3.1.2 Changes of State

In this lesson, we will explore how substances undergo physical changes when they change state, such as melting, freezing, boiling, evaporating, condensing, or sublimating. We will also discuss the concept of conservationThe professional care, preservation, and restoration of archaeological materials and sites, often requiring scientific expertise. of mass during these changes and how they differ from chemical changes.

Changes of State and Conservation of Mass

Changes of state refer to the physical transformations that substances undergo when transitioning between different states, such as solid, liquid, and gas. This happens through melting, freezing, boiling, evaporating, condensing, and sublimating.

Conservation of Mass:

The principle of conservation of mass states that mass is neither created nor destroyed during a physical or chemical change. When substances change state, the total mass of the system remains constant. The mass of the substance undergoing the change is conserved.

Reversibility of Changes of State

Now, let's explore the reversible nature of changes of state and how they differ from chemical changes.

Reversibility:

• Physical Changes: Changes of state are physical changes that can be reversed by altering the conditions such as temperature or pressure.
• Recovery of Original Properties: When a substance undergoes a change of state, it can recover its original properties if the change is reversed. For example, when ice melts to form water, it can freeze back into ice by reducing the temperature.

Differentiating from Chemical Changes:

• Chemical Changes: Chemical changes involve the rearrangement of atoms and the formation of new substances with different properties. These changes are not easily reversible, and the original substance cannot be recovered.
• Examples of Chemical Changes: Examples of chemical changes include combustion, rusting, and digestion, where the original substances are transformed into new substances with different properties.

Practical Examples

Let's discuss some practical examples that demonstrate changes of state and the conservation of mass.

Melting and Freezing:

• When ice (solid water) is heated, it melts and changes into liquid water. Conversely, when liquid water is cooled, it freezes and changes back into solid ice.
• Conservation of Mass: The total mass of the ice-water system remains constant during the melting and freezing processes.

Evaporation and Condensation:

• When water is heated, it evaporates and changes into water vapour (gaseous state). Conversely, when water vapour cools, it condenses and changes back into liquid water.
• Conservation of Mass: The total mass of the water-vapour system remains constant during the evaporation and condensation processes.

Sublimation and Deposition:

• Sublimation refers to the direct change of a solid into a gas, bypassing the liquid state. For instance, solid dry ice (frozen carbon dioxide) sublimates to form carbon dioxide gas. Conversely, deposition occurs when a gas changes directly into a solid, such as water vapour depositing onto a cold surface to form frost.
• Conservation of Mass: The total mass of the substance undergoing sublimation or deposition remains constant.

Conclusion

We explored changes of state, conservation of mass, and the reversible nature of these physical transformations. Changes of state involve the conversionThe act of changing someone's religion. between solid, liquid, and gas states, and during these changes, the mass of the substance is conserved. Unlike chemical changes, changes of state are reversible, and the substance can recover its original properties if the change is reversed.