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.

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.

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.

Basic Separation Techniques

Simple methods exploit clear differences in properties. Filtration separates insoluble solids from liquids using filter paper, which allows liquid particles to pass through but retains larger solid particles. Decantation is a quick, less precise method where a liquid is carefully poured away from a solid sediment. Evaporation is used to recover a soluble solid from its solution by heating the solution to remove the solvent, leaving the solid behind.

Advanced Separation Techniques

Distillation involves heating a solution to evaporate the solvent, which is then condensed back into a liquid and collected. This is useful when the liquid solvent is also needed. Fractional distillation separates liquids with different boiling points; the liquid with the lower boiling point evaporates and condenses first. Chromatography separates components based on their different affinities for a stationary phase (e.g., paper) and a mobile phase (e.g., solvent), causing them to travel at different rates.

Matching Technique to Mixture Properties

The choice of separation technique depends entirely on the properties of the components in the mixture. For example, to separate sand (insoluble solid) from salt solution (soluble solid in liquid), you would first filter out the sand, then evaporate the water to recover the salt. To separate two immiscible liquids (like oil and water), a separating funnel would be used. Understanding solubility, boiling points, and particle size is crucial.