Question

State whether each of the following is a physical or a chemical change: (a) baking soda fizzes in vinegar. (b) vinegar and oil separate into two layers. (c) a flaming wood log releases heat. (d) a helium balloon decreases in size.

Short answer

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

Step-by-step solution

Understanding Physical vs. Chemical Change

A physical change involves a change in a substance's physical properties without altering its composition. In contrast, a chemical change results in the formation of new substances with different properties. The key to identifying a chemical change is the formation of new substances.

Analyze (a) baking soda fizzes in vinegar

When baking soda fizzes in vinegar, it produces carbon dioxide gas, a new substance not present in the reactants. This indicates a chemical change because new substances are formed.

Analyze (b) vinegar and oil separate into two layers

The separation of vinegar and oil is a physical change. In this process, neither vinegar nor oil changes their chemical composition; they simply form two distinct layers due to their different densities.

Analyze (c) a flaming wood log releases heat

A flaming wood log burning is a chemical change. The wood reacts with oxygen in the air, producing new substances such as carbon dioxide and water, and releasing heat and light.

Analyze (d) a helium balloon decreases in size

A helium balloon decreasing in size is a physical change. The helium gas molecules may escape from the balloon over time, but no new substance is formed, and no chemical reaction occurs.

Key concepts

Chemical Reactions

Chemical reactions are processes where substances, known as reactants, undergo a transformation to produce new substances, called products. These reactions are essential to chemistry, as they explain how matter interacts and changes. A key aspect of chemical reactions is that they involve a change in the chemical composition of the substances involved.

When a chemical reaction occurs, bonds between atoms in the reactants break, and new bonds form to create the products. This results in substances that have different physical and chemical properties from the original substances. A classic sign of a chemical reaction is the formation of gas, the release or absorption of energy (heat, light), color changes, or the formation of a precipitate.

In the exercise example:

• The fizzing of baking soda in vinegar signifies a chemical reaction, as it produces new substances, including carbon dioxide gas.
• Similarly, when wood burns, it reacts with oxygen, leading to new substances such as carbon dioxide and water, signifying a chemical change.

Physical Properties

Physical properties are characteristics of a substance that can be observed or measured without changing the substance's chemical identity. These properties include aspects like color, melting point, boiling point, density, and state of matter (solid, liquid, gas).

Physical changes are changes that affect these properties, but do not alter the chemical structure of the substance. This means that even though the appearance or state of the substance might change, its core composition does not. An example from the exercise is the separation of vinegar and oil. They diverge based on density differences without changing their inherent composition.

Key physical properties you might observe include:

• Change of state, such as ice melting into water.
• Volume changes, such as a helium balloon decreasing in size over time.
• Separation processes, like mixture separation based on density or solubility differences.

Substances in Chemistry

In chemistry, substances refer to forms of matter that have a consistent composition and distinct properties. These can be elements or compounds. Substances play crucial roles in both chemical and physical changes. Understanding how they interact is foundational to understanding chemistry.

Elements like oxygen or compounds like water are substances that have specific chemical and physical properties. Knowing the characteristics of substances can help predict how they might behave in a given scenario, whether undergoing a chemical reaction or engaging in physical changes.

The substances in the exercise include vinegar and baking soda, both react to create new compounds in a chemical reaction, while others like helium in a balloon retain their properties even when experiencing a physical change.

Gas Formation

Gas formation is one hallmark of a chemical reaction. It occurs when a new gas is produced that was not present before, indicating a change in chemical composition. This can be quite visible in reactions involving acids and bases, where effervescence or bubbling can be seen as a new gas emerges.

A perfect instance of this in the exercise is when baking soda fizzes in vinegar, releasing carbon dioxide gas. This visible effervescence is a strong indicator of a chemical reaction, as a new gaseous substance, carbon dioxide, is formed.

Gas formation can also signal other reaction types, including combustion, where gases are often released. Recognizing gas production can significantly aid in identifying chemical changes and understanding the behavior of substances in various processes.