[Test for Ca2+] On adding aq NaOH, a white ppt is formed. The white ppt is Ca(OH)2.
Isn’t Ca(OH)2 an alkali/ soluble in water? Isn’t Ca(OH)2 limewater? So why would there be a precipitate?
Ca(OH)2 is sparingly/ slightly soluble in water (~1.7 g in 1 dm3 of water). Compare with NaOH (~1100 g in 1 dm3 of water) which is very soluble.
Under the conditions of the laboratory test, the amount of Ca(OH)2 produced is greater than the water can dissolve, hence it precipitates out as solid Ca(OH)2.
[Test for Ca2+] On adding aq NH3, no ppt is formed. Since NH3 can ionise in water to produce OH–, shouldn’t Ca(OH)2 (white ppt) be formed too?
For a precipitate to form, there must be sufficient amounts of the constituent ions in solution. Since aq NH3 is a weak alkali, the amount of OH– is very little and insufficient for solid Ca(OH)2 to form.
Test for Anions
[Test for CO32–] After the limewater turns chalky, if you continue to bubble in CO2, the white ppt will dissolve. Why?
The white ppt formed when CO2 is bubbled into limewater is CaCO3. In the presence of excess CO2, CaCO3 can undergo further reaction to form calcium hydrogen carbonate [Ca(HCO3)2] which is soluble in water.
[Test for Cl−/I−] Why is silver nitrate contained in a dark bottle?
Silver nitrate is photosensitive. In the presence of sunlight, silver nitrate can decompose to form metallic silver. Hence, it is important to store them in dark bottles to prevent light from entering and the chemical from decomposing.
[Test for NO3−] Why do we need to add aluminium foil in the test for NO3− ions?
Why does the aluminium foil turn black during the reaction?
Aluminium undergoes a redox reaction with nitrate ions, reducing them to ammonium ions. The ammonium ions then react with hydroxide present to form ammonia gas.
Since nitrate is reduced, aluminium must be oxidised. In the reaction, aluminium is oxidised to aluminium oxide which is the black substance observed.
Shouldn’t aluminium oxide be a white solid? Yes. But depending on the size of the particles and how they assemble on a surface, the appearance/ colour of a substance can vary. In this case, the aluminium oxide formed on the surface of aluminium appears black.
Test for Gases
[Test for H2] What causes the 'pop' sound?
On introducing the lighted splint, the heat from the flame can trigger a reaction between the hydrogen present with oxygen in the air.
The reaction releases a large amount of energy causing the air around it to expand rapidly which results in an explosion (similar to how a thunder is produced).
[Test for Cl2] What causes the moist litmus paper to become bleached?
Cl2 gas reacts with water to produce hypochlorous acid (HOCl) and the hypochlorous acid will then dissociate into hypochlorite ions (ClO–) and hydrogen ions (H+).
Litmus paper is filter paper stained with litmus indicator. Litmus indicator is a substance that exists in different forms under acidic and alkaline conditions, thereby appearing as red or blue.
Hypochlorite ions can react with the substance in litmus forming a product that has no colour. Hence, the litmus paper becomes bleached.
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