States of matter and mixtures Revision Notes

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.

The Principle of Paper Chromatography

Paper chromatography separates mixtures of soluble substances by using a solvent (mobile phase) to move them through chromatography paper (stationary phase). The unique rates at which different substances travel along the paper allow for their separation.

Components of Paper Chromatography

The stationary phase is the chromatography paper itself. The mobile phase is the solvent used, which can be water or another appropriate solvent. The mixture to be separated is applied as a spot onto the paper, and as the solvent moves through it, the components of the mixture are separated based on their differing affinities.

Separation Mechanism

Substances move at different rates because of their varying solubilities in the mobile phase and their strength of attraction to the stationary phase. Highly soluble substances with low attraction to the paper travel further, while less soluble substances with strong attraction to the paper travel shorter distances.

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.

Treating Waste and Ground Water

To make waste and groundwater potable (safe to drink), a three-step process is commonly used:

1. Sedimentation: Larger insoluble particles are allowed to settle at the bottom of a tank, removing significant debris. Chemicals may be added to help smaller particles clump together (coagulation) and settle more easily.
2. Filtration: Water passes through beds of sand and gravel, or other filter materials, to remove smaller suspended solids and some microorganisms that did not settle during sedimentation.
3. Chlorination: Chlorine gas or chlorine compounds are added to the filtered water. Chlorine is a disinfectant that kills harmful bacteria and other pathogens, ensuring the water is microbiologically safe for drinking. Sometimes, ozone or ultraviolet (UV) light are used as alternatives to chlorine for disinfection.

Making Seawater Potable via Distillation

Seawater contains a very high concentration of dissolved salts, making it unsuitable for direct consumption. Distillation is a process used to purify seawater.

1. Heating: Seawater is heated to its boiling point (approximately 100°C), turning the water into steam.
2. Evaporation: As the water evaporates, the dissolved salts are left behind in the boiling vessel because they have much higher boiling points and do not evaporate with the water.
3. Condensation: The steam is then cooled, usually by passing it through a condenser, causing it to condense back into liquid water.
4. Collection: This condensed liquid is pure water, free from dissolved salts, and is now potable. This process requires significant energy and is often more expensive than treating fresh water sources.

Importance of Pure Water in Analysis

In scientific analysis, it is crucial to use water that is free from dissolved salts and other impurities. This highly purified water is often referred to as deionised or distilled water.

• Avoidance of Interference: Dissolved ions (salts) in water can react with reagents, interfere with a chemical reaction, or form precipitates, leading to incorrect observations or results.
• Accuracy of Measurements: Impurities can affect the conductivity, pH, or concentration measurements, compromising the accuracy and precision of experiments.
• Prevention of Contamination: In trace analysis, even minute amounts of impurities in the water can lead to false positives or mask the presence of the substances being analysed. Therefore, analytical grade water ensures reliable and reproducible experimental outcomes.