Question

Which observation is consistent with a chemical reaction occurring? Why? (a) Solid copper deposits on a piece of aluminum foil when the foil is placed in a blue copper nitrate solution. The blue color of the solution fades. (b) Liquid ethyl alcohol turns into a solid when placed in a low-temperature freezer. (c) A white precipitate forms when solutions of barium nitrate and sodium sulfate are mixed. (d) A mixture of sugar and water bubbles when yeasts are added. After several days, the sugar is gone and ethyl alcohol is found in the water.

Short answer

Observations consistent with chemical reactions include: solid copper depositing on aluminum foil with color fade (a), a white precipitate forming when barium nitrate and sodium sulfate solutions are mixed (c), and sugar converting to ethyl alcohol with gas evolution when yeast is added (d).

Step-by-step solution

Identify Signs of a Chemical Reaction

To determine whether a chemical reaction is occurring, look for signs such as color change, formation of a precipitate, evolution of a gas, or creation of a new substance.

Evaluate Option (a)

In option (a), copper is depositing on aluminum foil from a copper nitrate solution, and the blue color of the solution fades. The change in color and the deposition of a metal on another metal indicate a chemical reaction.

Evaluate Option (b)

Option (b) describes the physical change of ethyl alcohol freezing. This is a change in state from liquid to solid and is not a chemical reaction.

Evaluate Option (c)

Option (c) indicates the formation of a white precipitate when two solutions are mixed. Formation of a precipitate is evidence of a chemical reaction.

Evaluate Option (d)

In option (d), bubbling is seen when yeast is added to a sugar solution, and after several days, sugar is converted to ethyl alcohol. The evolution of gas (bubbling) and the production of a new substance (ethyl alcohol) indicate a chemical reaction.

Conclusion

Options (a), (c), and (d) describe events that are consistent with a chemical reaction occurring, due to the evidence of new substances being formed, color changes, precipitate formation, and gas evolution.

Key concepts

Chemical Reaction Evidence

Understanding the evidence of a chemical reaction is crucial for any student studying chemistry. Signs that a chemical reaction has occurred include the formation of a new substance, change in color, production of a gas, and the appearance of a precipitate. In the observed exercise, we see examples such as the copper deposit on aluminum foil and a color change from blue to a lighter shade in (a), a white precipitate in (c), and the bubbling and creation of ethyl alcohol in (d). Each of these clues tells us that the original substances have undergone a transformation at the molecular level—indicating a chemical reaction.

It's essential for students to distinguish between simple physical changes, like the freezing of alcohol in (b), and the more significant changes occurring during chemical reactions. By examining changes that can't be easily reversed and involve alterations in the substances' structure, we can conclude that a chemical reaction has taken place.

Precipitate Formation

Precipitate formation is a tell-tale sign of a chemical reaction happening. A precipitate is an insoluble solid that emerges from a liquid solution, typically when two solutions are mixed and react chemically, as in example (c) from the exercise. This white precipitate which forms when barium nitrate and sodium sulfate solutions are combined is solid evidence of a chemical reaction.

The ions from each solution interact to form a new compound that is not soluble in the solution, creating the visible precipitate. This phenomenon is important not just as an indicator of chemical change but also has practical applications in fields like medicine, environmental science, and chemical engineering.

Color Change in Chemistry

Indicators of Chemical Changes

Color change is a powerful indicator of chemical change. In our exercise, we see the blue color of copper nitrate solution fade as it reacts with aluminum foil in example (a). This color change is due to the shift in the way electrons are arranged in the reacting substances, which changes the way they absorb and reflect light.

As the reaction proceeds, the formation of new compounds results in different color properties. It's important to understand that not all color changes signify a chemical reaction; some can be due to changes in temperature, pH, or concentration. However, when accompanied by other signs like the formation of new substances, color change is a reliable indicator of a chemical reaction.

Gas Evolution in Reactions

The evolution of a gas during a chemical reaction often provides a visible and sometimes audible cue that a chemical transformation is underway. In example (d) from the exercise, the addition of yeast to a sugar solution results in bubbling, which is the release of a gas. This gas is a byproduct of the reaction taking place between the yeast enzymes and the sugar, converting it to ethyl alcohol and carbon dioxide.

This process, known as fermentation, is just one instance where gas evolution indicates a chemical reaction. Other common reactions that release gases include acid-base reactions and decomposition reactions. Observing bubbles or smelling odors can help confirm that chemical reactions are occurring.

Physical vs Chemical Change

Understanding the Difference

It's vital for students to recognize the difference between physical and chemical changes. A physical change, such as the freezing of ethyl alcohol in (b), involves changes in the state or appearance of a substance without altering its chemical composition—these changes are usually reversible.

On the other hand, a chemical change involves creating new substances with different properties and compositions—these changes are generally irreversible. For example, the formation of copper on the aluminum foil (a), the precipitate in (c), and the production of ethyl alcohol in (d) are irreversible processes indicating that chemical reactions have taken place. Distinguishing between these types of changes is foundational to the study of chemistry and helps students understand the transformation of matter on a molecular level.