Question

Which of the following is evidence for a chemical reaction? (a) Mixing two aqueous solutions produces gas bubbles. (b) Mixing two solutions produces an insoluble solid.

Short answer

Both (a) and (b) indicate evidence of a chemical reaction.

Step-by-step solution

Understanding Chemical Reactions

A chemical reaction involves a change in the composition of substances, resulting in new substances being formed. Indicators include color change, temperature change, production of gas, and formation of a precipitate.

Evaluating Option (a)

Mixing two aqueous solutions to produce gas bubbles is often evidence of a chemical reaction. The formation of gas suggests that a new substance is being formed as a result of the reaction between the solutes.

Evaluating Option (b)

Mixing two solutions and producing an insoluble solid is indicative of a chemical reaction. This insoluble solid is called a precipitate, which forms because the product of the reaction is not soluble in the solvent.

Conclusion

Both options (a) and (b) are evidence of chemical reactions, as they involve the formation of new products: gas bubbles indicating gas formation, and an insoluble solid indicating precipitation.

Key concepts

Evidence of Chemical Reactions

Chemical reactions are all around us, occurring daily, often without us even realizing. Evidence of a chemical reaction indicates that something new is being formed at a molecular level. When two or more substances interact, the result can be a transformation that changes their physical and chemical properties. Several signs can highlight this transformation, such as:

• Color Change: If you observe a color change, it might mean that a new substance has been formed.
• Temperature Change: Exothermic reactions release heat while endothermic reactions absorb heat, causing temperature changes.
• Gas Formation: Bubbles or fizzing are a clear indication of a gas being produced.
• Precipitate Formation: A solid settling out of a liquid solution indicates a reaction.

In summary, if you notice any of these changes during an experiment or reaction process, it’s a good indication that a chemical reaction has taken place.

Gas Formation

Gas formation during a chemical reaction is often observed as the sudden appearance of bubbles or effervescence. This indicates that a new gaseous substance has formed as a result of a reaction. In the context of mixing two solutions, the formation of gas is a recognizable sign that a new chemical species that is gaseous is forming or evolving. Here's how you can recognize and understand gas formation:

• Visual Clues: Bubbles rising to the surface in a solution is an immediate visual sign that a gas is being produced.
• Common Experiments: An example includes the reaction between hydrochloric acid (HCl) and sodium carbonate (Na₂CO₃), which produces carbon dioxide (CO₂) gas, water, and sodium chloride as products. \[ \text{2 HCl} (aq) + \text{Na₂CO₃} (s) \rightarrow \text{CO₂} (g) + \text{H₂O} (l) + \text{2 NaCl} (aq) \]

Gas formation is not just limited to fizzing; sometimes, it can also be detected by an increase in pressure or a change in volume in a closed container responsive to the release of a gas.

Precipitate Formation

Precipitate formation is a fascinating evidence of a chemical reaction, involving the creation of a solid from a liquid solution. When two aqueous solutions are mixed, a solid product that forms and settles out of the solution is known as a precipitate. Here's a closer look at how this works:

• Understanding Insolubility: A precipitate forms when the product of the reaction is insoluble in the solvent, meaning it does not dissolve, and appears as a solid.
• Common Reactions: A typical reaction that results in a precipitate is the reaction between silver nitrate (AgNO₃) and sodium chloride (NaCl) forming silver chloride (AgCl), a white solid: \[ \text{AgNO₃} (aq) + \text{NaCl} (aq) \rightarrow \text{AgCl} (s) + \text{NaNO₃} (aq) \]

Precipitates are valuable in chemical analysis and in various industrial processes, including waste treatment and mineral extraction.