States of matter and mixtures Revision Notes

Recognising Physical Changes

Physical changes involve a change in appearance or state, but the chemical composition of the substance remains the same. Examples include dissolving, bending, breaking, and the interconversions between states of matter. These changes are typically reversible and do not involve the formation of new chemical substances. The particles themselves remain unchanged; only their arrangement and movement differ.

Interconversions Between States

The processes that transform a substance from one state to another are specific and named. Melting (solid to liquid) and freezing (liquid to solid) occur at the melting/freezing point. Boiling (liquid to gas) and condensation (gas to liquid) occur at the boiling/condensation point. Sublimation is the direct transition from solid to gas, and deposition is from gas directly to solid. These processes are driven by changes in temperature and pressure, which affect the kinetic energy of the particles.

Particle Energy Changes

Interconversions like melting, boiling, and sublimation are endothermic processes, meaning particles absorb energy from the surroundings. This absorbed energy increases the internal energy and kinetic energy of the particles, allowing them to move more vigorously and gain potential energy to overcome intermolecular forces. Conversely, freezing, condensation, and deposition are exothermic processes, releasing energy to the surroundings. This energy loss means particles have less kinetic energy and move less, allowing attractive forces to pull them into more ordered structures.

Arrangement and Movement of Particles

In a solid, particles are closely packed in fixed positions, vibrating about these positions. Upon melting, particles gain energy, move from their fixed positions, and become able to slide past each other, forming a liquid. They remain closely packed but are randomly arranged. When a liquid boils, particles gain enough energy to overcome all intermolecular forces, spreading out to fill the entire container as a gas. Gas particles are far apart, randomly arranged, and move rapidly and randomly. These changes are reversible: cooling a gas leads to condensation, then freezing, reducing particle movement and increasing order.

State Changes and Intermolecular Forces

The energy changes during state interconversions are directly related to overcoming or forming intermolecular forces. When a solid melts or a liquid boils, energy is supplied to break these forces. Stronger intermolecular forces require more energy to overcome, leading to higher melting and boiling points. When a gas condenses or a liquid freezes, energy is released as these forces reform, drawing particles closer together and establishing a more ordered structure. The identity of the substance determines the strength of these forces, impacting the energy required or released during state changes.

Solids

In solids, particles are arranged in a regular, fixed pattern and are very close to each other. They vibrate about fixed positions but do not move past each other. Due to this arrangement, solids have a definite shape and volume. Particles in solids possess the lowest kinetic energy among the three states, reflecting less overall movement.

Liquids

Liquid particles are still closely packed but are arranged randomly and can slide past one another. This allows liquids to flow and take the shape of their container, though they maintain a definite volume. The particles in a liquid have more kinetic energy than those in a solid, enabling them to move more freely.

Gases

In gases, particles are widely separated and move randomly and rapidly in all directions. They collide with each other and the container walls. Gases have no definite shape or volume, expanding to fill their container completely and are easily compressible. Particles in gases possess the highest kinetic energy among the three states, showing significant translational motion.

Relative Energy

The kinetic energy of particles increases from solid to liquid to gas. Solids have the lowest kinetic energy, as particles only vibrate. Liquids have intermediate kinetic energy, allowing particles to slide past each other. Gases have the highest kinetic energy, enabling rapid, random movement and large separations.

Understanding Physical States

Substances can exist in three primary physical states: solid, liquid, and gas. These states are determined by how closely particles are packed and how much kinetic energy they possess. Solids have particles fixed in a rigid structure, liquids have particles that can move past each other but remain in contact, and gases have particles that are far apart and move randomly and rapidly. Changes between these states are phase changes.

Using Melting and Boiling Points

To predict the physical state of a substance at a specific temperature, compare that temperature to the substance's melting and boiling points. If the temperature is below the melting point, the substance is a solid. If the temperature is between the melting point and the boiling point, it's a liquid. If the temperature is above the boiling point, it's a gas. For instance, water melts at 0 °C and boils at 100 °C. At 25 °C, it's a liquid because 0 °C < 25 °C < 100 °C.