States of matter
Why does ice float, why does water boil at 90 °C high in the mountains, and why does a syringe get harder to push? Move the temperature and watch particles lock into a crystal, slide as a liquid and fly apart as a gas, then follow a real heating curve and Boyle's law with numbers from OpenStax and NIST.
About this course
Put a beaker of ice at −12 °C on the bench. OpenStax shows what happens: after ten minutes the ice has warmed to 0 °C and a little has melted. Thirty minutes later more has melted, but the thermometer still reads 0 °C. Only when the last of the ice is gone does the temperature start to climb, reaching 22.2 °C. The ice kept absorbing heat from the room the whole time, and this course explains where that heat went.
The six units are what matter is, particles in solids, liquids and gases, melting and freezing, evaporation and boiling, heating curves, and gas pressure with Boyle's law. The water molecules do not change: H₂O is H₂O in ice, water and steam. What changes is how far apart the molecules are, how strongly they hold on to one another and how fast they move. Evaporation happens only at the surface and at any temperature, while boiling happens all through the liquid at one particular temperature. The heating curve is worked with real numbers: turning 135 g of ice at −15 °C into steam at 120 °C takes 416 kJ, and 305 kJ of that goes into boiling alone. OpenStax rounds each step and reports 416 kJ, while adding unrounded values gives 415.4 kJ, which the lab model shows.
The last unit explains gas pressure with moving molecules. Squeeze the gas into half the volume and each part of the wall is hit twice as often, so the pressure doubles. In a practice question a gas fills 2.0 L at 100 kPa and is squeezed to 0.50 L at the same temperature, and you find the new pressure.
Before you start
You can start here with no chemistry behind you, since the first unit asks what matter is at all. The heating curve unit counts heat in kilojoules per mole, so the section on moles in Chemical bonds helps, and Boyle's law questions need only multiplication and division.
When you finish
You will be able to tell mass from weight, describe how particles behave in a solid, a liquid and a gas, and explain why ice sits at 0 °C while it melts. You can say why water boils at about 90 °C in Leadville, calculate the heat needed to melt ice or boil water from its moles, and use Boyle's law to find a new gas volume.
- 1What matter isAn astronaut on the Moon weighs one sixth of what she weighs on Earth, but her mass does not change at all.
- 2Particles in solids, liquids and gasesNitrogen molecules in the air around you at 30 °C move at about 519 m/s, faster than a passenger jet.
- 3Melting and freezingA beaker of ice warmed by room air sat at exactly 0 °C for more than half an hour while it melted.
- 4Evaporation and boilingIn Leadville, Colorado, about 3,100 m up, air pressure is about 68 kPa and water boils at about 90 °C.
- 5Heating curvesTurning 135 g of ice at −15 °C into steam at 120 °C takes 416 kJ, and 305 kJ of that goes into boiling alone.
- 6Gas pressure and Boyle's lawIn 1662 Robert Boyle squeezed trapped air to a quarter of its volume and found that its pressure had gone up about four times.
Sources for this course (8)
- OpenStax Chemistry 2e (openstax.org/books/chemistry-2e): 1.2 Phases and Classification of Matter (matter, three states, plasma, mass and weight, Moon 1/6, conservation of matter, sand and clouds)
- 9.2 gas laws (Boyle's law P1V1 = P2V2, syringe 15.0 mL 13.0 psi to 7.5 mL 26 psi, breathing, divers: every 33 ft of sea water adds 1 atm and halves the air volume at 33 ft, Amontons ~1700, Gay-Lussac ~1800, Charles, STP 273.15 K and 101.325 kPa, -273 °C absolute zero)
- 9.5 kinetic-molecular theory (average kinetic energy proportional to kelvin temperature, KEavg = 3/2 RT, urms = sqrt(3RT/M), N2 at 30 °C 519 m/s, O2 at -23 °C 441 m/s)
- 10.3 phase transitions (vapor pressure, normal boiling point at 1 atm = 101.3 kPa, Leadville 68 kPa about 90 °C, ΔHvap 44.01 kJ/mol at 25 °C and 40.67 kJ/mol at 100 °C, 1.5 L sweat 3600 kJ, ΔHfus 6.01 kJ/mol, ice at -12 °C staying at 0 °C while melting, CO2 sublimation 26.1 kJ/mol, frost is deposition, heating curve example 135 g ice -15 °C to steam 120 °C = 416 kJ with c ice 2.09, water 4.18, steam 1.86 J/g °C, burner turned up does not raise boiling temperature)
- NIST Chemistry WebBook phase change data (webbook.nist.gov: ethanol Tfus 159 K, Tboil 351.5 K; nitrogen Tfus 63.3 K, Tboil 77.34 K; oxygen Tfus 54.8 K, Tboil 90.2 K; mercury Tfus 234.31 K, Tboil 629.81 K; CO2 triple point 216.58 K at 5.185 bar, solid vapour-pressure equation Giauque and Egan 1937 giving 194.7 K at 1 atm)
- USGS Water Science School, Water Density (0.99987 g/cm³ at 0 °C, 1.00000 at 4 °C, 0.95865 at 100 °C; ice about 9% less dense)
- Britannica, Boyle's law (Robert Boyle 1662, air compressed by mercury; Edme Mariotte 1676)
- Boyle's 1662 table (48 spaces at 29 2/16 in, 12 spaces at 117 9/16 in of mercury) as reprinted in J. Appl. Physiol. 87 (1999) 1543.
Checked: 2026-10-17