📚 Material Properties | 材料的性质
Materials are all around us, from the metals in a bicycle frame to the plastics in a phone case. A material's properties describe how it behaves under different conditions, such as when it is heated, stretched, or placed in water. By measuring and comparing these properties, scientists and engineers can choose the best material for each job.
我们周围到处都是材料，从自行车车架中的金属到手机壳中的塑料。材料的性质描述了它在不同条件下的表现，例如受热、拉伸或放入水中时。通过测量和比较这些性质，科学家和工程师可以为每项工作选择最合适的材料。
1. What Are Material Properties? | 什么是材料性质？
Material properties are the characteristics of a substance that can be observed or measured without changing its identity. Examples include hardness, colour, melting point, density, and electrical conductivity. Some properties are very useful for identifying an unknown material because they stay the same no matter how much of the material is present.
材料性质是物质可以被观察或测量、且不会改变其身份的特征。例子包括硬度、颜色、熔点、密度和导电性。有些性质非常有助于鉴别未知材料，因为这些性质无论材料数量多少都保持不变。
Properties can also help us predict how a material will behave in everyday use. For instance, a material with a low density might be used to make aircraft parts because it is lightweight, whereas a material with high electrical conductivity might be chosen for wiring.
性质还可以帮助我们预测材料在日常使用中的行为。例如，低密度材料可用于制造飞机部件，因为它重量轻；而高导电性材料可能被选作电线。
2. Physical Properties and Chemical Properties | 物理性质与化学性质
Physical properties can be measured or observed without changing the material into a new substance. They include boiling point, colour, density, hardness, and state at room temperature. For example, ice melting into water is a physical change, so melting point is a physical property.
物理性质可以在不把材料变成新物质的情况下测量或观察。它们包括沸点、颜色、密度、硬度和室温下的状态。例如，冰融化成水是物理变化，因此熔点是物理性质。
Chemical properties describe how a material reacts with other substances to form new products. Flammability, rusting, and reaction with acid are chemical properties. These properties are observed only during a chemical change, such as when iron reacts with oxygen and water to form rust.
化学性质描述材料如何与其他物质反应生成新的产物。可燃性、生锈和与酸反应都属于化学性质。这些性质只有在化学变化中才能观察到，例如铁与氧气和水反应生成锈。
3. Density | 密度
Density is the mass of a material packed into a certain volume. It tells us how heavy a material feels for its size. The formula for density is:
密度是材料在一定体积内所含的质量。它告诉我们在相同体积下材料有多重。密度的计算公式是：
Density = Mass ÷ Volume ρ = m ÷ V
The unit of density is usually grams per cubic centimetre (g/cm³) for solids and liquids, or kilograms per cubic metre (kg/m³) for gases. Materials with a density less than water (1.0 g/cm³) tend to float, while denser materials sink.
密度的单位通常是固体和液体的克每立方厘米（g/cm³），或气体的千克每立方米（kg/m³）。密度小于水（1.0 g/cm³）的材料通常会漂浮，而密度更大的材料会下沉。
| Material | 密度 (g/cm³) | Material | 密度 (g/cm³) |
|---|---|---|---|
| Air 空气 | 0.0013 | Aluminium 铝 | 2.7 |
| Wood 木材 | 0.6-0.9 | Iron 铁 | 7.9 |
| Water 水 | 1.0 | Copper 铜 | 8.9 |
Knowing density helps engineers compare materials. For example, aluminium is used in aircraft because it has a lower density than steel, so aircraft can be lighter and use less fuel.
了解密度有助于工程师比较材料。例如，铝用于飞机是因为它的密度比钢低，因此飞机可以更轻并节省燃料。
4. Melting Point and Boiling Point | 熔点与沸点
The melting point is the temperature at which a solid changes into a liquid. The boiling point is the temperature at which a liquid changes into a gas. Pure substances have sharp melting and boiling points, which makes these values useful for identification.
熔点是固体变为液体的温度。沸点是液体变为气体的温度。纯净物具有确定的熔点和沸点，这使得这些数值可用于鉴别物质。
Different materials melt and boil at very different temperatures. Water melts at 0°C and boils at 100°C, while iron melts at about 1538°C. Plastics and waxes often have much lower melting points than metals, so they are not used for oven trays or engine parts. Published by TutorHao | KS3 Chemistry Revision Series | aleveler.com
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