Explaining Density | 解释密度

📚 Explaining Density | 解释密度

Density is a property that links mass and volume. In KS3 chemistry, understanding density helps you explain why some objects float, why gases rise or sink, and how the particle model accounts for differences between solids, liquids and gases.

密度是把质量和体积联系起来的一种性质。在 KS3 化学中，理解密度可以帮助你解释为什么有些物体会漂浮、为什么气体会上升或下沉，以及粒子模型如何解释固体、液体和气体之间的差异。

1. What Density Means | 密度是什么

Density describes how much mass is packed into a given volume. A material has high density when a large mass is squeezed into a small space, and low density when a small mass is spread over a large volume.

密度描述的是在一定体积内所包含的质量有多少。当较大质量被挤压在较小空间里时，材料密度就高；当较小质量分布在较大体积中时，密度就低。

For example, a kilogram of lead feels small and heavy because lead has a high density. The same mass of expanded polystyrene would take up much more space because it has a very low density.

例如，一千克铅拿起来又小又重，因为铅的密度很高。同样质量的发泡聚苯乙烯会占据大得多的空间，因为它的密度非常低。

2. The Density Equation | 密度公式

Density can be calculated using the relationship between mass and volume:

密度可以用质量与体积之间的关系来计算：

density = mass ÷ volume

This equation is often written using symbols as:

这个公式常用符号写成：

ρ = m ÷ V

Here ρ stands for density, m stands for mass, and V stands for volume. To find mass or volume, rearrange the equation: mass = density × volume, and volume = mass ÷ density.

其中 ρ 代表密度，m 代表质量，V 代表体积。要求质量或体积时，可以变形公式：质量 = 密度 × 体积，体积 = 质量 ÷ 密度。

Remember that in calculations the units must match. If mass is in grams and volume is in cubic centimetres, density will be in grams per cubic centimetre.

请记住，计算时单位必须一致。如果质量以克为单位、体积以立方厘米为单位，那么密度的单位就是克/立方厘米。

3. Units of Density | 密度的单位

The most common units in KS3 chemistry are grams per cubic centimetre, written g/cm³. For larger masses, kilograms per cubic metre, written kg/m³, is also used.

KS3 化学中最常用的单位是克/立方厘米，写作 g/cm³。对于较大的质量，也使用千克/立方米，写作 kg/m³。

To convert between them, remember that 1 g/cm³ is equal to 1000 kg/m³. This is because 1 kg = 1000 g and 1 m³ = 1 000 000 cm³.

两者之间的换算要记住：1 g/cm³ 等于 1000 kg/m³。这是因为 1 kg = 1000 g，而 1 m³ = 1 000 000 cm³。

Always include the unit cube (³) when writing density. The small ‘3’ tells you that volume is three-dimensional.

写密度时一定要带上立方单位（³）。这个小 ‘3’ 表示体积是三维的。

4. Comparing Densities of Substances | 比较物质的密度

Different substances have different densities. Knowing these values helps predict floating, sinking and layering.

不同物质具有不同的密度。了解这些数值有助于预测漂浮、下沉和分层现象。

Substance	Density (g/cm³)
Air	0.0013
Wood (typical)	0.6-0.9
Ice	0.92
Water	1.0
Aluminium	2.7
Iron	7.9
Mercury	13.6
Gold	19.3

The table shows that air is much less dense than water or metals. That is why a balloon filled with air can float on water, but a block of iron sinks.

该表显示空气的密度远小于水或金属。这就是为什么充气的气球可以浮在水面上，而铁块会下沉。

5. Density and the Particle Model | 密度与粒子模型

The particle model explains density in terms of how closely particles are arranged. In solids, particles are packed tightly in a regular arrangement, giving high density.

粒子模型从粒子排列的紧密程度来解释密度。在固体中，粒子紧密、规则地排列，因此密度较高。

In liquids, particles are still close but can slide past each other, so most liquids have densities close to their solids, though usually slightly lower.

在液体中，粒子仍然很接近，但可以彼此滑动，所以大多数液体的密度接近其固体，不过通常略低。

In gases, particles are very far apart and move rapidly. This means gases have much lower densities than solids or liquids.

在气体中，粒子相距很远且快速运动。这意味着气体的密度远低于固体或液体。

The same substance can have different densities in different states because the particles are arranged differently.

同一种物质在不同状态下可能具有不同的密度，因为粒子的排列方式不同。

6. Why Ice Floats on Water | 为什么冰浮在水上

Most substances are denser as solids than as liquids. Water is unusual: ice is less dense than liquid water.

大多数物质在固态时比液态时密度更大。水却很不寻常：冰的密度比液态水小。

When water freezes, the particles arrange into an open, hexagonal lattice. This structure holds the particles further apart on average than in liquid water.

水结冰时，粒子排列成开放的六边形晶格。这种结构使粒子的平均距离比液态水中更远。

Ice has a density of about 0.92 g/cm³, while liquid water has a density of about 1.0 g/cm³. Because ice is less dense, it floats.

冰的密度约为 0.92 g/cm³，而液态水的密度约为 1.0 g/cm³。由于冰的密度较小，所以它会漂浮。

This property is important for aquatic life: ice forms an insulating layer on top of ponds and lakes, preventing the water below from freezing solid.

这一性质对水生生物很重要：冰在池塘和湖泊表面形成一层隔热层，防止下面的水完全冻结。

7. Floating and Sinking | 浮与沉

An object floats in a liquid if its average density is less than the density of the liquid. It sinks if its average density is greater.

如果物体的平均密度小于液体的密度，它就会漂浮；如果平均密度更大，它就会下沉。

For example, wood has a lower density than water, so wooden blocks float. Iron has a higher density than water, so iron nails sink.

例如，木材的密度比水小，所以木块会漂浮。铁的密度比水大，所以铁钉会下沉。

A ship made of iron floats because its overall shape contains a large volume of air. The average density of the whole ship, including the air inside, is less than the density of water.

用铁制造的轮船能漂浮，是因为它的整体形状中包含大量空气。整艘船的平均密度（包括内部空气）小于水的密度。

8. Calculating Density: Worked Example | 密度计算示例

Suppose a metal block has a mass of 200 g and a volume of 50 cm³. Its density is calculated as:

假设一个金属块的质量为 200 g，体积为 50 cm³。其密度计算如下：

ρ = 200 g ÷ 50 cm³ = 4 g/cm³

This means every cubic centimetre of the metal has a mass of 4 grams.

这意味着每一立方厘米的金属质量为 4 克。

If a liquid has a density of 0.8 g/cm³ and a volume of 250 cm³, its mass can be found using mass = density × volume:

如果一种液体的密度为 0.8 g/cm³，体积为 250 cm³，则可以用质量 = 密度 × 体积来求质量：

mass = 0.8 g/cm³ × 250 cm³ = 200 g

Always check that the units cancel correctly and that the final answer has the correct unit.

务必检查单位是否能正确约去，并确保最终答案带有正确单位。

9. Measuring Density in the Lab | 在实验室测量密度

To find the density of a regular solid, first measure its mass using a balance. Then measure its length, width and height to calculate volume.

要测量规则固体的密度，首先用天平测量它的质量。然后测量它的长、宽、高来计算体积。

For an irregular solid, use a displacement method. Fill a Eureka can or measuring cylinder with water, add the object, and measure the volume of water displaced.

对于不规则固体，可以使用排水法。把尤里卡罐或量筒装满水，放入物体，测量排出的水的体积。

For a liquid, measure the mass of an empty measuring cylinder, pour in a known volume, and measure the mass again. The difference is the mass of the liquid.

对于液体，先测量空量筒的质量，倒入已知体积的液体，再测量总质量。两次质量之差就是液体的质量。

Finally divide mass by volume to obtain density.

最后用质量除以体积即可得到密度。

10. Density of Gases | 气体的密度

Gases have very low densities because their particles are spread far apart. However, different gases still have different densities.

气体的密度非常低，因为它们的粒子相隔很远。不过，不同气体的密度仍然不同。

Carbon dioxide is denser than air, so it sinks and can collect in low places. This is why carbon dioxide can fill a trough or a lower container.

二氧化碳的密度比空气大，所以它会下沉并聚集在低处。这就是为什么二氧化碳能填满槽或较低容器的原因。

Helium is less dense than air, so a helium balloon rises. Hydrogen is also less dense than air, which is why it was historically used in airships, although it is very flammable.

氦气的密度比空气小，所以氦气球会上升。氢气的密度也比空气小，因此历史上曾被用于飞艇，尽管它非常易燃。

11. Density, Temperature and Expansion | 密度、温度与膨胀

When a substance is heated, its particles gain energy and move further apart. The volume increases while the mass stays the same, so the density decreases.

当物质受热时，粒子获得能量并彼此远离。体积增大而质量不变，因此密度减小。

When a substance cools, the particles move closer together, volume decreases, and density increases.

当物质冷却时，粒子靠得更近，体积减小，密度增大。

This explains convection currents in liquids and gases: warmer, less dense fluid rises, while cooler, denser fluid sinks.

这解释了液体和气体中的对流：较暖、密度较小的流体会上升，而较冷、密度较大的流体会下沉。

12. Common Misconceptions | 常见误区

‘Heavy’ and ‘dense’ are not the same thing. A large block of wood can be heavy but still less dense than a small steel ball.

‘重’和’密度大’不是一回事。一大块木头可能很重，但密度仍可能小于一个小钢球。

Size alone does not determine density. Density depends on both mass and volume together.

仅凭大小

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Explaining Density | 解释密度