Thursday, 22 January 2015

7.1.1 - Laboratory Activity : Role of water to show properties of acids


Laboratory Activity 7.1.1:
Role of water to show properties of acids
Aim: To study the role of water in showing the properties of an acid
Problem statement: Does water need to be present to enable an acid to show its properties?
Hypothesis: Water needs to be present to enable an acid to show its properties.
Variable:
» Fixed variable : Ethanoic acid, test on acid
» Manipulated variable : Presence of water
» Responding variable : Acidic characteristics

Material:
» Ethanoic acid glacial
» Ethanoic acid aqueous solution
» Litmus paper

Apparatus:
» Test tube
» Dropper
» Carbon electrodes
» Connecting wires
» Crocodile clip
» Electric bulb
» Beakers
» Batteries
Procedure:

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

(A) Litmus paper test
1. 10cm3 of glacial ethanoic acid and aqueous ethanoic acid is poured in 2 separate test tubes.
2. Both acids are tested with litmus paper. The results are recorded.

(B) Electrical conductivity test
1. Some glacial ethanoic acid is poured in a beaker.
2. The switch is then turned on. All observations are recorded.
3. Repeat the conductivity test for ethanoic acid aqueous.
Results:

Test Glacial ethanoic acid Ethanoic acid in water
Litmus paper No change Blue litmus turns red
Electrical conductivity No change Electric bulb light up
Discussion:

Without water, dry acid exists in the form of molecules(CH3COOH) . There is no ion, therefore it does not exhibit the characteristics of an acid.

In the presence of water, the acid ionizes:
CH3COOH ⇔ CH3COO- + H+

The presence of the ethanoate, CH3COO- and hydrogen ion, H+ cause blue litmus paper to turn red.

Electricity is also conducted by the aqueous solution because of the presence of freely moving ions in the solution. On the other hand, glacial ethanoic acid cannot conduct electricity because of the presence of covalent molecules in it.
Conclusion:

Acid can only exhibit their acidic characteristics in the presence of water. As aqueous solution, they ionise to form freely moving H+ ions and anions which can conduct electricity and turn blue litmus red.

The hypothesis is accepted.


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