States and properties of matter Notes

The Particle Model Explains the Behavior of All States of Matter

The particle model of matter is a simplified but powerful framework for understanding the behavior of solids, liquids and gases on a microscopic level.

At its core, the model assumes that:

1. All substances are made of tiny particles
   1. These particles may be atoms, molecules or ions.
2. Particles are in constant motion
   1. The higher the temperature, the faster the particles move (they have more kinetic energy).
3. Forces of attraction exist between particles
   1. These forces can be stronger or weaker, holding particles closely together (as in many solids) or allowing them to move more freely (as in liquids and gases).
4. There are spaces between particles
   1. The amount of space between particles helps explain properties such as density and compressibility.

The Relationship Between Particle Arrangements and Properties

Understanding how particles are arranged and how they move helps explain three key properties of solids, liquids and gases:

1. Density
2. Compressibility
3. Diffusion

You can think about the three states of matter like this:

Density

$$
\text { Density }=\frac{\text { mass }}{\text { volume }}
$$

1. In solids, particles are packed closely together, so solids often have high density.
2. Liquids are usually slightly less dense than solids because there is a little more space between particles.
3. Gases have very low density because their particles are far apart.

Compressibility

1. In solids, particles are locked in fixed, close positions with very little empty space between them. Solids are therefore almost incompressible.
2. Liquids also have particles close together, with only slightly more space than in solids, so they are nearly incompressible.
3. In gases, particles are widely spaced, so they can be pushed closer together. Gases are therefore highly compressible.

Diffusion

1. In solids, diffusion is extremely slow because particles can only vibrate in fixed positions.
2. In liquids, diffusion occurs slowly as particles can slide past each other.
3. In gases, diffusion is rapid because particles have high kinetic energy and large spaces between them, allowing them to spread out quickly.

The Kinetic Theory Explains Particle Motion

The kinetic theory of gases extends the particle model to explain the behaviour of gases in terms of the motion of their particles.

It states that:

1. Gas particles are in random, constant motion.
   1. They move in straight lines until they collide with each other or with the walls of the container.
2. Collisions between gas particles are (ideally) perfectly elastic.
   1. In a perfectly elastic collision, no kinetic energy is lost overall.
   2. Energy can be transferred between particles, but the total kinetic energy remains constant (as long as temperature stays constant).
3. The average kinetic energy of gas particles is directly proportional to the absolute temperature (in Kelvin).
   1. If the temperature in Kelvin doubles, the average kinetic energy of the particles also doubles.

Changes of State: From Solid to Gas and Back Again

Solids, Liquids, and Gases: The Three Classical States of Matter

Solids, liquids and gases have distinct properties because their particles are arranged and move differently.

Solids

1. Particles are tightly packed in a fixed, regular arrangement (a lattice in many solids).
2. Forces of attraction between particles are very strong.
3. Particles can only vibrate about fixed positions.
4. Solids have a fixed shape and a fixed volume.

Liquids

1. Particles are close together, but not in fixed positions.
2. Forces of attraction are strong enough to keep particles close, but weak enough to allow them to move past each other.
3. Liquids have a fixed volume, but they take the shape of their container and can flow.

Gases

1. Particles are far apart, with very weak (often negligible) forces of attraction between them.
2. Particles move randomly and rapidly in all directions.
3. Gases have no fixed shape and no fixed volume – they fill the entire container.

From Melting to Sublimation: The Processes of State Changes

1. When matter changes state, it undergoes physical changes such as melting, boiling, condensation, freezing, sublimation and deposition.
2. These changes can usually be reversed by heating or cooling.
3. They are physical changes because the chemical composition of the substance remains the same.

Latent Heat – Energy During State Changes

1. During a change of state, the temperature of the substance stays constant even though energy is being transferred.
2. This extra energy is called latent heat.
3. At the plateaus, average kinetic energy (temperature) stays the same, but potential energy of the particles changes as the arrangement becomes more or less ordered.

Melting and Freezing

Melting and freezing are opposite processes between the solid and liquid states.

Melting (solid → liquid)

1. When a solid is heated, particles gain kinetic energy and vibrate more strongly.
2. At the melting point, particles have enough energy to overcome some of the forces holding them in fixed positions.
3. The regular structure breaks down and the solid becomes a liquid.

During the process:

1. Energy is absorbed from the surroundings (endothermic).
2. Temperature stays constant while the solid melts – the energy goes into breaking intermolecular forces (latent heat), not raising temperature.

Freezing (liquid → solid)

1. When a liquid is cooled, particles lose kinetic energy and move more slowly.
2. At the freezing point, particles no longer have enough energy to move freely and settle into fixed positions.
3. The liquid becomes a solid with a more ordered arrangement.

During the process:

1. Energy is released to the surroundings (exothermic).
2. Temperature stays constant while the liquid solidifies – energy is released as intermolecular forces form and particles become more ordered.

Boiling and Condensation

Boiling (liquid → gas)

1. When a liquid is heated, particles gain kinetic energy.
2. At the boiling point, particles have enough energy to break the forces of attraction and form a gas.
3. Bubbles of gas form throughout the liquid and rise to the surface.

During the process:

1. Energy is absorbed from the surroundings (endothermic).
2. Temperature remains constant while all the liquid changes to gas.
3. The extra energy increases the potential energy of the particles as they move much further apart.

Condensation (gas → liquid)

1. When a gas cools, particles lose kinetic energy and move more slowly.
2. They come closer together and the forces of attraction become more important.
3. Particles form a liquid with a more ordered arrangement.

During the process:

1. Energy is released to the surroundings (exothermic).
2. Temperature stays constant while gas turns into liquid – energy is released as particles form stronger interactions in the liquid.

Evaporation

1. Evaporation is a slower process than boiling and occurs at the surface of a liquid at any temperature below the boiling point.
   1. Some surface particles have enough kinetic energy to escape into the gas phase.
   2. These high-energy particles leave the liquid, so the average kinetic energy of the remaining particles decreases.
2. As a result, evaporation causes cooling.

Sublimation and Deposition

1. Sublimation: A change from solid directly to gas without passing through the liquid state.
2. Deposition: The reverse process, where a gas changes directly to a solid without becoming a liquid.

The Role of Gases in Diffusion, Compressibility, and Density

Gases show some extreme forms of the properties we introduced with the particle model.

Diffusion – spreading out evenly

In gases, particles move rapidly and randomly, so they spread out to fill the entire available space.

Compressibility – gases can be squashed

1. Gas particles are far apart, with a lot of empty space between them.
2. When a gas is compressed, particles are pushed closer together, reducing the volume.
3. This is why gases are much more compressible than liquids and solids.

Density – less mass in more space

1. Gases have low density because their particles are spread far apart.
2. There are fewer particles per unit volume compared with liquids and solids.