📚 Elements, Mixtures and Compounds | 元素、混合物和化合物
Everything around us is made of matter, and chemistry helps us understand what matter is made of. In KS3 science, we group materials into elements, mixtures and compounds. These three categories look different at the particle level, and recognising them is a key skill for practical work and exams.
我们周围的一切都是由物质组成的，而化学帮助我们理解物质的构成。在KS3科学中，我们把材料分为元素、混合物和化合物三大类。这三类物质在粒子层面上看起来不同，能够识别它们是实验操作和考试中的关键技能。
1. Matter and the Particle Model | 物质与粒子模型
All substances are made of tiny particles that are too small to see. The particle model describes how particles are arranged and how they move in solids, liquids and gases.
所有物质都是由小到看不见的粒子组成的。粒子模型描述了粒子在固体、液体和气体中的排列方式和运动方式。
In a solid, particles are packed closely in a regular pattern and only vibrate in fixed positions. In a liquid, particles are still close but can slide past each other. In a gas, particles are far apart and move quickly in all directions.
在固体中，粒子紧密排列成规则的结构，只能在固定位置上振动。在液体中，粒子仍然靠得很近，但可以相互滑动。在气体中，粒子相距很远，向各个方向快速运动。
2. Elements: The Simplest Substances | 元素：最简单的物质
An element is a pure substance made of only one type of atom. There are about 118 known elements, and each element has its own unique atoms. Examples include oxygen, carbon, iron, gold and hydrogen.
元素是由同一种原子组成的纯净物。目前已知大约有118种元素，每种元素都有自己独特的原子。例子包括氧、碳、铁、金和氢。
Elements cannot be broken down into simpler substances by chemical reactions. For example, heating iron does not turn it into any simpler substance; it stays as iron atoms.
元素不能通过化学反应分解成更简单的物质。例如，给铁加热不会使它变成任何更简单的物质；它仍然是铁原子。
The smallest particle of an element that still has the properties of that element is called an atom. Some elements exist as single atoms, such as helium, while others exist as pairs of atoms, such as oxygen gas, O₂.
仍然具有该元素性质的最小粒子称为原子。有些元素以单个原子存在，如氦；而有些元素以原子对存在，如氧气 O₂。
3. The Periodic Table and Chemical Symbols | 元素周期表与化学符号
The periodic table lists all known elements in order of increasing atomic number. Each box gives the chemical symbol, atomic number and relative atomic mass. The table is arranged in rows called periods and columns called groups.
元素周期表按原子序数递增的顺序列出了所有已知元素。每个方框中都给出化学符号、原子序数和相对原子质量。周期表按行（周期）和列（族）排列。
Chemical symbols are short ways of writing element names. Most symbols have one or two letters. The first letter is always a capital letter; if there is a second letter, it is lowercase. For example, H for hydrogen, O for oxygen, Na for sodium and Fe for iron.
化学符号是书写元素名称的简写方式。大多数符号由一或两个字母组成。第一个字母总是大写；如果有第二个字母，则为小写。例如，H表示氢，O表示氧，Na表示钠，Fe表示铁。
Some symbols come from Latin names, so they do not match the English name. For example, sodium is Na from ‘natrium’, and gold is Au from ‘aurum’.
有些符号来自拉丁语名称，因此与英文名称不一致。例如，钠的符号Na来自’natrium’，金的符号Au来自’aurum’。
4. Atoms and Molecules | 原子与分子
Atoms are the smallest particles of an element. When two or more atoms join together chemically, a molecule is formed. A molecule can contain the same type of atoms, like O₂, or different types of atoms, like H₂O.
原子是元素最小的粒子。当两个或更多原子以化学方式结合时，就形成了分子。分子可以包含同种原子，如O₂；也可以包含不同种原子，如H₂O。
Elements, compounds and even some mixtures can be described in terms of molecules. However, mixtures do not have fixed molecule types because the substances are not chemically bonded.
元素、化合物甚至一些混合物都可以用分子来描述。然而，混合物没有固定的分子类型，因为其中的物质并不是以化学键结合的。
5. Compounds: Chemical Combination | 化合物：化学结合
A compound is a pure substance made of two or more different elements chemically joined together. The properties of a compound are usually different from the properties of the elements that make it. For example, sodium chloride (table salt) is made from sodium, a reactive metal, and chlorine, a toxic green gas.
化合物是由两种或更多不同元素以化学方式结合而成的纯净物。化合物的性质通常与组成它的元素的性质不同。例如，氯化钠（食盐）由活泼金属钠和有毒的绿色气体氯气组成。
Compounds always contain elements in fixed proportions. Water, H₂O, always has two hydrogen atoms bonded to one oxygen atom. Carbon dioxide, CO₂, always has one carbon atom bonded to two oxygen atoms.
化合物中各元素总是以固定的比例存在。水H₂O中总是两个氢原子与一个氧原子结合。二氧化碳CO₂中总是有一个碳原子与两个氧原子结合。
To separate a compound into its elements, a chemical reaction is needed. Physical methods such as filtering or evaporating cannot break the chemical bonds in a compound.
要把化合物分解成元素，需要发生化学反应。过滤或蒸发等物理方法不能破坏化合物中的化学键。
6. Writing Chemical Formulae | 化学式的书写
A chemical formula uses symbols and numbers to show which elements are in a compound and how many atoms of each element are present. The number is written as a subscript after the symbol.
化学式用符号和数字来表示化合物中有哪些元素以及每种元素有多少个原子。数字作为下标写在符号之后。
For example, CO₂ contains one carbon atom and two oxygen atoms. H₂O contains two hydrogen atoms and one oxygen atom. If there is only one atom of an element, no number is written.
例如，CO₂ 含有1个碳原子和2个氧原子。H₂O含有2个氢原子和1个氧原子。如果某种元素只有1个原子，则不写数字。
Common formulae you should know at KS3 include: H₂O (water), CO₂ (carbon dioxide), O₂ (oxygen), N₂ (nitrogen), NaCl (sodium chloride), CH₄ (methane) and HCl (hydrochloric acid).
KS3阶段应掌握的常见化学式包括：H₂O（水）、CO₂（二氧化碳）、O₂（氧气）、N₂（氮气）、NaCl（氯化钠）、CH₄（甲烷）和HCl（盐酸）。
7. Mixtures and Pure Substances | 混合物与纯净物
A mixture contains two or more substances that are not chemically joined. The substances in a mixture keep their own properties and can usually be separated by physical methods.
混合物含有两种或更多未经化学结合的物质。混合物中的物质保持各自的性质，通常可以用物理方法分离。
Air is a mixture of nitrogen, oxygen, carbon dioxide, argon and water vapour. Seawater is a mixture of water, salt and other dissolved substances. The composition of a mixture is not fixed.
空气是氮气、氧气、二氧化碳、氩气和水蒸气的混合物。海水是水、盐和其他溶解物质的混合物。混合物的组成不是固定的。
A pure substance is either a single element or a single compound. It has a definite composition and sharp melting and boiling points. A mixture often melts or boils over a range of temperatures.
纯净物要么是单质，要么是单一的化合物。它具有确定的组成和固定的熔点、沸点。混合物往往在一个温度范围内熔化或沸腾。
8. Comparing Elements, Compounds and Mixtures | 元素、化合物与混合物的比较
Elements contain one type of atom; compounds contain two or more types of atom chemically bonded; mixtures contain different substances physically mixed but not bonded.
元素只含一种原子；化合物由两种或更多种原子以化学键结合；混合物由不同物质物理混合但未结合。
The particles in an element are all identical atoms or molecules made of the same type of atom. In a compound, all particles are identical molecules or formula units with fixed ratios. In a mixture, different particles are mixed together randomly.
元素中的粒子都是相同的原子或由同种原子构成的分子。化合物中的粒子都是相同的分子或以固定比例构成的化学式单元。混合物中的不同粒子随机混合在一起。
| Feature | Element | Compound | Mixture |
|---|---|---|---|
| Type of atoms | One type | Two or more types bonded | Two or more substances not bonded |
| Properties | Fixed | Fixed, often different from elements | Each substance keeps its own properties |
| Separation | Cannot be broken down | Chemical reaction needed | Physical methods |
| Example | Iron, oxygen | Water, carbon dioxide | Air, seawater |
9. Separating Mixtures: Physical Techniques | 混合物的分离：物理方法
Because the substances in a mixture are not chemically combined, they can be separated by physical techniques. The method chosen depends on the states and properties of the substances.
由于混合物中的物质没有化学结合，因此可以用物理方法进行分离。选择哪种方法取决于物质的状态和性质。
Filtration is used to separate an insoluble solid from a liquid, such as sand from water. The liquid passes through the filter paper and the solid stays behind.
过滤用于将不溶性固体与液体分离，例如分离沙子和水。液体会通过滤纸，而固体则留在滤纸上。
Evaporation is used to separate a soluble solid from a solution, such as salt from salt water. Heating the solution removes the liquid, leaving the solid behind.
蒸发用于将可溶性固体从溶液中分离出来，例如将盐从盐水中分离。加热溶液可以除去液体，留下固体。
Distillation separates a solvent from a solution by boiling and condensing the liquid. It is used to obtain pure water from seawater. Simple distillation collects the liquid, while the dissolved solid remains in the flask.
蒸馏通过蒸发和冷凝液体，将溶剂从溶液中分离出来。它用于从海水中获得纯水。简单蒸馏收集液体，而溶解的固体留在烧瓶中。
Chromatography separates coloured substances dissolved in a solvent. As the solvent moves up the paper, different dyes travel at different speeds and separate into spots.
色谱法用于分离溶解在溶剂中的有色物质。当溶剂沿纸张上升时，不同染料以不同速度移动，从而分离成不同斑点。
Magnetic separation can separate magnetic materials such as iron from non-magnetic materials such as sand or sulfur. A magnet attracts the iron but not the sand.
磁选可以将铁等磁性材料与沙子或硫等非磁性材料分离。磁铁会吸引铁，而不会吸引沙子。
10. Everyday Examples and Key Skills | 日常实例与关键技能
Understanding elements, mixtures and compounds helps explain materials in daily life. Salt and water are compounds; air and seawater are mixtures; iron and aluminium are elements.
理解元素、混合物和化合物有助于解释日常生活中的材料。盐和水是化合物；空气和海水是混合物；铁和铝是元素。
When a piece of iron rusts, iron reacts with oxygen and water to form hydrated iron(III) oxide, a compound. This is a chemical change, not simply mixing.
当一块铁生锈时，铁与氧气和水反应生成水合氧化铁，这是一种化合物。这是化学变化，而不仅仅是混合。
In exams, you may be asked to identify whether a diagram shows an element, compound or mixture. Look carefully at whether the particles are all the same type, whether they are joined in groups, and whether different particles are mixed randomly.
在考试中，你可能会被要求判断某幅图表示的是元素、化合物还是混合物。仔细观察粒子是否全部为同一种类型，它们是否成组结合，以及不同粒子是否随机混合。
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