Chapter 8: Salts
1. Salts A salt is an ionic compound. The anion part comes from the acid while the cation part comes from a base. Example: KCl, KOH(aq) + HCl(aq) KCl(aq) + H2O(l) A salt is a compound formed when the hydrogen ion. H+ from an acid is replaced by a metal ion or an ammonium ion, NH4+. Salts Nitrate salts Carbonate salts Chloride salts Soluble All nitrate salts Potassium carbonate, K2CO3 Ammonium carbonate, (NH4)2CO3 Sodium carbonate, Na2CO3 All chloride salts Except Sulphate salts All sulphate salts Except 2. Preparation and purification of soluble salts Lead (II) chloride, PbCl2 Silver chloride, AgCl Mercury Chloride, HgCl Lead (II) sulphate, PbSO4 Barium sulphate, BaSO4 Cacium sulphate, CaSO4 Persatuan Bahasa Cina Pb Ag Hg P A S All other carbonate salts Insoluble
i. ii. iii. iv.
The salts formed during preparation of soluble salts contain impurities. These salts need to be purified through a process known as recrystallisation. Physical characteristics of crystals: Crystals have fixed geometrical shapes such as a cuboid, rhombic or prism. Crystals of the same substance have same shapes but may be in different sizes. Crystals have flat surfaces, straight edges and sharp angles. Crystals have fixed angles between two neighboring surfaces.
Example 1: Preparation of Soluble Salts Na2SO4
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Example 2: Preparation of Soluble Salt MgSO4
Preparation of insoluble salts An insoluble salts is prepared through precipitation reaction. Aqueous solutions containing the ions of the insoluble salt are mixed together to from the salt. The insoluble salt is formed as a precipitate and can be obtained by filtration. Example: BaCl2(aq) + Na2SO4(aq) BaSO4(s) + 2NaCl(aq) Insoluble salts can prepared by double decomposition reaction through precipitation.
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Procedure for the selection of the method preparing 1 specified salt.
Qualitative Analysis of Salts Qualitative analysis is a chemical technique used to determine what substance is present in a mixture but not their quantities. In the quantitative analysis of salts, we need to identify the ions that are present in salts. This can be done by analyzing their physical and chemical properties. Cations and anions in salts can be identified through: a) Colour and solubility of the salt c) Effect of heat on salt e) Confirmatory test for cations b) Gas Test d) Confirmatory test for anions
Colour and solubility of the salt Cation / anion / salt / compounds which may be present 2+ 2+ 3+ 2+ + Salts of Ca , Mg , Al , Pb , NH4 CuSO4 / Cu(NO3)2 CuCl2 CuCO3 FeSO4 / Fe(NO3)2 / FeCl2 Fe(SO4)3 / Fe(NO3)3 / FeCl3 PbO CuO ZnO PbCl2 PbI2
Colour Solid White Blue Green Green Green Brown Brown when hot and yellow when cooled Black Yellow when hot and white when cooled White Yellow Solution Colourless Blue Blue Insoluble Green Yellow / Browndish-yellow / Brown (depending on concentration) Insoluble Insoluble Insoluble Insoluble in cold water but soluble in hot water Insoluble in cold water but soluble in hot water
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Gas Test Gas Oxygen, O2 Colour Colourless Smell Odourless Confirmatory test Lights up a glowing wooden splinter Diagram
Produce a ‘pop’ sound with a lighted wooden splinter
Carbon dioxide, CO2
Turns limewater cloudy
a) Turns moist red litmus paper blue b) Produces thick white fumes with hydrogen chloride, HCl gas
a) Bleaches moist red litmus paper b) Turns moist blue litmus paper to red and then bleaches it
Hydrogen Chloride, HCl Nitrogen dioxide, NO2 Sulphur dioxide, SO2 Water vapour, H2O
Produces thick white fumes with ammonia, NH3 gas
Turns moist blue litmus paper red
Bleaches the purple...
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