Understanding the State of Matter

The state of matter (solid, liquid, gas) is determined by the relationship between pressure and temperature. While volume can change, it is not an independent determinant; it is a consequence of pressure and temperature changes, especially in gases. For a given substance, the phase (state) is defined by specific pressure and temperature conditions.

Step-by-Step Explanation:

Step 1: Consider the phase diagram of a substance. A phase diagram plots pressure on the y-axis and temperature on the x-axis, with lines separating the solid, liquid, and gas regions. This shows that pressure and temperature directly determine the state.

Step 2: For example, water at 1 atm pressure:

• Below 0°C, it is solid (ice).
• Between 0°C and 100°C, it is liquid.
• Above 100°C, it is gas (steam).

Changing the pressure changes these transition temperatures.

Step 3: Volume is not a primary determinant because it depends on pressure and temperature. For instance, compressing a gas (increasing pressure) at constant temperature can liquefy it (change state), but the key variable is pressure, not volume itself.

Step 4: Temperature alone is insufficient because pressure also matters. Ice can melt either by increasing temperature (at constant pressure) or by decreasing pressure (at constant temperature, like in ice skating).

Final Answer: Pressure and temperature determine the state of matter.

Related Topics:

• Phase Diagrams: Graphs showing the states of a substance at different pressures and temperatures.
• Triple Point: The unique pressure and temperature where all three states coexist.
• Critical Point: The point beyond which liquid and gas phases are indistinguishable.
• Ideal Gas Law: Relates pressure, volume, and temperature for gases, but does not define state changes; it applies only within a single phase.

Key Formulae and Theory:

Clapeyron Equation (for phase boundaries): Describes how the pressure-temperature relationship changes at a phase transition.

$\frac{dP}{dT}=\frac{\Delta H}{T\Delta V}$

where $\Delta H$ is the enthalpy change of transition, $T$ is temperature, and $\Delta V$ is the volume change.

Theory: The state of matter is governed by the balance between intermolecular forces and thermal energy. Pressure affects intermolecular distances, and temperature affects thermal motion. Together, they determine whether molecules form a solid, liquid, or gas.