Urgent... F.4 Chemistry(Acid)

a)
Add some sodium hydrogencarbonate to the unknown aqueous solution. If some colourless bubbles are evolved, the unknown aqueous solution is an acid. (The colourless gas evolved would turn lime water milky.)
NaHCO3(s) + H+(aq) → Na+(aq) + H2O(l) + CO2(g)
or HCO3-(aq) + H+(aq) → H2O(l) + CO2(g)

b)
The experiment can be divided into two parts. Water is absent in the first part, but water is present in the second part.

In the first part of experiment, some solid citric acid and calcium carbonate are added to a test tube and are mixed well. Record the observations. It is found that there is no observable change. The acidic properties are due to the presence of hydrogen ions. Because of the absence of water, the citric acid is not ionized and exists as molecules. There is no hydrogen ion, and thus there are no acidic properties.

In the second part of experiment, some distilled water is then added to the reaction mixture in the first part. It is observed that some colourless bubbles are evolved. Citric acid dissolves in water, and is then ionized to give hydrogen ions. The hydrogen ions then react with hydrogencarbonate to give a colourless gas of carbon dioxide.
NaHCO3(s) + H+(aq) → Na+(aq) + H2O(l) + CO2(g)
or HCO3-(aq) + H+(aq) → H2O(l) + CO2(g)

This experiment shows that the citric acid does not show acidic properties in the absence of water, but it does show acidic properties in the presence of water.
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Thank you very much , Gabriella Montez

You may take a look at http://hk.knowledge.yahoo.com/question/question?qid=7008121300517
I've answered a similar Q b4.

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