Chemistry: Atoms to AP Chemistry · Module 5 of 15
States of Matter & Gas Laws
How particle behavior explains the states of matter, and the laws that describe how gases respond to pressure and temperature.
Kinetic Molecular Theory & States of Matter
- Particles of matter are always in motion: solids have particles tightly packed and vibrating in place, liquids have particles close but able to slide past each other, and gases have particles far apart and moving freely.
- Temperature is a measure of the average kinetic energy of a substance's particles.
Why does a gas expand to fill its entire container while a solid keeps its shape?
Solution. Gas particles have high kinetic energy and negligible attraction to each other, so they spread out to fill all available space; solid particles are held in fixed positions by strong attractive forces.
Gas Laws
- Boyle's Law: at constant temperature, pressure and volume are inversely related (P₁V₁ = P₂V₂).
- Charles's Law: at constant pressure, volume and temperature (in Kelvin) are directly related (V₁/T₁ = V₂/T₂).
A gas occupies 4.0 L at 2.0 atm. What is its new volume if pressure increases to 4.0 atm at constant temperature?
Solution. Boyle's law: P₁V₁ = P₂V₂ → (2.0)(4.0) = (4.0)(V₂) → V₂ = 8.0 ÷ 4.0 = 2.0 L.
The Ideal Gas Law
- PV = nRT relates pressure, volume, moles, and temperature (in Kelvin) using the gas constant R.
- The ideal gas law lets you solve for any one variable if the other three are known.
How many moles of gas occupy 22.4 L at standard conditions (1 atm, 273 K), using R = 0.0821 L·atm/(mol·K)?
Solution. n = PV/RT = (1)(22.4) ÷ (0.0821 × 273) ≈ 22.4 ÷ 22.4 ≈ 1.0 mol — matching the fact that 1 mole of gas occupies 22.4 L at STP.
Phase Changes & Heating Curves
- Phase changes (melting, freezing, boiling, condensing) occur at constant temperature while energy is added or removed to break or form intermolecular forces, not to raise or lower temperature.
- A heating curve shows flat plateaus during phase changes and rising or falling slopes while the temperature is actually changing within one phase.
While ice is melting into water at 0°C, does the temperature rise?
Solution. No — the temperature stays at 0°C until all the ice has melted; the added energy goes into breaking the solid's intermolecular forces, not raising temperature.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. According to kinetic molecular theory, which state of matter has the most freely moving particles?
2. Using Boyle's Law, if a gas's volume doubles at constant temperature, what happens to its pressure?