Thursday, 22 January 2015

7.1.2 - Laboratory Activity : Role of water to show properties of alkalis

Laboratory Activity 7.1.2:
Role of water to show properties of alkalis
Aim: To study the role of water in showing the properties of an alkali
Problem statement: Does water needed to be present to enable a base to show its alkaline properties?
Hypothesis: Water must be present for a base to show its alkaline properties.
» Fixed variable : Type of base
» Manipulated variable : Presence of water
» Responding variable : Alkaline characteristics

» Dry calcium hydroxide
» Calcium hydroxide solution
» Ammonia solution
» Litmus paper
» Ammonia in trichloromethane

» Test tube
» Switch
» Carbon electrodes
» Connecting wires
» Crocodile clip
» Electric bulb
» Beakers
» Batteries

The animation below shows the arrangement and results of the experiment.

(A) Litmus paper test
1. A few pieces of dry calcium hydroxide are placed in a dry test tube.
2. 10cm3 of calcium hydroxide solution poured into another test tube.
3. Both are tested with litmus paper.

(B) Electrical conductivity test
1. Dry calcium hydroxide and calcium hydroxide solution are poured separately in two beakers.
2. The switch is then turned on. All observations are recorded.
3. Repeat the conductivity test for ammonia in trichloromethane and ammonia solution.

Test Dry Ca(OH)2 Ca(OH)2(aq) NH3 in trichloromethane NH3(aq)
Litmus paper No change Red litmus turns blue No change Red litmus turns blue
Electrical conductivity No change Electric bulb light up No change Electric bulb light up

Without water, alkalis cannot exhibit alkaline characteristics as they cannot dissociate to form ions.

In water, alkalis dissociate to form cations and OH-.

OH- ions change red litmus to blue colour.

Hydroxide ion, OH-, which is free to move in water allows electricity to pass through it.

Ammonia that is dissolved in an organic solvent does not show its alkaline characteristics because ammonia exists in molecules form.

In water, ammonia dissociate to form NH+ ions and OH- ions which is free to move.
NH3 + H2O → NH4+ + OH-

An alkali only shows its alkaline properties in the presence of water.

The hypothesis is accepted.

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