States of Matter

Theory — Matter, Temperature, and Phase Changes

The Kinetic Theory of Matter

All matter is made of particles (atoms or molecules) in constant motion. Temperature is a measure of the average kinetic energy of those particles: the hotter the substance, the faster its particles move on average. The three common states — solid, liquid, gas — differ in how tightly the particles are held together and how freely they move.

Temperature and Molecular Motion Temperature ∝ average kinetic energy of the particles
Absolute (Kelvin) scale: T(K) = T(°C) + 273
At 0 K (absolute zero, −273 °C) molecular motion is at a minimum.

The Three States

In a solid, particles are locked in a fixed arrangement and only vibrate in place, so a solid keeps its shape and volume. In a liquid, particles are still close together but can slide past one another, so a liquid keeps its volume but takes the shape of its container. In a gas, particles are far apart and move freely at high speed, so a gas expands to fill any container.

Solid

Particles vibrate in a fixed lattice. Definite shape and volume. Strongest attractions, lowest energy.

Liquid

Particles touch but flow past each other. Definite volume, takes container's shape. Moderate energy.

Gas

Particles far apart, moving freely and fast. No fixed shape or volume; fills the container. Highest energy.

Phase Changes

Adding heat to a substance raises its temperature — until it reaches a phase-change point. At the melting point a solid becomes a liquid; at the boiling point a liquid becomes a gas. The reverse changes (freezing, condensation) release the same energy.

The Two Energy Equations Heating within a phase: Q = m · c · ΔT (temperature changes)
Changing phase: Q = m · L (temperature constant)

c = specific heat · L = latent heat (L_f melting, L_v boiling)

The Heating Curve and Latent Heat

If you heat ice steadily and plot temperature against time, the graph rises, then flattens at 0 °C while the ice melts, rises again through the liquid range, flattens again at 100 °C while the water boils, then rises through the steam range. During each flat plateau the temperature does not change even though heat is still being added. This "hidden" energy is the latent heat — it goes into breaking the attractions between particles (rearranging them into the new phase) rather than speeding them up.

Why Temperature Holds Steady During a Phase Change During melting/boiling, added heat breaks intermolecular bonds,
not raises kinetic energy — so the temperature stays constant.

Sloped part = temperature rising · Flat part = phase changing

Water property (at 1 atm)	Value
Melting / freezing point	0 °C (273 K)
Boiling / condensation point	100 °C (373 K)
Specific heat of ice	2.09 J/g·°C
Specific heat of liquid water	4.18 J/g·°C
Latent heat of fusion (melting)	334 J/g
Latent heat of vaporization (boiling)	2260 J/g

Instructions — Running the Virtual Experiment

The Particle View tab lets you watch molecules respond to heat; the Heating Curve tab records temperature versus heat added. Record your observations in your lab notebook.

Experiment 1 — Watching the Phases (Particle View tab)

Open Simulation → Particle View. Start at a low temperature and observe the particles locked in a vibrating lattice (solid).

Slowly raise the temperature with the Heat control. Note the temperature at which the lattice breaks up and the particles begin to flow past one another (melting), and the temperature at which they spread out to fill the container (boiling).

Use the Cool control to reverse the process. Confirm the gas condenses back to a liquid and the liquid freezes back to a solid at the same temperatures.

Experiment 2 — The Heating Curve of Water (Heating Curve tab)

Open Heating Curve. The sample is 1 g of ice starting at −20 °C. Press Add heat steadily and watch the temperature trace build up.

Identify the two flat plateaus — one at 0 °C (melting) and one at 100 °C (boiling). Note that during these plateaus heat is still being added but the temperature does not rise.

Click Label the regions. Confirm the five regions (heating ice, melting, heating water, boiling, heating steam) and relate the flat parts to latent heat and the sloped parts to specific heat.

Simulation — Molecular Motion & the Heating Curve

molecules

Faster, more spread out = hotter. Slow lattice = solid.

Controls

Temperature −20 °C

Readout

Temperature— °C

In Kelvin— K

State—

—

Temperature vs heat added for 1 g of water, starting as ice at −20 °C.

Heating curve

Current point

Heat added0 J

Temperature−20 °C

Phasesolid (ice)

Team Questions

Question 1. Convert 25 °C to the Kelvin scale using T(K) = T(°C) + 273. (Type just the number in K)

Question 2. In which state — solid, liquid, or gas — are the particles locked in a fixed arrangement, vibrating but not moving past one another? (One word)

Question 3. While ice is melting at 0 °C, you keep adding heat. What happens to the temperature during the melting — does it rise, fall, or stay the same? (One word)

Question 4. How much heat is needed to melt 50 g of ice at 0 °C? Use Q = mL with L_f = 334 J/g. (Type just the number in J)

Question 5. How much heat is needed to raise the temperature of 50 g of liquid water from 0 °C to 100 °C? Use Q = mcΔT with c = 4.18 J/g·°C. (Type just the number in J)

Question 6. Temperature is a measure of the average ___ energy of the particles in a substance. (One word)

Question 7 — Challenge. It takes 334 J to melt 1 g of ice but 2260 J to boil 1 g of water. Why is so much more energy needed to boil than to melt? (One or two words for the key idea — what must the molecules do during boiling?)

Example Lab Report

Sample report demonstrating the expected format and level of detail. Use as a guide for your own submission.

States of Matter: Molecular Motion, Phase Changes, and the Heating Curve

Physics | Section: [Your Section] | Date: [Date]

Lab Members: [Names of all members present]

Purpose

To relate temperature to molecular motion across the solid, liquid, and gas states, and to investigate the heating curve of water — explaining why the temperature stays constant during melting and boiling while heat continues to be added. The lab connects the sloped portions of the curve to specific heat and the flat portions to latent heat.

Theory

Temperature measures the average kinetic energy of particles. Within a single phase, adding heat raises the temperature according to Q = mcΔT. At a phase-change point, added heat goes into breaking intermolecular bonds rather than raising kinetic energy, so the temperature stays constant while the substance changes phase, requiring Q = mL.

T(K) = T(°C) + 273
Q = mcΔT (within a phase) · Q = mL (phase change)
Water: melt 0 °C, boil 100 °C; L_f = 334 J/g, L_v = 2260 J/g

Calculations — Heating 1 g of water from −20 °C to steam at 120 °C

1. Heat ice −20 → 0 °C: Q = mcΔT = (1)(2.09)(20) = 41.8 J

2. Melt ice at 0 °C: Q = mL_f = (1)(334) = 334 J (flat plateau)

3. Heat water 0 → 100 °C: Q = (1)(4.18)(100) = 418 J

4. Boil water at 100 °C: Q = mL_v = (1)(2260) = 2260 J (flat plateau)

5. Heat steam 100 → 120 °C: Q = (1)(2.01)(20) = 40.2 J

Total: 41.8 + 334 + 418 + 2260 + 40.2 = 3094 J

Results Table — Regions of the Heating Curve (1 g water)

Region	What happens	Equation	Heat (J)	Temperature
1	Heating ice	Q = mcΔT	41.8	rises −20→0 °C
2	Melting (fusion)	Q = mL_f	334	constant at 0 °C
3	Heating water	Q = mcΔT	418	rises 0→100 °C
4	Boiling (vaporization)	Q = mL_v	2260	constant at 100 °C
5	Heating steam	Q = mcΔT	40.2	rises 100→120 °C

Discussion

The particle simulation showed the expected progression: at low temperature the molecules vibrated in a fixed lattice (solid); as heat was added they broke free and flowed past one another (liquid); with more heat they spread apart and moved rapidly to fill the container (gas). Cooling reversed the sequence at the same temperatures, confirming that freezing and condensation occur at the melting and boiling points.

The heating curve showed two flat plateaus, at 0 °C and 100 °C, where the temperature held constant even though heat was still being added. These plateaus correspond to melting and boiling, where the added energy (latent heat) breaks intermolecular attractions instead of increasing molecular speed. The sloped regions, where the temperature rose, correspond to heating within a single phase and obey Q = mcΔT. Boiling required far more energy than melting (2260 J/g versus 334 J/g) because vaporization must separate the molecules completely, overcoming essentially all of their mutual attraction, whereas melting only loosens the lattice.

Conclusion

The experiment confirmed that temperature reflects average molecular kinetic energy and that the three states of matter differ in molecular arrangement and motion. The heating curve demonstrated that temperature rises within a phase (Q = mcΔT) but holds constant during a phase change (Q = mL), with the latent heat of vaporization much larger than the latent heat of fusion.

Practice Questions

Show all work and include units in your answers.

Question 1

Convert the following temperatures to Kelvin: 0 °C, 37 °C (body temperature), and −40 °C. Then convert 350 K back to Celsius.

Hint: T(K) = T(°C) + 273, and T(°C) = T(K) − 273.

Question 2

How much heat is required to raise the temperature of 200 g of liquid water from 25 °C to 75 °C? (c = 4.18 J/g·°C)

Hint: Q = mcΔT, with ΔT = 50 °C.

Question 3

How much heat is needed to completely melt 30 g of ice already at 0 °C? How much to then boil that 30 g of water away once it reaches 100 °C? (L_f = 334 J/g, L_v = 2260 J/g)

Hint: Q = mL for each phase change.

Question 4

Sketch the heating curve for ice taken from −10 °C to steam at 110 °C. Label the two plateaus and state what is happening to the molecules during each.

Hint: plateaus at 0 °C (melting) and 100 °C (boiling); during each, bonds break and temperature is constant.

Question 5

Explain, in terms of particle motion and spacing, why a gas fills its entire container while a liquid does not.

Hint: gas particles are far apart with negligible attraction and high speed; liquid particles still attract and stay together.

Question 6 — Challenge

Calculate the total heat needed to convert 25 g of ice at −15 °C into steam at 110 °C. Use c_ice = 2.09, c_water = 4.18, c_steam = 2.01 J/g·°C, L_f = 334, L_v = 2260 J/g.

Hint: five steps — heat ice, melt, heat water, boil, heat steam — then add them all.