States of Matter

Predict the state of bromine at $20^\circ\text{C}$ and chlorine at $25^\circ\text{C}$. (Bromine: melting point = $-7^\circ\text{C}$, boiling point = $59^\circ\text{C}$) (Chlorine: melting point = $-101^\circ\text{C}$, boiling point = $-34^\circ\text{C}$)

Step 1: Compare the target temperature for bromine to its given points.

• $20^\circ\text{C}$ is greater than $-7^\circ\text{C}$ but less than $59^\circ\text{C}$.
• Answer: Bromine is a liquid at $20^\circ\text{C}$.

Step 2: Compare the target temperature for chlorine to its given points.

• $25^\circ\text{C}$ is significantly higher than the boiling point of $-34^\circ\text{C}$.
• Answer: Chlorine is a gas at room temperature ($25^\circ\text{C}$).

When writing chemical equations, state symbols are used to show these physical states: (s) for solid, (l) for liquid, (g) for gas, and (aq) for an aqueous solution (dissolved in water).

Explaining Changes of State

• You can easily melt an ice cube in your hand, but melting table salt requires an industrial furnace.
• This difference exists because the amount of energy needed to change state depends entirely on the strength of the forces holding the particles together.
• Stronger forces require more energy to overcome, resulting in higher melting and boiling points.

Changes of state happen due to energy transfers between a substance and its surroundings.

• Melting and Boiling (Endothermic): Thermal energy is transferred to the substance. Heating increases the kinetic energy of the particles, allowing them to move fast enough to overcome the attractive forces holding them together.
• Freezing and Condensing (Exothermic): Energy is transferred from the substance to the surroundings. As particles lose kinetic energy, they slow down, allowing attractive forces to reform.

The type of force overcome depends on the structure of the substance.

• In simple molecular substances (like $CO_2$ or water), heating overcomes weak intermolecular forces between the molecules. The strong covalent bonds inside the molecules do NOT break.
• In giant structures (like metals or ionic lattices), large amounts of energy are required to break the strong electrostatic forces or chemical bonds throughout the entire giant lattice.

Evaluating the Simple Particle Model (Higher Tier)

• The atoms that make up a solid block of metal are actually almost entirely empty space.
• However, the simple particle model (or kinetic theory) represents atoms, ions, or molecules as small, solid, inelastic spheres.
• While this model is successful at showing how particle arrangement and movement change when energy is added, it has several major limitations.

When evaluating the simple particle model, you must balance its usefulness against its flaws.

• Limitation 1: The model assumes there are NO forces between the spheres. In reality, melting and boiling points are entirely determined by the strength of intermolecular or electrostatic forces, which the model completely ignores.
• Limitation 2: The model represents particles as solid inelastic spheres. In reality, particles consist of a tiny nucleus surrounded by electron shells, meaning they are mostly empty space and do not bounce off each other perfectly without deforming.
• Limitation 3: The model depicts all particles as having uniform size and shape. In reality, particles vary significantly in size (e.g., small atoms versus large polymers) and shape (e.g., bent water molecules).
• Limitation 4: The model does not distinguish between different types of particles. It draws single atoms, ions, and complex molecules identically.