stearate

Normal Salts

A Normal salt is formed when all the hydrogen ions(H⁺) of an acid, have been replaced by metal ions or by the ammonium ions(NH4⁺), All the salts listed in table 1 are normal salts. 

Normal salts may be soluble or insoluble in cold water.

• all nitrates salts are soluble
• all common sodium, potassium and ammonium salts are soluble
• all chlorides salts all soluble except lead, silver and mercury 
• all sulfates salts are soluble except lead, barium and calcium

Table 2 Normal Salts

Acid	Type of salt	Example
Carbonic acid H2CO3	Carbonates	Sodium carbonate Na2CO3
Sulfuric acid H2SO4	sulphates	Sodium sulfate Na2SO4
Ethanoic acid CH3COOH	Ethanoates	Sodium ethanoate CH3COONa
Hydrochloric acid HCl	Chlorides	Potassium chloride KCl
Nitric acid HNO3	Nitrates	Potassium nitrate KNO3

Acid Salts

An acid salt is formed where not all of the hydrogen ions of an acid have been replaced by metal ions or the ammonium ions. For examples:

Acids such as sulfuric acid and carbonic acid which have two hydrogen ions per molecule, when only one of the hydrogen ions is replaced with a metal ion, the salts formed still contain hydrogen ions.

Table 3 Acid Salts

Acid	Type of acid salt	Example
Carbonic acid H2CO3	Hydrogencarbonate	Sodium hydrogencarbonate NaHCO3
Sulfuric acid HsSO4	Hydrogensulfate	Potassium hydrogensulfate KHSO4

Acids

Example of acidic products:

Citric acid in lemon can
conduct electricity

 Carbonated drinks (carbonic acid)

Acetic/ethanoic acid in vinegar

Tomato ketchup

Sulfuric acid, 
a strong acid

Hydrochloric acid found 
in stomach digestive juices

Testing with Litmus Paper

Acid turn blue litmus to red

Testing with Universal indicator

To find the out how acidic a substance is, one or two drops of universal indicator is mixed into the substance.

Universal indicator solution

Universal indicator paper/ pH paper

The colour change is matched to a pH scale ranged from 0 to 14 as below. A substance with a pH of less than 7 is an acid. The lower the pH(below 7), the more acidic the substance. One with a pH of 7 is neutral. One with a pH of greater than 7 is an alkaline, the higher the pH(above 7), the more alkaline a substance.

Acids	Molecules Formula	Strong/Weak Acid	pH
Hydrochloric acid	HCl (aq)	Strong acid	0 to 1
Sulfuric acid	H2SO4 (aq)	Strong acid	1 to 2
Nitric acid	HNO3 (aq)	Strong acid	1 to 2
Acetic/Ethanoic acid (vinegar acid)	CH3COOH (aq)	Weak acid	3 to 5
Citric acid (lemon/lime juice)	C6H8O7	Weak acid	2.5-6.0
Carbonic acid (fizzy drink)	H2CO3 (aq)	Weak acid	2.4
Methanoic acid/ Formic Acid (ant stings)	HCOOH /HCO2H	Weak acid	3.7
Sulfurous acid (acid rain)	H2SO3 (aq)	Weak acid	varied
Ascorbic acid (vitamin C)	C6H8O6	Weak acid	4.1

Properties of Acids

1. taste sour 
2. soluble in water
3. corrosive
4. conduct electricity
5. react with certain metals and give out hydrogen gas
6. react with metal carbonates and give out carbon dioxide gas
7. react with bases to form salts and water

A victim of acid attack

Sulfurous acid(acid rain) a weak acid,
when oxidised to sulfuric acid it becomes strong  acid and
 damages the environment quite badly. 

Hydrogen ions is responsible for the 'acidic' condition

In order to show acidity, the substance must be dissolved in water.  For an example, gaseous hydrogen  chloride, HCl(g), is not acidity but when it dissolves in water an acidic solution is produced.

Strong Acid or Weak Acid

Solubility of the H⁺ ions in water indicates the strength of the acid. Strong acid means the acid can easily dissociates(ionises) to produce hydrogen ions in water.

It is important to note that a concentrated acid does not mean it is a strong acid and a dilute acid does not mean it is a weak acid. Concentration of an acid only indicate the proportions of water and acid present in aqueous solution. For examples hydrochloric acid is a strong acid, virtually 100% hydrogen ionised; even a diluted hydrochloric acid is still a strong acid. For the same reason, ethanoic acid found in the vinegar is a weak acid, only partial hydrogen ionised; concentrated ethonoic acid is still a weak acid.

An acid is strong acid if all of its H⁺ ions dissociates(ionises) to form hydrogen ions in the water. An acid is weak acid if only small amount of its H⁺ ions dissociates(ionises) to form hydrogen ions in the water.

Hydrochloric acid is a strong acid because all the hydrogen chloride molecules break up to form H⁺ ions and  Cl^-  ions.

                               water
hydrogen chloride -------------> hydrogen ions + chloride ions

                                water
        HCl (g)        ------------->  H⁺ (aq)     +    Cl^- (aq)




Ethanoic acid (vinegar) is a weak acid because it produces few hydrogen ions when dissolves in water. Only partial of the hydrogens in ethanoic acid dissociate to form hydrogen ions in the water. the hydrogen ions and ethonate ions dissociated will react together to re-form the ethanoic acid molecule again since it is a reversible reaction.

                             water
ethanoic acid                hydrogen ions +    ethanoate ions

                              water
CH₃COOH(aq)                H⁺ (aq)     +    CH₃COO^- (aq)