States of Matter and Changes of State
- You are holding a piece of ice in your hand.
- As it begins to melt, the solid ice transforms into liquid water.
- If you leave it out long enough, the water might evaporate, becoming an invisible gas in the air.
These changes illustrate the states of matter and their transformations, a cornerstone of chemistry.
The Three States of Matter: Solids, Liquids, and Gases
- Matter exists in three primary states: solids, liquids, and gases.
- The state of a substance depends on the arrangement and energy of its particles.
Solids: Fixed Shape and Volume
Solids
In solids, particles are tightly packed in a fixed arrangement. They vibrate in place but do not move freely, giving solids a definite shape and volume.
A cube of ice retains its shape unless it melts.
TipThink of a solid as a group of dancers standing in fixed positions on a stage: they can sway slightly but cannot move around.
Key properties of solids:
- Shape: Fixed.
- Volume: Fixed.
- Particle Movement: Particles vibrate in place.
Liquids: Fixed Volume, Variable Shape
Liquids
Liquids have particles that are close together but not in a fixed arrangement. This allows them to flow and take the shape of their container while maintaining a constant volume.
Water in a glass conforms to the shape of the glass but does not expand to fill it.
Analogy- Consider liquid particles as marbles in a shallow bowl.
- They can slide past each other but remain confined to the bowl.
Key properties of liquids:
- Shape: Takes the shape of the container.
- Volume: Fixed.
- Particle Movement: Particles can move and slide past one another.
Gases: No Fixed Shape or Volume
Gases
In gases, particles are far apart and move freely in all directions. This explains why gases expand to fill the shape and volume of their container.
The air in a balloon spreads evenly to occupy the entire space.
Key properties of gases:
- Shape: Takes the shape of the container.
- Volume: Expands to fill the container.
- Particle Movement: Particles move rapidly and independently.
- Do not confuse gases with vapors.
- A vapor refers to the gaseous state of a substance that is typically a liquid or solid at room temperature, like water vapor.
State Symbols in Chemical Equations
In chemical equations, the physical state of a substance is indicated by a state symbol in parentheses:
- (s): Solid
- (l): Liquid
- (g): Gas
- (aq): Aqueous (dissolved in water)
$$\text{H}_2\text{O}(s) \rightarrow \text{H}_2\text{O}(l)$$ This equation represents the melting of ice into liquid water.
HintRemember that (aq) is used only for substances dissolved in water, such as NaCl(aq) for saltwater.
Changes of State: Transformations Between Solids, Liquids, and Gases
- Changes of state occur when a substance absorbs or releases thermal energy.
- These transformations are physical changes, meaning the chemical composition of the substance remains unchanged.
- The energy involved affects the potential energy of the particles, not their temperature during the phase change.
