Problem 31 - Chemistry Principles and Practice

Question

Which of the following processes describe physical changes, and which describe chemical changes? (a) Alcohol burns (b) Sugar crystallizes (c) Gas bubbles rise out of a glass of soda (d) A tomato ripens

Step-by-Step Solution

Answer

(a) Chemical, (b) Physical, (c) Physical, (d) Chemical

Step 1: Identify the Nature of Alcohol Burning

When alcohol burns, it reacts with oxygen in the air to form new products, primarily water and carbon dioxide. This transformation involves breaking chemical bonds and forming new ones, thus constituting a chemical change.

Step 2: Consider the Process of Sugar Crystallization

Sugar crystallizing from a solution involves a physical change where sugar molecules arrange themselves in a solid, ordered structure from a solution. No new chemical substances are formed, so this is a physical change.

Step 3: Examine Gas Bubbles in Soda

Gas bubbles rising out of a soda is due to the release of carbon dioxide that was dissolved in the soda. This release is physical as it involves changing phases from dissolved gas to gaseous state, with no new substances being produced.

Step 4: Analyze the Ripening of a Tomato

A tomato ripening involves chemical changes because it encompasses complex biochemical reactions, leading to changes in color, texture, and flavor, and results in new chemical products.

Key Concepts

Understanding Chemical Changes

Chemical changes occur when substances undergo a transformation to form different substances. This process involves the breaking and forming of chemical bonds, resulting in new substances with unique properties.

Key features of chemical changes include:

• Formation of new substances
• Changes in energy, which can be either absorbed or released
• Irreversibility in most cases

For example, when alcohol burns (as noted in step 1 of the solution), it combines with oxygen and transforms into carbon dioxide and water. This change is accompanied by energy release in the form of heat and flame. Hence, burning alcohol signifies a chemical change due to these characteristic transformations.

Recognizing Physical Changes

Physical changes involve alterations in the physical state or appearance of a substance without any change in its chemical composition. These changes are usually reversible.

• Common examples include changes in state, size, or shape
• No new substances are formed
• Often reversible, like melting or freezing

For instance, sugar crystallizing involves sugar molecules forming a solid crystal structure from a liquid solution. This is purely a physical change as the chemical composition of sugar remains the same. Similarly, gas bubbles arising from soda are another physical change, where carbon dioxide simply transitions from being dissolved to a gaseous state without altering its chemical form.

Exploring Reaction Processes

Reaction processes are the pathways that lead to either chemical or physical changes. Understanding these processes helps us determine the nature of the changes occurring.

• Chemical reactions lead to the formation of new compounds.
• Physical processes can involve phase changes, like evaporation or crystallization.
• Biological processes, such as a tomato ripening, involve complex chemical reactions.

When a tomato ripens (as described in step 4), it undergoes numerous chemical reactions, which alter its taste, color, and texture, resulting in new chemical substances. Therefore, analyzing the specific reactions and processes can discern whether a change is chemical or physical.