Question

Which of the following is evidence for a chemical reaction? (a) Mixing two aqueous solutions produces a precipitate. (b) Mixing two colorless solutions gives a blue solution.

Short answer

Both options (a) and (b) indicate a chemical reaction.

Step-by-step solution

Understanding Chemical Reaction Indicators

A chemical reaction is indicated by observable changes that suggest a new substance or substances have been formed. Common indicators include the formation of a precipitate, a color change, gas evolution, or temperature change.

Analyzing Option (a)

Option (a) states that mixing two aqueous solutions produces a precipitate. A precipitate is a solid that forms from a solution during a chemical reaction, indicating that a new substance has been formed. Therefore, this is evidence of a chemical reaction.

Analyzing Option (b)

Option (b) describes mixing two colorless solutions to produce a blue solution. A color change from combining two solutions typically indicates the formation of a new substance, which is evidence of a chemical reaction.

Concluding the Evidence

Both options (a) and (b) demonstrate evidence of a chemical reaction. Option (a) is evidenced by precipitate formation, and option (b) by color change.

Key concepts

Precipitate Formation

When we talk about precipitate formation, we're referring to the process where a solid forms and separates from a liquid mixture. This phenomenon occurs during certain chemical reactions, particularly when two aqueous solutions react. In these reactions, the ions in the two solutions combine to form a new compound that is not soluble in water, thus falling out of the solution as a solid.
This solid is called a precipitate. It's one of the clearest indicators of a chemical reaction because it shows that new bonds have formed to create a new substance.
To identify a precipitate formation, look for:

• An insoluble solid forming at the bottom of a container.
• A cloudy or milky appearance in a previously clear solution.
• Changes in the consistency or texture of the solution, sometimes forming clumps or settling sediments.

Understanding precipitate formation helps chemists predict the outcomes of reactions and determine the possible substances produced.

Color Change

Color change is another vital clue that signifies a chemical reaction has taken place. When two substances mix and a new color emerges, this usually indicates a change in the composition. The color change happens because the molecular structure of substances alters during the reaction, creating compounds that absorb light differently.
Remember that color changes can be dramatic, like mixing two colorless solutions to produce a vivid blue, or subtle, such as a slight yellowing from a reaction.
Key points about color changes:

• Visual changes like these provide immediate and easy-to-spot evidence of a chemical reaction.
• The starting materials, called reactants, often look entirely different when new products are formed.
• Colors are associated with specific elements or compounds. Thus, recognizing color patterns can aid in identifying which substances are present in the reaction.

Whether you're in a chemistry lab or observing reactions at home, noting color changes is an essential skill for verifying chemical activity.

Reaction Indicators

Indicators of chemical reactions are observable changes that allow us to conclude whether a new substance has formed. Besides precipitate formation and color changes, several other indicators can signal a chemical process:

• **Gas Evolution**: The release of gas bubbles is a tell-tale sign. This can be seen in everyday reactions, like the fizz in a carbonated drink.
• **Temperature Change**: Reaction enthalpy leading to heat release (exothermic) or absorption (endothermic) is often felt as a temperature change.
• **Light or Sound Emission**: Some reactions produce energy as light and sound, think of fireworks or the glow from combustion.

These indicators are important because they provide us with concrete, observable evidence that a chemical reaction has taken place.
Teaching and learning about these indicators is crucial in chemistry, as they help bridge the gap between theoretical concepts and real-world observations.