Physical states of matter: Class 9 Chemistry

Key points

Matter exists mainly as solid, liquid and gas. A fourth state, plasma, is a hot ionised gas found in the Sun, stars and lightning. The state of a substance depends on how strongly its particles attract each other and how fast they move.

The kinetic molecular theory of gases says:

• Gases are made of very small particles with large spaces between them.
• The particles move constantly in straight lines and in random directions.
• Collisions between particles and with the walls are elastic, so no energy is lost.
• There are no attractive or repulsive forces between particles.
• The average kinetic energy is proportional to the absolute (kelvin) temperature.

Gas pressure is caused by the particles hitting the walls of the container. Pressure is force per unit area. The SI unit is the pascal (Pa), which is 1 N/m². Other units are: 1 atm = 101325 Pa = 760 mmHg (torr). Volume is measured in dm³ (1 dm³ = 1000 cm³ = 1 litre). Temperature must be in kelvin: T (K) = t (°C) + 273.

Boyle's law: at constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure. So P × V is constant and P₁V₁ = P₂V₂. When pressure doubles, volume halves, because the same particles hit the walls more often in a smaller space. The graph of P against V is a curve (hyperbola), and the graph of P against 1/V is a straight line through the origin.

Charles's law: at constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature. So V/T is constant and V₁/T₁ = V₂/T₂. A graph of V against T in kelvin is a straight line through the origin. Extending the graph of V against t in °C shows that volume would become zero at −273 °C. This temperature is absolute zero, or 0 K.

Avogadro's law: equal volumes of all gases at the same temperature and pressure contain the same number of molecules. One mole of any gas occupies 22.4 dm³ at STP (0 °C and 1 atm). One mole contains 6.02 × 10²³ particles.

Diffusion is the spreading of a gas to fill the space available, due to random motion. Effusion is the escape of a gas through a tiny hole. Graham's law of diffusion states that at the same temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its density or molar mass. Light gases such as hydrogen diffuse faster than heavy gases such as oxygen.

Gases show real behaviour at low temperature and high pressure. Under these conditions the particles are close together and the attractions between them matter, so gases can be liquefied. A gas can be liquefied by cooling and by applying pressure. Below a certain temperature called the critical temperature, pressure alone is enough.

Properties of liquids:

• Fixed volume but no fixed shape: they take the shape of the container.
• Particles are close but can slide past each other, so liquids flow and are hard to compress.
• Evaporation is the escape of faster molecules from the surface at any temperature. It increases with temperature, surface area and wind, and it causes cooling.
• Vapour pressure is the pressure of the vapour in equilibrium with its liquid in a closed container. It rises with temperature.
• Boiling point is the temperature at which the vapour pressure equals the external pressure. At high altitude the pressure is lower, so water boils below 100 °C.

Other liquid properties: viscosity is the resistance to flow (honey is more viscous than water). Surface tension is the tendency of a liquid surface to contract and act like a stretched skin, due to attraction between molecules. It lets insects walk on water and makes drops spherical.

Solids have fixed shape and volume. Their particles vibrate about fixed positions. Crystalline solids (sodium chloride, sugar, diamond) have a regular repeating arrangement, sharp melting points and flat faces. Amorphous solids (glass, rubber, plastic) have no regular pattern and soften gradually over a range of temperature.

Allotropy is the existence of an element in two or more forms in the same physical state. Carbon forms diamond (hard, each atom bonded to four others, an insulator), graphite (layers, soft, conducts electricity) and fullerenes. Sulphur has rhombic and monoclinic forms. Sublimation is the change from solid directly to gas, as shown by iodine, naphthalene and dry ice.

Study tip: For every gas law problem, write the law, convert the temperature to kelvin first, substitute with units, then solve. Keep the units of pressure and volume the same on both sides. Learn the kinetic theory points in a list, because explanation questions use them as reasons.

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Common mistakes

• Using °C instead of K in Charles's law calculations.
• Writing Boyle's law without saying that temperature and mass are constant.
• Saying that evaporation happens only at the boiling point.
• Inverting the molar masses in Graham's law, so the heavier gas looks faster.
• Calling glass a crystalline solid. It is amorphous.

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