Problem 24 - Chemistry The Central Science In Si Units

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 or a mixture?

Step-by-Step Solution

Answer

(a) Chemical change; (b) The solution will be a mixture.

Step 1: Identifying the Change

When a silvery metal is placed in water and bubbles are formed on its surface, accompanied by the metal dissolving, it is indicative of a chemical change. Chemical changes often involve the formation of new substances, such as gases (bubbles) that weren't present initially.

Step 2: Determining the Type of Solution

After the metal reacts with water and dissolves, the resulting solution cannot be a pure substance, because a chemical reaction has occurred. The presence of bubbles suggests a gas has been formed, and the solution now contains different ions or compounds resulting from the metal-water reaction. This indicates that the remaining solution is a mixture.

Key Concepts

Physical Changes

Physical changes are transformations that do not alter the basic identity of a substance. These changes are usually reversible and do not involve the formation of new substances.
An example of a physical change is the melting of ice. When ice melts, it turns into water. The form changes from solid to liquid, but the chemical composition, \(H_2O\), remains the same.
It's important to note that physical changes do not produce any new chemical products.

• They can involve a change in state, like going from liquid to gas.
• They might affect an object's physical properties such as shape or size.
• Examples include dissolving sugar in water or breaking a piece of chalk.

Mixtures

A mixture is a combination of two or more substances in which each substance retains its own chemical identity and properties. Mixtures can be either homogeneous or heterogeneous.
Let's break it down:

• **Homogeneous mixtures** are uniform in composition, like salt water, where the dissolved salt is not visible.
• **Heterogeneous mixtures** have visibly different components, such as a salad with lettuce, tomatoes, and cucumbers.

Mixtures can be separated by physical means, such as filtration or distillation. For example, you can separate sand from water by using a filter.
This is different from chemical reactions, where new substances form that require different methods to separate.

Chemical Reactions

Chemical reactions involve the breaking and forming of chemical bonds, resulting in new substances with different chemical properties. These changes are indicators of chemical reactions:

• Formation of gas (e.g., bubbles forming)
• Change in temperature
• Formation of a precipitate (a solid that forms in a solution)
• Color change

An example of a chemical reaction is the rusting of iron when it combines with oxygen to form iron oxide.
In chemical reactions, the original substances change into entirely different substances, and this process is usually not reversible by simple physical means.
It's important to understand chemical reactions because they are the foundation of many processes in chemistry and are integral to changes we observe in the laboratory and in the world around us.