Principles of Chemistry: States of Matter and Solubility Study on States of Matter and Solubility

THE ARRANGEMENT OF THE PARTICLES

Everything in the physical world is composed of particles that are invisible to the naked eye due to their diminutive size. These particles are arranged differently depending on whether the substance is a solid, a liquid, or a gas.

• Solids:

  • Arrangement: The particles are usually arranged regularly and packed closely together.

  • Movement: The particles are only able to vibrate about fixed positions; they cannot move around from place to place.

  • Forces: There are strong forces of attraction between the particles, which keep them held together in a fixed structure.

  • Kinetic Energy: The particles in a solid have less kinetic (movement) energy than the particles in a liquid or a gas.

• Liquids:

  • Arrangement: The particles are still mostly touching, but some gaps have appeared between them. This accounts for why liquids are usually less dense than the solids they originate from. The arrangement is random.

  • Movement: The forces between the particles are less effective than in a solid, allowing the particles to move around and slide over each other.

  • Kinetic Energy: The particles in a liquid have more kinetic energy than those in a solid, but less than those in a gas.

• Gases:

  • Arrangement: The particles are much further apart and arranged randomly. There are almost no forces of attraction between the particles.

  • Movement: The particles move randomly at high speeds in all directions.

  • Kinetic Energy: The particles in a gas have the highest kinetic energy of the three states of matter.

OBSERVATIONS EXPLAINED BY PARTICLE ARRANGEMENT

• Resistance and Permeability: You cannot walk through a brick wall (solid) because the strong forces of attraction mean the particles cannot move out of your way. You can swim through water (liquid) because you can push the particles aside. Moving through air (gas) is very easy because there are virtually no forces between the particles.

• Density and Volume: When most solids melt, their volume increases slightly because the particles are no longer as tightly packed. If you boil approximately $5\,cm^3$ of water, the resulting steam will fill an average bucket (demonstrating the large space between gas particles).

INTERCONVERSIONS BETWEEN THE THREE STATES OF MATTER

Interconversions are physical changes that can be achieved by heating or cooling, which alters the energy, movement, and arrangement of the particles.

• Solid to Liquid (Melting): As a solid is heated, the heat energy causes the particles to vibrate faster. Eventually, the vibrations become so vigorous that the forces of attraction are no longer strong enough to hold them in a fixed position. The particles begin to move around each other. The specific temperature at which this occurs is the melting point.

• Liquid to Solid (Freezing): As a liquid is cooled, the particles move slower and slower. Eventually, the forces of attraction hold them in a fixed position, and they pack closely into a regular solid. The temperature of this change is the freezing point. Note: For any given substance, the melting point and freezing point are at exactly the same temperature.

• Liquid to Gas (Boiling): This occurs when a liquid is heated so strongly that the particles move fast enough to overcome all forces of attraction between them. Stronger forces of attraction result in a higher boiling point. Boiling happens at one specific temperature.

• Liquid to Gas (Evaporation): Unlike boiling, evaporation can occur at any temperature below the boiling point. In a liquid, particles have a range of speeds. Some very fast-moving particles at the surface have enough energy to break away and form a gas. Puddles in the UK may evaporate at $5\,^{\circ}C$ even though water boils at $100\,^{\circ}C$.

• Gas to Liquid (Condensing): When a gas is cooled, the particles move slowly enough that forces of attraction begin to form, holding them together as a liquid.

• Solid to Gas (Sublimation) and Gas to Solid (Deposition): A small number of substances change directly between solid and gas without becoming a liquid. The transition from solid to gas is sublimation; the reverse is deposition (though some also refer to this as sublimation or de-sublimation).

  • Example: Carbon dioxide (dry ice) sublimes at $-78.5\,^{\circ}C$. The white "smoke" seen around dry ice is not the CO2 gas itself (which is invisible) but water vapor in the air condensing due to the extreme cold.

WORKING OUT PHYSICAL STATES

To determine the state of a substance at a specific temperature (such as room temperature, typically defined in science as between $20\,^{\circ}C$ and $25\,^{\circ}C$), compare that temperature to the substance's melting and boiling points:

• Solid: Temperature is below the melting point.

• Liquid: Temperature is between the melting and boiling points.

• Gas: Temperature is above the boiling point.

Example: Bromine

• Melting point: $-7\,^{\circ}C$

• Boiling point: $59\,^{\circ}C$

• Result: Since room temperature ($20-25\,^{\circ}C$) falls between these values, bromine is a liquid at room temperature.

DIFFUSION

Diffusion is the spreading out of particles from an area of high concentration (where there are many particles in a volume) to an area of low concentration (where there are fewer particles).

• Diffusion in Gases: This is relatively fast because gas particles move quickly. However, it takes longer than expected because particles collide with air particles, bouncing in different directions. Ammonia particles travel at roughly $600\,m/s$, but due to collisions, a particle may travel $30\,km$ just to move across a single lab.

• Demonstration (Bromine and Air): When a jar of air is placed over a jar of brown bromine gas and the lids are removed, the brown color diffuses upward until both jars are a uniform shade. Air particles also diffuse downward.

• Demonstration (Hydrogen and Air): Hydrogen (less dense) and air will diffuse until they are evenly mixed in both jars, leading to identical explosions in both if ignited.

SPEED OF DIFFUSION AND PARTICLE MASS

A classic experiment involves the reaction between ammonia gas ($NH_3$) and hydrogen chloride gas ($HCl$): $NH_3(g) + HCl(g) \rightarrow NH_4Cl(s)$

• The Experiment: Cotton wool soaked in concentrated ammonia is placed at one end of a glass tube; cotton wool soaked in concentrated hydrochloric acid is placed at the other.

• Observation: A white ring of solid ammonium chloride forms.

• Result: The ring forms closer to the hydrochloric acid end.

• Explanation: Ammonia particles are lighter ($\text{Relative Molecular Mass} = 17$) than hydrogen chloride particles ($\text{Relative Molecular Mass} = 36.5$). Lighter particles move and diffuse faster, so the ammonia travels further in the same amount of time.

DIFFUSION AND DILUTION IN LIQUIDS

• Diffusion in Liquids: This is much slower than in gases. For example, potassium manganate(VII) in water can take days to diffuse completely. This is because particles in a liquid move more slowly and are closer together, resulting in more constant collisions and less empty space to move into.

• Dilution of Coloured Solutions: Dissolving $0.01\,g$ of potassium manganate(VII) in a small volume and diluting it to $10000\,cm^3$ shows that even a tiny mass contains a vast number of particles.

  • Calculated estimate: A single "particle" of potassium manganate(VII) weighs approximately $0.00000000000000000000026\,g$.

  • There are approximately $38\,000\,000\,000\,000\,000\,000$ particles in $0.01\,g$.

SOLUTES, SOLVENTS, AND SOLUTIONS

• Solute: The substance that dissolves (e.g., the solid).

• Solvent: The liquid in which the solute dissolves.

• Solution: The liquid mixture formed when a solute dissolves in a solvent.

• Process: Dissolving occurs when the attractive forces between solute particles are broken and replaced by new attractive forces between solute and solvent particles.

MEASURING SOLUBILITY (CHEMISTRY ONLY)

Solubility is defined as the maximum mass of solute (in grams) that can dissolve in $100\,g$ of solvent at a specific temperature to form a saturated solution.

• Saturated Solution: A solution containing the maximum amount of dissolved solid possible at a specific temperature. Undissolved solid must be present.

• Units: $g\,per\,100\,g\,of\,solvent$.

Investigating Solubility of Potassium Nitrate ($KNO_3$) at $40\,^{\circ}C$:

1. Weigh an empty evaporating basin.

2. Prepare a saturated solution by adding excess potassium nitrate to water heated slightly above $40\,^{\circ}C$ and stirring.

3. Cool to exactly $40\,^{\circ}C$.

4. Pour only the liquid (solution) into the evaporating basin and weigh it.

5. Heat the basin to evaporate the water completely.

6. Re-weigh the basin to find the mass of the dry crystals. This is done repeatedly until the mass remains constant (heating to constant mass).

7. Calculation:

   • $\text{Mass of water} = \text{Mass of (basin + solution)} - \text{Mass of (basin + crystals)}$

   • $\text{Mass of solute} = \text{Mass of (basin + crystals)} - \text{Mass of empty basin}$

   • $\text{Solubility (g/100 g)} = \frac{\text{mass of solute}}{\text{mass of solvent}} \times 100$

SOLUBILITY CURVES (CHEMISTRY ONLY)

A solubility curve plots solubility against temperature. Most solids become more soluble as temperature increases.

• Crystallization: If a saturated solution is cooled, the solubility decreases and the "excess" solute must leave the solution, forming crystals (precipitating out).

• Example Calculation: If a solution at $90\,^{\circ}C$ contains $200\,g$ of $KNO_3$ in $100\,g$ of water, and is cooled to $30\,^{\circ}C$ (where solubility is only $50\,g$), then $200 - 50 = 150\,g$ of crystals will forms.

• Calculation for different water masses: $\text{Mass of water (g)} / 100 \times \text{solubility at temperature} = \text{maximum mass that dissolves}$.

QUESTIONS AND DISCUSSION

• What is the name for gas to solid? Deposition (or sublimation/de-sublimation).

• How does temperature affect diffusion? Heating a gas increases the kinetic energy and speed of particles, leading to faster diffusion. At $2\,^{\circ}C$, the ammonia/HCl ring would take longer to form than at $25\,^{\circ}C$.

• Why does the ring form closer to the HCl? Because ammonia particles are lighter and move faster.

• If hydrobromic acid (HBr) is used instead of HCl: A white ring of ammonium bromide forms. Since HBr is twice as heavy as HCl, it moves even slower, and the ring would form even closer to the acid end of the tube.

• Identifying states at specific temperatures:

  • Substance A (MP $-259\,^{\circ}C$, BP $-253\,^{\circ}C$) is a gas at $30\,^{\circ}C$.

  • Substance B (MP $0\,^{\circ}C$, BP $100\,^{\circ}C$) is a liquid at $30\,^{\circ}C$.

  • Substance E (MP $801\,^{\circ}C$, BP $1413\,^{\circ}C$) is a solid at $30\,^{\circ}C$.