Question

State whether each of the following is a physical or a chemical change: (a) burning (b) condensing (c) freezing (d) rusting

Short answer

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

Step-by-step solution

Understanding Burning

Burning is a process where a substance reacts with oxygen to produce heat and light, often resulting in new substances being formed. Typically, the original material is transformed into ash, gases, and other substances. This is a chemical change because new substances with different properties are produced.

Examining Condensing

Condensing is when a gas changes into a liquid state, typically by cooling. The substance's chemical composition remains unchanged during this phase transition. This is a physical change because it involves a change of state without forming a new substance.

Evaluating Freezing

Freezing is the process where a liquid becomes solid as a result of temperature decrease. The chemical makeup of the substance does not alter, only its physical state. This is a physical change because the composition of the material remains the same.

Analyzing Rusting

Rusting occurs when iron reacts with oxygen and moisture to form a new substance called iron oxide (rust). This involves a change in the chemical composition and the formation of a new substance. Therefore, rusting is a chemical change.

Key concepts

Chemical Change

Chemical changes occur when substances undergo transformations that result in the formation of new substances with different properties. These changes are usually irreversible without another chemical reaction.
An example of a chemical change is rusting, where iron reacts with oxygen and moisture to form iron oxide, a different substance entirely from the starting materials. Rusting involves a rearrangement of the bonds between atoms, leading to changes in chemical composition.

• Reaction with oxygen: Rusting and burning both involve reactions with oxygen, but at different scales and speeds.
• Formation of new substances: In chemical changes, such as burning, new materials (like carbon dioxide and water vapor in the case of burning wood) are created from the reactants.
• Irreversibility: Once a chemical change occurs, the process is difficult, if not impossible, to reverse without undergoing another chemical reaction.

A characteristic signal of a chemical change can include the emission of heat and light, the formation of a gas, or a color change.

Physical Change

Physical changes involve alterations to the state or physical form of a substance without changing its chemical identity. During these changes, the molecular composition of the material remains the same.
A common example is the freezing of water. When water freezes, it changes from a liquid to a solid state, but chemically, it remains H₂O.
Key characteristics of physical changes include:

• State change: The transition from one state to another (e.g., liquid to solid, as in freezing) is a classic example of a physical change.
• Reversibility: Most physical changes are reversible, meaning the original state can be restored, like melting ice to create liquid water again.
• No new substance: The composition of the material does not change; only the form does.

Physical changes contrast with chemical changes in that the substance itself remains unchanged at the molecular level.

Phase Transition

Phase transitions are specific types of physical changes where a substance changes from one state of matter to another—solid, liquid, or gas—due to temperature or pressure changes.
These transitions are significant in our daily lives and highlight the reversible nature of physical changes.
Examples of phase transitions include:

• Condensing: When water vapor condenses into liquid water, it experiences a physical change involving the movement of molecules from a gaseous to a liquid state.
• Freezing: The transformation of liquid water into ice is another example of a phase transition where the molecules slow down and arrange into a structured solid form.
• Boiling: This occurs when a liquid changes into gas upon heating, like when water boils to form steam.

During phase transitions, energy is either absorbed or released, but the molecular structure of the substance remains unchanged, ensuring that the transition is reversible.