Identifying Reactants Producing Gas that Turns Limewater Milky

The question describes a chemical reaction occurring at room temperature in a flask, where bubbles of a gas are produced. A key piece of information is that this gas turns limewater milky. This observation is a standard test used in chemistry to identify a specific gas.

What Gas Turns Limewater Milky?

Limewater is an aqueous solution of calcium hydroxide, $\text{Ca(OH)}_2$. When certain gases are bubbled through limewater, a reaction occurs. The gas that specifically reacts with limewater to produce a white, insoluble precipitate of calcium carbonate, $\text{CaCO}_3$, is carbon dioxide ($\text{CO}_2$). This precipitate makes the solution appear milky or cloudy. The chemical equation for this reaction is:

$\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}$

Therefore, the problem is asking us to identify the pair of reactants from the options that produces carbon dioxide gas at room temperature.

Analyzing the Given Reactants and Their Reactions

Let's examine each option to see what gas, if any, is produced and under what conditions.

• Option 1: Zinc and hydrochloric acid

  Zinc ($\text{Zn}$) is a metal, and hydrochloric acid ($\text{HCl}$) is an acid. When reactive metals react with acids, they typically produce hydrogen gas ($\text{H}_2$). This reaction happens readily at room temperature.

  The chemical equation is: $\text{Zn(s)} + 2\text{HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)}$

  Hydrogen gas is colourless and odourless. It does not turn limewater milky; it burns with a 'pop' sound.

• Option 2: Magnesium carbonate and hydrochloric acid

  Magnesium carbonate ($\text{MgCO}_3$) is a metal carbonate, and hydrochloric acid ($\text{HCl}$) is an acid. A characteristic reaction of metal carbonates with acids is the production of a salt, water, and carbon dioxide gas. This reaction typically occurs quickly at room temperature.

  The chemical equation is: $\text{MgCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

  This reaction produces carbon dioxide gas, which is known to turn limewater milky.

• Option 3: Methane and oxygen

  Methane ($\text{CH}_4$) is a hydrocarbon, and oxygen ($\text{O}_2$) is a gas required for combustion. The reaction between methane and oxygen is combustion, which produces carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$).

  The chemical equation is: $\text{CH}_4\text{(g)} + 2\text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + 2\text{H}_2\text{O(l)}$

  However, combustion reactions like this require ignition (a spark or flame) to start and sustain. They do not occur spontaneously at room temperature in a flask simply by mixing the gases (unless under very specific, unusual conditions not implied here). Therefore, this option does not fit the description of a reaction producing gas at room temperature.

• Option 4: Copper and dilute hydrochloric acid

  Copper ($\text{Cu}$) is a metal, and dilute hydrochloric acid ($\text{HCl}$) is an acid. Copper is a relatively unreactive metal located below hydrogen in the reactivity series. This means that copper generally does not react with non-oxidizing acids like dilute $\text{HCl}$ to displace hydrogen gas at room temperature.

  No significant reaction producing gas would occur between copper and dilute hydrochloric acid under these conditions.

Conclusion

Comparing the analyses:

• Zinc and hydrochloric acid produce hydrogen gas, not carbon dioxide.
• Magnesium carbonate and hydrochloric acid produce carbon dioxide gas at room temperature.
• Methane and oxygen produce carbon dioxide and water, but typically require ignition, not just room temperature mixing.
• Copper and dilute hydrochloric acid do not react to produce gas at room temperature.

Only the reaction between magnesium carbonate and hydrochloric acid fits the description of producing a gas (carbon dioxide) that turns limewater milky at room temperature.

Therefore, the reactants that could be in the flask are magnesium carbonate and hydrochloric acid.

Revision Table: Key Concepts

Additional Information: Identifying Gases in Chemistry

Identifying gases produced during chemical reactions is a common practice in chemistry. Various tests are used:

• Hydrogen ($\text{H}_2$): Tested by bringing a burning splint near the gas; it extinguishes the flame with a 'pop' sound.
• Oxygen ($\text{O}_2$): Tested by bringing a glowing splint into the gas; it relights the splint.
• Carbon Dioxide ($\text{CO}_2$): Tested by bubbling the gas through limewater ($\text{Ca(OH)}_2$ solution); it turns milky.
• Ammonia ($\text{NH}_3$): Tested by its pungent smell and by using damp red litmus paper, which turns blue.
• Chlorine ($\text{Cl}_2$): Tested by its pungent smell and by using damp blue litmus paper, which turns red, then bleaches white.

Understanding these simple tests helps in identifying the products of many chemical reactions conducted in the lab.