Fundamentals of Bulk Properties

Step 1: Compare the given temperature to the melting point.

• $20^\circ C$ is higher than $-105^\circ C$, meaning the substance has already melted (it is not a solid).

Step 2: Compare the given temperature to the boiling point.

• $20^\circ C$ is also higher than $-25^\circ C$, meaning the substance has already boiled.

Step 3: State the final answer.

• Substance Y is a gas.

Bulk Properties of Giant Ionic and Metallic Structures

• Dropping a ceramic plate shatters it into tiny pieces, but dropping a metal spoon just leaves a dent.
• Ionic compounds form a giant ionic lattice. They have incredibly high melting points because strong electrostatic attractions act in all directions between oppositely charged ions, requiring massive amounts of thermal energy to overcome.
• However, ionic lattices are highly brittle. If a physical force is applied, the regular layers of ions shift. This causes ions of the same charge to suddenly align side-by-side, forcing them to violently repel each other and shatter the crystal.
• Metals also form a giant structure, consisting of positive metal ions surrounded by a "sea" of delocalised electrons.
• Metals are malleable rather than brittle. When hammered, the regular layers of metal atoms can easily slide over one another without breaking the overall metallic bond, because the constantly moving delocalised electrons maintain the electrostatic attraction.
• To conduct electricity, a substance must contain mobile charge carriers. Solid ionic lattices do not conduct because their ions are locked in fixed positions. They only conduct when molten or dissolved in water, which frees the ions to move. Metals conduct in all physical states because their delocalised electrons are always free to move through the entire structure.

Bulk Properties of Simple and Giant Covalent Structures

• Every time you boil a kettle, you are overcoming weak attractive forces, but leaving the actual water molecules perfectly intact.
• A simple molecule (like $H_2O$ or $CH_4$) contains a small, fixed number of non-metal atoms. These have a dual bonding nature: they contain strong intramolecular bonds (covalent bonds) within the molecules, but only weak intermolecular forces between the separate molecules.
• Simple molecules have low melting and boiling points because only the weak intermolecular forces need to be overcome. The strong covalent bonds absolutely do not break during state changes. They are also electrical insulators in all states because they completely lack delocalised electrons or free ions to carry a charge.
• As molecules increase in size and mass (like longer polymer chains compared to small gases), their intermolecular forces become much stronger. This means more energy is required to separate them, resulting in higher melting and boiling points, though these forces always remain weaker than true chemical bonds.
• Giant covalent structures consist of a continuous 3D network of atoms. Their melting points are incredibly high because every single atom is connected by strong covalent bonds that must all be broken. For example, diamond has four strong bonds per carbon atom in a rigid shape, making it extremely hard. Graphite only has three bonds per carbon atom, forming flat layers. The weak intermolecular forces between these layers allow them to slide easily, making graphite soft and slippery.