States of matter

Boiling turns liquid water into steam — a change between states of matter.

Where gas pressure comes from

Gas molecules move fast in all directions. They keep hitting — colliding 碰撞 with — the walls of their container. Each hit gives the wall a tiny push. The pressure 压强 of the gas is the overall result of these many collisions on the walls.

Gas pressure: fast molecules move in all directions and collide with the walls; the many tiny pushes add up to the pressure

Ideal gases

An ideal gas 理想气体 is a simple model. We assume two things:

• the particles themselves take up zero volume.
• there are no intermolecular forces 分子间作用力 of attraction between the particles.

A real gas 实际气体 follows this model closely at low pressure and high temperature. It behaves least like an ideal gas at high pressure and low temperature, when the particles are squeezed close together and the forces between them start to matter.

An ideal gas is a model: point particles with no forces between them. A real gas behaves least like this at high pressure and low temperature, when the particles are crowded and their real size and attractions start to matter

The ideal gas equation

The ideal gas equation 理想气体方程 links pressure, volume, amount and temperature:

where $p$ is the pressure in Pa, $V$ is the volume in $\text{m}^3$, $n$ is the amount in moles, $T$ is the temperature in kelvin 开尔文 (K), and $R$ is the gas constant 气体常量 ($8.31\ \text{J K}^{-1}\,\text{mol}^{-1}$).

Always change the units first: °C to K (add 273), and $\text{cm}^3$ or $\text{dm}^3$ to $\text{m}^3$.

Worked example. Find the volume of $0.50\ \text{mol}$ of an ideal gas at $27\ ^{\circ}\text{C}$ and $100\ \text{kPa}$. ($R = 8.31\ \text{J K}^{-1}\,\text{mol}^{-1}$.)

Convert first: $T = 300\ \text{K}$, $p = 1.00 \times 10^{5}\ \text{Pa}$. Then

You can also use the equation to find a molar mass 摩尔质量. Since $n = m/M$:

This lets you work out $M_r$ from the mass (or the density) of a gas.

Worked example. A flask holds $0.96\ \text{g}$ of a gas in $600\ \text{cm}^3$ at $100\ \text{kPa}$ and $27\ ^{\circ}\text{C}$. Find the molar mass of the gas. ($R = 8.31\ \text{J K}^{-1}\,\text{mol}^{-1}$.)

Convert: $V = 6.00 \times 10^{-4}\ \text{m}^3$, $T = 300\ \text{K}$. Then

Quartz is a giant covalent structure of silicon and oxygen.

How a substance behaves depends on how its particles are joined. There are four main structures of a crystalline solid 晶体.

The four structures of a crystalline solid — the structure decides the melting point, conductivity and solubility

Giant ionic

A giant ionic 离子晶体 structure is a huge regular lattice 晶格 of positive and negative ions 离子, held together by strong attraction in every direction. Examples are sodium chloride and magnesium oxide.

Simple molecular

A simple molecular 分子晶体 structure is made of small molecules 分子. The bonds inside each molecule are strong, but the intermolecular forces between the molecules are weak. Examples are iodine ($\text{I}_2$), fullerene 富勒烯 ($\text{C}_{60}$) and ice.

Giant molecular

A giant molecular 原子晶体 structure (also called giant covalent) is a huge network of atoms joined by strong covalent bonds 共价键. Examples are silicon(IV) oxide 二氧化硅, graphite 石墨 and diamond 金刚石.

Two giant covalent forms of carbon: diamond is a rigid 3D network (very hard); graphite has sliding layers and spare electrons that conduct

A real diamond in the rock it grew in. That whole crystal is one giant molecule — a single unbroken network of carbon atoms, each bonded to four others. Breaking it means breaking countless strong covalent bonds, which is why diamond is the hardest natural material

Giant metallic

A giant metallic 金属晶体 structure is a lattice of positive metal ions in a "sea" of delocalised electrons 离域电子. An example is copper.

A piece of pure copper metal. The shine, and the way metals conduct and bend, all come from that giant lattice of copper ions sitting in a shared sea of delocalised electrons

Physical properties

The structure decides the physical properties:

• melting point 熔点 and boiling point 沸点 are high when strong forces (ionic, covalent or metallic) must be broken, and low when only weak intermolecular forces break.
• electrical conductivity 导电性 needs charged particles that can move — ions that are free (when molten or dissolved) or delocalised electrons. Graphite conducts because some of its electrons are delocalised.
• solubility 溶解度 in water is usually high for ionic solids and low for molecular and giant covalent solids.

You can work backwards too: from the melting point, conductivity and solubility of an unknown substance, deduce the type of structure and bonding it has.

• Use SI units in $pV = nRT$: pressure in Pa, volume in $\text{m}^3$ ($\text{cm}^3 \times 10^{-6}$), temperature in K ($^\circ\text{C} + 273$).
• State the ideal-gas assumptions (negligible molecular volume, no intermolecular forces) and when real gases deviate (high pressure, low temperature).
• Link each property to structure: giant ionic (high m.p., conducts molten), giant covalent (very high m.p.), simple molecular (low m.p.), giant metallic (conducts, malleable).
• Graphite conducts because each carbon has a delocalised electron; diamond does not — a favourite comparison.