States of Matter | 物质的态

📚 States of Matter | 物质的态

States of matter is one of the most fundamental topics in A-Level Chemistry. It connects the observable behaviour of solids, liquids and gases with the microscopic motion of particles, and it provides the basis for understanding gas laws, kinetic theory and phase changes.

物质的态是 A-Level 化学中最基础的主题之一。它将固体、液体和气体的宏观行为与微观粒子运动联系起来，并为理解气体定律、分子运动论和相变提供基础。

1. The Three States and the Particle Model | 三态与粒子模型

Matter exists in three common physical states: solid, liquid and gas. The differences between these states can be explained by the arrangement, motion and separation of particles. In a solid, particles are closely packed in a regular or irregular lattice and can only vibrate about fixed positions. In a liquid, particles are still close together but can slide past one another, so a liquid has a fixed volume but no fixed shape. In a gas, particles are widely separated, move rapidly in random directions and fill the entire container.

物质通常以三种物理状态存在：固态、液态和气态。这些状态之间的差异可以用粒子的排列方式、运动方式和间距来解释。在固体中，粒子紧密排列在规则或非规则的晶格中，只能在固定位置附近振动。在液体中，粒子仍然靠得很近，但可以相互滑动，因此液体有固定的体积但没有固定的形状。在气体中，粒子间距很大，快速向各个方向随机运动，并充满整个容器。

State / 状态	Arrangement / 排列	Motion / 运动	Shape and volume / 形状与体积	Compressibility / 可压缩性
Solid / 固体	Closely packed, regular or irregular / 紧密排列，规则或非规则	Vibrate about fixed positions / 在固定位置振动	Fixed shape and volume / 形状和体积固定	Very low / 很低
Liquid / 液体	Close together but disordered / 相互靠近但无序	Slide past one another / 相互滑动	Fixed volume, shape of container / 体积固定，形状随容器	Low / 低
Gas / 气体	Widely separated, random / 间距很大，随机分布	Rapid, random straight-line motion / 快速、随机直线运动	No fixed shape or volume / 无固定形状和体积	High / 高

2. Changes of State and Energy | 状态变化与能量

Changes of state are physical processes in which the arrangement and motion of particles change but the chemical composition does not. Melting, boiling and sublimation are endothermic because energy must be supplied to overcome attractive forces between particles. Freezing, condensation and deposition are exothermic because energy is released as particles become more ordered.

状态变化是物理过程，粒子排列和运动方式发生改变，但化学组成不变。熔化、沸腾和升华是吸热过程，因为必须提供能量来克服粒子之间的吸引力。凝固、冷凝和凝华是放热过程，因为粒子变得更有序时会释放能量。

During melting and boiling, the temperature remains constant even though heat is still being supplied. The energy absorbed is used to break intermolecular forces, not to increase the average kinetic energy of the particles. This energy is called latent heat.

在熔化和沸腾过程中，即使持续供热，温度也保持不变。吸收的能量用于克服分子间作用力，而不是增加粒子的平均动能。这种能量称为潜热。

ΔH_fus = enthalpy change of fusion (melting)

ΔH_vap = enthalpy change of vaporisation (boiling)

熔化焓变用 ΔH_fus 表示，汽化焓变用 ΔH_vap 表示。

3. Heating and Cooling Curves | 加热与冷却曲线

A heating curve shows how temperature changes when a solid is heated at a steady rate until it becomes a gas. The curve has sloping sections where heating raises the temperature, and horizontal plateaus where a change of state occurs at constant temperature.

加热曲线表示固体以恒定速率加热直至变为气体时温度随时间的变化。曲线中倾斜段表示加热使温度升高，水平段表示在恒定温度下发生状态变化。

On a heating curve, the first sloping region corresponds to warming the solid. The first plateau is melting, where solid and liquid coexist. The next sloping region warms the liquid, and the second plateau is boiling, where liquid and gas coexist. The final sloping region warms the gas.

在加热曲线上，第一段倾斜区域对应固体升温。第一个平台是熔化过程，此时固体和液体共存。下一个倾斜区域使液体升温，第二个平台是沸腾过程，此时液体和气体共存。最后一段倾斜区域使气体升温。

For the sloping sections, the energy change can be calculated using q = m c ΔT, where c is the specific heat capacity. For the plateaus, the energy change is q = n ΔH_fus or q = n ΔH_vap.

对于倾斜段，能量变化可用 q = m c ΔT 计算，其中 c 为比热容。对于平台段，能量变化为 q = n ΔH_fus 或 q = n ΔH_vap。

q = m c ΔT

q = n ΔH_fus (melting) or n ΔH_vap (boiling)

4. Gas Pressure and Temperature | 气体压强与温度

Gas pressure is caused by gas particles colliding with the walls of their container. Each collision exerts a small force, and the total force per unit area is the pressure. If the temperature increases, particles move faster, collide more frequently and with greater force, so pressure increases if volume is constant.

气体压强由气体粒子与容器壁碰撞产生。每次碰撞施加一个微小的力，单位面积上的总力即为压强。如果温度升高，粒子运动更快，碰撞更频繁且更有力，因此在体积不变时压强增大。

The Kelvin temperature scale is essential in gas calculations because it is directly proportional to the average kinetic energy of particles. Zero kelvin (0 K) is absolute zero, at which particle motion has its minimum possible energy.

开尔文温标在气体计算中至关重要，因为它与粒子的平均动能成正比。零开尔文（0 K）为绝对零度，此时粒子运动具有最小可能能量。

The three historical gas laws can be stated as follows: Boyle’s law gives pV = constant at constant temperature; Charles’s law gives V ∝ T at constant pressure; and the pressure law gives p ∝ T at constant volume.

三条历史上的气体定律可表述为：玻意耳定律指出在恒温下 pV 为常数；查理定律指出在恒压下 V ∝ T；压强定律指出在恒体积下 p ∝ T。

p₁ V₁ = p₂ V₂ (Boyle’s law, constant T and n)

V₁ / T₁ = V₂ / T₂ (Charles’s law, constant p and n)

p₁ / T₁ = p₂ / T₂ (pressure law, constant V and n)

5. The Ideal Gas Equation | 理想气体状态方程

Combining Boyle’s law, Charles’s law and the pressure law gives the ideal gas equation. An ideal gas is a theoretical gas that follows this equation exactly under all conditions. The equation is pV = nRT, where p is pressure, V is volume, n is amount in moles, R is the gas constant and T is temperature in kelvin.

将玻意耳定律、查理定律和压强定律合并即可得到理想气体状态方程。理想气体是一种在所有条件下都严格遵循该方程的理论气体。方程为 pV = nRT，其中 p 为压强，V 为体积，n 为物质的量（摩尔），R 为气体常数，T 为开尔文温度。

pV = nRT

R = 8.31 J K⁻¹ mol⁻¹

Units must be consistent: pressure in pascals (Pa), volume in cubic metres (m³), amount in moles (mol) and temperature in kelvin (K). To convert from degrees Celsius to kelvin, use T(K) = t(°C) + 273.15.

单位必须一致：压强用帕斯卡（Pa），体积用立方米（m³），物质的量用摩尔（mol），温度用开尔文（K）。从摄氏度换算为开尔文温度，使用 T(K) = t(°C) + 273.15。

Example: Calculate the volume occupied by 0.50 mol of an ideal gas at 298 K and 100 kPa.

示例：计算 0.50 mol 理想气体在 298 K 和 100 kPa 下所占的体积。

p = 100 kPa = 1.00 × 10⁵ Pa

V = nRT / p = (0.50 mol × 8.31 J K⁻¹ mol⁻¹ × 298 K) / (1.00 × 10⁵ Pa) = 0.0124 m³ = 12.4 dm³

6. Molar Volume and Gas Stoichiometry | 摩尔体积与气体化学计量

The molar volume of a gas is the volume occupied by one mole of the gas under specified conditions. At room temperature and pressure (RTP, about 298 K and 100 kPa), the molar volume of an ideal gas is approximately 24.5 dm³ mol⁻¹, often rounded to 24 dm³ mol⁻¹ in A-Level calculations. At standard temperature and pressure (STP, 273.15 K and 100 kPa), it is 22.7 dm³ mol⁻¹.

气体的摩尔体积是指在特定条件下一摩尔气体所占的体积。在常温常压下（RTP，约 298 K 和 100 kPa），理想气体的摩尔体积约为 24.5 dm³ mol⁻¹，在 A-Level 计算中常取 24 dm³ mol⁻¹。在标准温度和压强下（STP，273.15 K 和 100 kPa），摩尔体积为 22.7 dm³ mol⁻¹。

Because equal volumes of gases at the same temperature and pressure contain equal numbers of moles, gas volumes can be used directly in stoichiometric calculations. For example, in the reaction 2H₂(g) + O₂(g) → 2H₂O(g), the volumes react in the ratio 2:1:2 at constant temperature and pressure.

由于在相同温度和压强下，相同体积的气体含有相同的物质的量，因此气体体积可以直接用于化学计量计算。例如，在反应 2H₂(g) + O₂(g) → 2H₂O(g) 中，在恒温恒压下气体体积按 2:1:2 的比例反应。

2H₂(g) + O₂(g) → 2H₂O(g)

Volume ratio = 2 : 1 : 2

7. Dalton’s Law of Partial Pressures | 道尔顿分压定律

Dalton’s law states that the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the partial pressures of the individual gases. The partial pressure of a gas is the pressure it would exert if it alone occupied the whole volume.

道尔顿定律指出，非反应气体混合物的总压强等于各组分气体分压之和。某气体的分压是指该气体单独占据整个体积时所施加的压强。

p_total = p₁ + p₂ + p₃ + …

总压强等于各分压之和。

For a mixture of gases, the partial pressure of gas 1 is equal to its mole fraction multiplied by the total pressure. The mole fraction is the amount of that gas divided by the total amount of all gases present.

对于气体混合物，气体 1 的分压等于其摩尔分数乘以总压强。

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States of Matter | 物质的态