Phases of Matter and Heating Curves

Phases of Matter at the Particle Level

Phase questions appear in the Structures and Properties of Matter and Energy core ideas and frequently use a graph or diagram as the cluster stimulus. You must connect the macroscopic observation (it melted, it boiled) to the particle-level cause (attractions, spacing, energy).

• Solid: particles are packed in fixed positions and only vibrate; a solid has definite shape and volume.
• Liquid: particles are close but can slide past one another; a liquid flows and takes the shape of its container while keeping a definite volume.
• Gas: particles are far apart with negligible attractions; a gas has no definite shape or volume and fills its container.

As you move solid to liquid to gas, particle spacing and energy increase while the strength of the attractions holding particles together effectively decreases.

A common cluster stimulus shows a particle diagram of each phase and asks you to identify which is the gas. The cue is spacing: gas particles are drawn far apart and randomly scattered, liquid particles are touching but disordered, and solid particles sit in a neat repeating pattern. Density usually follows the same order, with solids and liquids much denser than gases. Water is the famous exception, because solid ice is less dense than liquid water and floats, a result of its hydrogen-bonded open structure.

Phase Changes and Energy

A phase change is a physical change between states. Naming both directions matters on the exam:

A key Regents idea: a substance melts and freezes at the same temperature, and boils and condenses at the same temperature. For water these are 0 degrees Celsius and 100 degrees Celsius at standard pressure. Notice the energy symmetry in the table: every change that absorbs energy (melting, boiling, sublimation) has an exact reverse that releases the same amount of energy (freezing, condensation, deposition). The exam often pairs these, asking which change is exothermic, and the answer is always a particle becoming more ordered.

During any phase change the temperature does not change because the energy is rearranging particle attractions, not speeding particles up. This single fact links phase changes to the flat parts of a heating curve described below.

Reading a Heating Curve

A heating curve plots temperature (y-axis) versus heat added or time (x-axis) as a solid is steadily warmed into a gas. The graph has two kinds of segments, and telling them apart is the single most tested skill here.

Sloped (diagonal) segments

When the line slopes upward, temperature is rising. The added heat increases the average kinetic energy (faster particles). Only one phase is present. There are three sloped segments: warming the solid, warming the liquid, and warming the gas.

Flat (plateau) segments

When the line is horizontal, temperature is constant even though heat is still being added. The energy is going into potential energy (breaking the attractions between particles) during a phase change. The lower plateau is melting; the upper plateau is boiling. During a flat segment, kinetic energy does not change and two phases coexist.

A classic trap: students say the substance "stops absorbing energy" on a plateau. It is still absorbing energy; that energy just changes potential energy instead of temperature.

Vapor Pressure and Boiling (Vapor Pressure table, legacy Table H)

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid. A liquid boils when its vapor pressure equals the surrounding atmospheric pressure. The Vapor Pressure table (the section the legacy edition labeled Table H) graphs vapor pressure versus temperature for four liquids: propanone, ethanol, water, and ethanoic acid.

Key ways to use the Vapor Pressure table (legacy Table H):

1. The dotted horizontal line marks 101.3 kPa (standard pressure). Where a liquid's curve crosses that line is its normal boiling point (water = 100 degrees Celsius).
2. A liquid with higher vapor pressure at a given temperature is more volatile and has a lower normal boiling point.
3. At higher elevation, atmospheric pressure is lower, so liquids boil at a lower temperature.

Stronger intermolecular forces lower vapor pressure and raise the boiling point. Water, with hydrogen bonding, sits to the right of propanone and ethanol on the Vapor Pressure table (legacy Table H).

Reading a Vapor Pressure (legacy Table H) question

A typical item gives you a temperature and asks for the vapor pressure of one liquid, or gives a pressure and asks for the boiling temperature. The procedure is the same: find the value on one axis, trace straight to the named curve, then read straight across to the other axis. If a question lowers the external pressure (for example to 80 kPa), draw a new horizontal line at 80 kPa and read where each curve crosses it; every liquid now boils at a lower temperature than it did at 101.3 kPa. Practice this trace until it is automatic, because Vapor Pressure (legacy Table H) clusters are quick points if you can read the graph.

Choosing the Right Heat Equation

The equation depends on which segment you are on. The 2025 reference tables supply water's heat constants (specific heat, heat of fusion, heat of vaporization — formerly grouped as Table B) and the Mathematical and Computational Models section (legacy Table T) supplies the formulas.

• Sloped segment (temperature changing): use q = mC(delta)T, where C is specific heat. For water C = 4.18 J/(g.K).
• Melting plateau: use q = mHf, where Hf is the heat of fusion (water: 334 J/g).
• Boiling plateau: use q = mHv, where Hv is the heat of vaporization (water: 2260 J/g).

Never use q = mC(delta)T on a flat segment, because delta-T is zero there and you would calculate q = 0, which is wrong. Match the equation to the segment first, then substitute with units.