Problem 24 - Chemistry: The Central Science in SI Units, Global Edition

Question

A silvery metal is put inside a beaker of water. Bubbles form on the surface of the metal and it dissolves gradually. (a) Is this an example of a chemical or a physical change? (b) Do you expect the remaining solution to be a pure substance of a mixture?

Step-by-Step Solution

Answer

(a) This is an example of a chemical change, as new substances are produced during the process. (b) The remaining solution is a mixture, as it contains components from both the original metal and the water.

Step 1: Identify the type of change

Analyze the given clues: bubbles form on the metal's surface, and it dissolves gradually in the water. These observations indicate that new substances are produced in the process. Therefore, this is an example of a chemical change.

Step 2: Determine the nature of the resulting solution

Since a chemical change occurred, new substances were formed during the process. This means that the resulting solution contains components from both the original metal and the water. Hence, the remaining solution is a mixture. So, to sum up: (a) The observed change is a chemical change. (b) The remaining solution is a mixture.

Key Concepts

Understanding Mixtures

Mixtures are fascinating combinations of substances where each retains its own properties. Unlike chemical compounds, which have new and sometimes unique properties, mixtures are more straightforward.
For example, if we dissolve sugar in water, we create a homogeneous mixture, or a solution. The sugar and water components can still be separated by physical means like evaporation.

• Homogeneous mixtures: These have uniform composition throughout, like saltwater.
• Heterogeneous mixtures: The different components are easily distinguishable, such as a salad.

In the case of the chemical change in the exercise, even though the metal dissolved, the resulting mixture formed is likely to be homogeneous. This means substances are evenly distributed, yet can contain different elemental compounds. Being able to identify if a solution is homogeneous or heterogeneous assists in understanding the properties and behaviors of mixtures.

Bubbles in Reactions

Bubbles often indicate that a chemical reaction is occurring. When a solid substance produces bubbles in a liquid, it's a visual cue that a gas is being released.
This is common in several chemical reactions. For instance, when baking soda reacts with vinegar, carbon dioxide bubbles are produced:

• Many times, bubbles form due to the release of gases such as oxygen, carbon dioxide, or hydrogen.
• Bubbles can also inform us about the rate and vigor of the reaction, as faster bubbling might indicate a more vigorous reaction.

Understanding the formation of bubbles helps identify whether a process is a physical or chemical change. If bubbles result from chemical bonds breaking and forming, as with the metal in water, it typically signals a chemical change where new substances are being created.

Dissolving Metals

When metals dissolve in a solution, a chemical change often takes place, leading to the formation of different molecules or ions. In most cases, these are not purely disappeared, but rather integrated into the solution.
For instance, when a reactive metal like the one in our exercise combines with water, it forms a compound, often releasing hydrogen gas:

• Depending on the nature of the metal, it may form metal oxide or metal hydroxide, altering the properties of both the metal and the solvent.
• The rate at which a metal dissolves can depend on factors such as temperature, concentration of the solvent, and the metal's reactivity.

This process showcases a chemical change, given that new atomic or ionic arrangements occur which differ from the original substances. Recognizing the effects of dissolving metals can help predict the outcome of similar chemically reactive interactions.