States of Matter and the Kinetic Molecular Theory
States of matter
Matter exists in three common states: solid, liquid and gas. The arrangement, motion and energy of particles in each state explain the macroscopic properties we observe. Changes between these states occur when heat is added or removed; these are called changes of state (phase changes).
Diffusion and Brownian motion
Diffusion is the process by which particles spread from a region of higher concentration to a region of lower concentration until they are evenly distributed. A common classroom example is a drop of food colouring placed in water: the colour gradually spreads through the water without any stirring. Diffusion occurs because particles are in constant random motion.
In 1828, Robert Brown observed that tiny pollen grains suspended in water moved in a rapid, irregular fashion. This random motion of small particles suspended in a fluid is called Brownian motion. Brownian motion is a visible consequence of collisions between the suspended particles and the much smaller, rapidly moving molecules of the fluid; it is closely related to diffusion.
Definitions of common phase-change terms
- Melting - the change from solid to liquid. The melting point is the temperature at which this phase change occurs for a pure substance.
- Freezing - the change from liquid to solid. The freezing point is the temperature at which a liquid becomes a solid.
- Evaporation - the process by which molecules escape from the surface of a liquid and become a gas. Evaporation can occur at temperatures below the boiling point.
- Boiling - a rapid form of vapour formation that occurs throughout a liquid when its vapour pressure equals the external pressure. The boiling point is the temperature at which a liquid changes to a gas under a given pressure.
- Condensation - the change from gas to liquid.
- Sublimation - the change from solid directly to gas. The reverse is called deposition.
If the melting point and boiling point of a substance are known, one can determine whether the substance will be a solid, liquid or gas at any given temperature under the same pressure.
Heating and cooling curves - experimental observation
A heating curve shows how temperature changes with time as a substance is heated from solid to liquid to gas. A cooling curve shows temperature changes as the substance cools from gas to liquid to solid. An important experimental observation is that during a phase change (melting or boiling) the temperature remains constant until the whole sample has changed state. The added heat is used to break intermolecular forces rather than to increase kinetic energy; this absorbed energy during a phase change is called latent heat. Aim: To investigate the heating and cooling curve of water and to observe constant-temperature plateaux during phase changes. Apparatus: beakers, ice, a heat source (e.g., Bunsen burner or hot plate), a thermometer, water, stopwatch. Method (heating ice to steam): Safety: Handle hot beakers and the heat source carefully. Do not touch hot glassware with bare hands. Expected results and interpretation: As ice is heated, the temperature rises until melting begins. During melting the temperature remains approximately constant until all ice has become liquid. After melting, temperature rises again until boiling begins; during boiling the temperature remains approximately constant again. These flat portions correspond to the energy being used to overcome intermolecular forces (latent heat) rather than to raise temperature.Formal experiment: Heating and cooling curve of water
The kinetic molecular theory
The kinetic molecular theory (KMT) explains the macroscopic behaviour of matter in terms of the motion, energy and interactions of particles (atoms or molecules). The core ideas are:
- All matter is made of a large number of very small particles (atoms or molecules).
- These particles are in continuous random motion; the average kinetic energy of the particles increases with temperature.
- There are forces of attraction between particles that vary in strength depending on the substance and on particle separation.
- The spaces between particles and the strengths of attractive forces determine the physical state (solid, liquid, gas).
Using these ideas we can explain characteristic features of each state of matter and why substances change state when heated or cooled.
Particle description of the three states
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Particle arrangement | Closely packed in a regular pattern (often a lattice) | Close together but with no fixed arrangement | Far apart with random arrangement |
| Particle motion | Vibrate about fixed positions | Can move/slide past one another (flow) | Move rapidly in all directions |
| Energy of particles | Lowest (relative) | Higher than solids but lower than gases | Highest |
| Spaces between particles | Very small | Moderate | Large |
| Interparticle forces | Strong - maintain fixed shape | Weaker than solids but still significant | Very weak - particles act independently |
| Compressibility | Nearly incompressible | Nearly incompressible | Highly compressible |
Example (copper): In solid copper the atoms vibrate about fixed positions in a lattice. On heating, atomic energy increases and atoms can break some bonds and move past one another to form liquid copper. Further heating provides enough energy for atoms to separate widely and form copper vapour (gas). Gas particles, having high energy and weak attractions, fill the available container volume.
Activity: Model the three phases of water
Use marbles, small balls of clay or playdough to represent water molecules and arrange them to show ice, liquid water and steam.
- Arrange marbles in a close-packed regular pattern to represent the solid (ice). Discuss fixed shape and low particle motion.
- Arrange marbles with small gaps and allow them to move past one another to represent the liquid. Discuss flow and intermediate particle energy.
- Scatter marbles far apart to represent a gas (steam). Discuss large spaces and high particle motion.
Summary
- There are three common states of matter: solid, liquid and gas.
- Diffusion is movement of particles from regions of high concentration to low concentration; Brownian motion is the random motion of visible particles caused by collisions with molecules.
- Melting point and freezing point are temperatures of solid-liquid transitions. Boiling point is the temperature for liquid-gas transition at a given pressure.
- Evaporation occurs at the surface of a liquid; boiling occurs throughout a liquid when vapour pressure equals external pressure. Sublimation is solid to gas directly; deposition is gas to solid.
- During a phase change, temperature remains constant while energy is used to overcome or form intermolecular attractions; this energy is known as latent heat.
- The kinetic molecular theory explains macroscopic properties in terms of particle motion, energy and interparticle forces.
End of chapter exercises
1. Give one word or term for each of the following descriptions.
a. The change in phase from a solid to a gas.
b. The change in phase from liquid to gas.
2. Water has a boiling point of 100°C.
a. Define boiling point.
b. What change in phase takes place when a liquid reaches its boiling point?
3. Describe a solid in terms of the kinetic molecular theory.
4. Refer to the table below which gives the melting and boiling points of a number of elements and then answer the questions that follow.
