Question

State whether each of the following is a physical or a chemical change: (a) steam condenses and freezes to ice. (b) hydrogen and oxygen gases explode. (c) bromine vaporizes to a brown gas. (d) sodium metal changes to a white powder.

Short answer

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

Step-by-step solution

Analyze the Nature of the Change - Part (a)

Steam condenses and freezes to ice. This process involves the change of state from a gas to a liquid and then to a solid. Since the chemical identity of water remains unchanged throughout these transformations, it is a physical change.

Analyze the Nature of the Change - Part (b)

Hydrogen and oxygen gases explode. During this reaction, hydrogen and oxygen gases combine to form water. This involves a change in the chemical composition of the substances, resulting in a chemical change.

Analyze the Nature of the Change - Part (c)

Bromine vaporizes to a brown gas. This process involves a change in state from liquid to gas, without altering the chemical structure of bromine. Thus, it is a physical change.

Analyze the Nature of the Change - Part (d)

Sodium metal changes to a white powder. This transformation indicates the formation of a new substance, typically involving reaction with other elements like oxygen, implying a change in chemical composition. Therefore, it is a chemical change.

Key concepts

State of Matter

Matter typically exists in three primary states: solid, liquid, and gas. Each state is characterized by distinct properties, such as shape and volume. For instance, solids maintain a fixed shape and volume, while liquids adjust to the shape of their container but retain a constant volume. Gases, on the other hand, fill the entire space available to them, changing both their shape and volume smoothly.
When matter changes from one state to another, such as water vapor (steam) condensing into liquid water and freezing into ice, it undergoes a physical change. The molecules of the substance rearrange to occupy different volumes and shapes, but the chemical structure remains the same.
This means the process affects only the physical state, not the chemical composition of the substance. Examples of state changes include melting, freezing, condensation, and evaporation. These transformations are typically reversible, and no new substances are formed, which is key to identifying them as physical changes.

Chemical Reaction

A chemical reaction involves the transformation of one or more substances into new substances with different chemical properties. During a chemical reaction, the molecules interact and bonds are broken and formed, leading to changes in the chemical structure.
For example, when hydrogen and oxygen gases explode to form water, a chemical reaction occurs. The chemical bonds between hydrogen atoms and oxygen atoms are rearranged, resulting in the formation of water molecules. This process is irreversible under normal conditions, as new substances with distinct properties are formed.
Indicators like color change, gas production, heat release, or the formation of a precipitate often signal that a chemical reaction has occurred. These changes are critical as they demonstrate the reorganization of chemical bonds and the creation of new material.

Physical Transformation

Physical transformations involve changes that affect a substance's form but not its chemical identity. Common physical changes include changes in shape, size, and state of matter, like the vaporization of bromine from a liquid to a gas.
These transformations generally do not create new substances. Instead, they alter physical characteristics, such as phase or size, without affecting the molecular structure.
Physical transformations are often reversible since the material's intrinsic chemical properties remain unchanged. Understanding the distinction between physical and chemical changes helps in predicting how substances will behave under different conditions. For example, crushing a can or dissolving sugar in water indicates a change that is physical, because no new product is formed.