A gas that turns lime water milky is likely to be from

1. Analyze the question

The question asks for the identity of a gas that turns lime water milky. This is a classic qualitative analysis test in Chemistry used to identify a specific gas produced during a reaction.

2. Explain the concept

• Lime water is a clear, saturated solution of calcium hydroxide, $Ca(OH)_2$.
• When carbon(IV) oxide ($CO_2$) gas is bubbled through lime water, it reacts to form an insoluble white precipitate of calcium trioxocarbonate(IV) ($CaCO_3$).
• The chemical equation for this reaction is: $Ca(OH)_{2(aq)} + CO_{2(g)} \rightarrow CaCO_{3(s)} + H_2O_{(l)}$
• The "milky" appearance is caused by the formation of the solid $CaCO_3$ particles suspended in the water.
• Therefore, any substance that releases carbon(IV) oxide gas upon reaction (usually with an acid) will turn lime water milky. Compounds containing the trioxocarbonate(IV) ion ($CO_3^{2-}$) are the primary sources of $CO_2$ gas.

3. Show the solution

• Option A (Trioxocarbonate(IV)): When a trioxocarbonate(IV) salt (like $Na_2CO_3$ or $CaCO_3$) reacts with a dilute acid, it releases carbon(IV) oxide gas ($CO_2$). This gas turns lime water milky.
• Option B (Chloride): Chlorides react with acids to produce hydrogen chloride gas ($HCl$), which is acidic but does not turn lime water milky.
• Option C (Trioxonitrate(V)): These salts generally produce nitrogen oxides or other gases depending on the reaction, but they do not produce $CO_2$ to turn lime water milky.
• Option D (Tetraoxosulphate(VI)): These salts do not release $CO_2$ gas.

Since the gas responsible for the milky effect is carbon(IV) oxide, and trioxocarbonate(IV) compounds are the source of this gas, Option A is the correct choice.

4. Identify the answer

The correct option is A.

5. Exam Tip

Remember the "Lime Water Test": Whenever you see "milky" or "white precipitate" in a question involving lime water, immediately think of Carbon(IV) Oxide ($CO_2$). If you are asked what produces it, look for the Trioxocarbonate(IV) group ($CO_3^{2-}$). This is a very common question in JAMB and WASSCE practical chemistry!