📚 Year 7 WJEC Chemistry: High-Frequency Topics and Common Errors Analysis | Year 7 WJEC 化学：高频考点与易错题分析
Welcome to this targeted revision resource for Year 7 WJEC Chemistry. We have analysed past exam trends and classroom performance to identify the topics that appear most often in assessments and the common mistakes that can trip you up. Use this guide to focus your revision and sharpen your exam technique.
欢迎使用这份针对 Year 7 WJEC 化学的复习指南。我们根据考试趋势和课堂表现，梳理出最常考的知识点以及学生容易踏入的误区。通过本文，你可以更有针对性地复习，避开常见的失分陷阱。
1. States of Matter and the Particle Model | 物质三态与粒子模型
The three states of matter – solid, liquid and gas – are explained by the particle model. In a solid, particles are packed closely in a regular arrangement and can only vibrate in fixed positions. In a liquid, particles are still close together but are able to move past each other, taking the shape of the container. In a gas, particles are far apart, move rapidly in all directions and fill any available space. One of the most common errors is thinking that particles themselves expand when a substance is heated. In reality, the particles remain the same size; it is the spaces between them that increase. For example, when ice melts, the water particles do not grow larger – they simply gain enough energy to break free from the rigid lattice, allowing them to flow. Another mistake is to confuse the random motion of gas particles with currents or ‘upward’ movement, whereas it is truly random and independent of orientation.
物质的三态（固态、液态和气态）可用粒子模型解释。固体中粒子紧密排列，只在固定位置振动；液体中粒子仍然靠近，但能自由滑动并容器的形状；气体中粒子相距很远，高速无规则运动并充满整个空间。学生常犯的一个错误是认为加热时粒子本身会膨胀。实际上，粒子大小不变，改变的是它们之间的距离。例如冰融化时，水分子并没有变大——它们只是获得足够能量脱离固定晶格，从而能够流动。另一个常见错误是将气体粒子的无规则运动误解为“向上”或某种定向流动，其实扩散运动是完全随机的。
2. Physical and Chemical Changes | 物理变化与化学变化
A physical change is one in which no new substances are made – changes of state, dissolving or mixing are typical examples. A chemical change produces at least one new substance with different properties, and it is usually irreversible by simple physical means. Students often wrongly classify boiling water as a chemical change because bubbles are observed, but those bubbles are just steam (still water). Similarly, melting, freezing and dissolving salt in water are physical changes. Evidence for a chemical change includes a colour change, a gas given off (effervescence), a temperature change, or the formation of a precipitate. In WJEC assessments, you may be asked to identify which process is chemical from a set of observations. An error to avoid is assuming that any fizzing must be chemical – if you dissolve a fizzy tablet in water, the fizz is release of pre‑dissolved gas which is often a physical change.
物理变化不产生新物质，如状态改变、溶解或混合；化学变化至少产生一种性质不同的新物质，通常无法用简单物理手段逆转。学生经常错误地把水沸腾归为化学变化，因为观察到气泡，但这些气泡是水蒸气，仍是水。同样，融化、冻结和食盐溶于水都是物理变化。判断化学变化的证据包括颜色变化、放出气体、温度改变或生成沉淀。在 WJEC 考试中，你可能会需要根据观察结果判断哪一种过程是化学变化。需注意的是，不是所有冒泡都是化学反应——比如泡腾片在水中溶解释放出预先溶解的气体，这通常属于物理变化。
| Physical Change | 化学变化 |
| No new substance formed | 新物质生成 |
| Usually easy to reverse | 通常不可逆 |
| e.g. melting ice, dissolving sugar | 例如：燃烧、生锈 |
3. Elements, Compounds and Mixtures | 元素、化合物与混合物
An element contains only one type of atom and cannot be broken down into simpler substances by chemical means. A compound consists of two or more different elements chemically bonded together in fixed proportions, and it has properties distinct from its constituent elements. A mixture contains two or more substances (elements or compounds) that are not chemically combined; each part keeps its own properties and the composition can vary. A very common misconception is to label air as a compound – air is actually a mixture of gases (mainly nitrogen, oxygen, argon and carbon dioxide). Similarly, many students think that all pure water is a mixture because it contains hydrogen and oxygen, but water is a compound because the hydrogen and oxygen are chemically bonded as H₂O molecules. When salt is dissolved in water, a mixture (salt solution) is formed, and the salt can be recovered by evaporation. In WJEC questions, you must be able to recognise pure substances and mixtures from particle diagrams and descriptions.
元素只包含一种原子，不能用化学方法分解成更简单的物质。化合物由两种或以上不同元素以化学键结合，有固定组成，其性质与组成元素不同。混合物包含两种或以上未发生化学结合的物质，各成分保留自身性质且比例可变。一个很常见的误解是认为空气是化合物——实际上空气是多种气体（主要是氮气、氧气、氩气和二氧化碳）的混合物。同样，很多学生认为纯水是混合物，因为它含有氢和氧，但水是化合物，因为氢和氧以化学键结合成 H₂O 分子。食盐溶于水形成混合物（盐溶液），通过蒸发可以回收盐。WJEC 题目常要求你根据粒子图或描述区分纯净物和混合物。
4. Basic Atomic Structure | 原子结构基础
Atoms are made of three subatomic particles: protons (positive charge, mass 1), neutrons (neutral, mass 1) and electrons (negative charge, negligible mass). Protons and neutrons are found in the nucleus, while electrons orbit in shells. The atomic number (Z) tells you the number of protons and, in a neutral atom, also the number of electrons. The mass number is the total of protons plus neutrons. A typical error is to confuse atomic number with mass number or to assume that the number of neutrons equals the number of protons. For instance, sodium has atomic number 11 and mass number 23, so it has 11 protons, 11 electrons and 12 neutrons (23 - 11). Another mistake is to think electrons have significant mass or that they reside inside the nucleus. In WJEC Year 7, you will be expected to interpret the representation of an element such as ¹²₆C, identifying the numbers of protons, neutrons and electrons.
原子由三种亚原子粒子组成：质子（带正电，质量数1）、中子（不带电，质量数1）和电子（带负电，质量可忽略）。质子和中子位于原子核内，电子分层排布。原子序数（Z）等于质子数，在中性原子中也等于电子数。质量数是质子数与中子数之和。常见错误是将原子序数与质量数混淆，或认为中子数一定等于质子数。例如，钠的原子序数是11、质量数是23，所以它有11个质子、11个电子和12个中子（23 – 11）。另一个错误是以电子质量很大，或者电子位于原子核内。在 Year 7 WJEC 化学中，你需要能够根据符号如 ¹²₆C 推断出质子、中子和电子的数量。
5. Introduction to the Periodic Table | 元素周期表入门
The periodic table arranges all known elements in order of increasing atomic number. Horizontal rows are called periods, and vertical columns are called groups. Elements in the same group have similar chemical properties because they have the same number of electrons in their outer shell. For example, Group 1 elements (lithium, sodium, potassium) are all very reactive metals that form alkaline solutions when they react with water. Group 7 elements (fluorine, chlorine, bromine) are reactive non-metals. A common mistake is to confuse periods and groups: a period tells you the number of electron shells an atom has, while the group often indicates the number of outer electrons (with some exceptions in transition metals). Also, hydrogen sits alone at the top, sometimes placed in Group 1, but it is not a metal. When labelling the table, remember that metals are found on the left and centre, and non-metals on the right.
元素周期表按原子序数递增排列所有已知元素。横排叫周期，竖列叫族。同一族的元素化学性质相似，因为它们最外层电子数相同。例如第1族元素（锂、钠、钾）都是非常活泼的金属，与水反应后生成碱性溶液。第7族元素（氟、氯、溴）是活泼的非金属。学生经常混淆周期和族：周期号代表原子的电子层数，而族号通常代表最外层电子数（过渡金属部分例外）。另外，氢独自位于表顶，有时放在第1族，但它不是金属。在指认周期表时，请记住金属在左侧和中部，非金属在右侧。WJEC 可能会要求你在空白周期表中指出金属/非金属分界以及各族的位置。
6. Metals and Non-metals | 金属与非金属
Metals generally share properties such as shiny appearance (lustre), good conduction of heat and electricity, malleability (can be hammered into sheets) and ductility (can be drawn into wires). Most have high melting points. Non-metals are typically dull, brittle as solids, and poor conductors of heat and electricity. One important exception is graphite, a form of carbon, which conducts electricity and is used in electrodes. A related chemical property is that metal oxides tend to be basic (they neutralise acids), while non-metal oxides are acidic. Students often misclassify substances like salt (sodium chloride) as metals because it is a solid crystal, but salt is a compound. Another error is to assume that all elements that look shiny are metals – iodine is a non-metal but it forms shiny grey crystals.
金属通常有光泽、是热和电的良导体、具有延展性和可锻性，熔点一般较高。非金属通常无光泽，固态时脆性大，导热导电性差。一个重要的例外是石墨——它是一种碳单质，却能导电，常用于电极。相关的化学性质是：金属氧化物大多显碱性（能中和酸），而非金属氧化物则显酸性。学生常把食盐（氯化钠）当作金属，因为它呈固态晶体，但食盐是化合物。另一个错误是认为所有看起来有光泽的单质都是金属——碘是非金属，但呈闪亮的灰色晶体。WJEC 考试中可能会让你根据性质判断一种未知元素属于金属还是非金属。
7. Acids and Alkalis | 酸与碱
Acids are substances that release hydrogen ions (H⁺) in water. Common laboratory acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃). Alkalis are soluble bases that release hydroxide ions (OH⁻) in water; examples are sodium hydroxide (NaOH) and potassium hydroxide (KOH). Indicators like litmus change colour in acids and alkalis: blue litmus turns red in acid, red litmus turns blue in alkali. Universal indicator gives a range of colours corresponding to the pH scale (0 to 14). A frequent mistake is reading the pH scale backwards: a low pH (0-3) indicates a strong acid, while a high pH (11-14) indicates a strong alkali; pH 7 is neutral. Students may wrongly think that a pH 1 is a weak acid because it is a small number. Also, remember that not all bases are alkalis – only bases that dissolve in water are alkalis.
酸是在水中释放氢离子（H⁺）的物质，实验室常见酸有盐酸（HCl）、硫酸（H₂SO₄）和硝酸（HNO₃）。碱是可溶性碱，在水中释放氢氧根离子（OH⁻），例如氢氧化钠（NaOH）和氢氧化钾（KOH）。指示剂如石蕊在酸和碱中变色：蓝色石蕊遇酸变红，红色石蕊遇碱变蓝。通用指示剂显示与 pH 值（0-14）对应的颜色渐变。一个常见错误是颠倒 pH 值的含义：pH 越小（0-3）代表酸性越强，越大（11-14）碱性越强，pH 7 为中性。有的学生误以为 pH 1 是弱酸，因为数字小。还需注意，并非所有的碱都是碱——只有能溶于水的碱才称为“碱”（alkali）。WJEC 常考 pH 值与物质腐蚀性强弱的关系以及安全标志的辨识。
8. Neutralisation | 中和反应
ample, hydrochloric acid + sodium hydroxide → sodium chloride + water. The name of the salt comes from the metal (from the alkali) and the acid used: sulfuric acid gives sulfate salts, nitric acid gives nitrate salts. A common error is to write the salt name incorrectly – for hydrochloric acid, the salt ends in ‘chloride’, not ‘ lution is always neutral without checking the acid and alkali strength. In WJEC practical questions, you might need to describe how to neutralise a named acid safely, or interpret temperature changes during the reaction.
酸与碱发生中和反应，生成盐和水。通用的文字方程式为：酸 + 碱 → 盐 + 水。例如：盐酸 + 氢氧化钠 → 氯化钠 + 水。盐的名称由碱中的金属和所用酸决定：硫酸生成硫酸盐，硝酸生成硝酸盐。常见错误是写错盐名——盐酸生成的盐以“氯化物”结尾，而不是“氯酸盐”。另一个错误是忘记中和反应是放热反应，或未考虑酸和碱的强弱就武断地认为生成的盐溶液一定是中性的。在 WJEC 的实验题中，你可能需要描述如何安全地中和某种酸，或解读反应中的温度变化。
9. Chemical Formulae and Word Equations | 化学式与文字方程式
s of atoms, such as H₂O (two hydrogen atoms bonded to one oxygen atom), CO₂, and NaCl. In Year 7 WJEC, you are mainly expected to write word equations rather than balanced symbol equations. A word equation uses the names of reactants and products with an arrow ( → ) to show the direction of the reaction. Common errors include writing incorrect subscripts – for example, writing HO₂ for water instead of H₂O – using an equals sign instead of an arrow. Students also sometimes confuse capitalisation: ‘CO’ is carbon monoxide, but ‘ cobalt. Always check the order of elements in a compound: MgO is magnesium oxide, not OMg.
化学符号代表元素（如 C 表示碳，Na 表示钠）。化合物的化学式表示原子的种类和比例，例如 H₂O（两个氢原子与一个氧原子结合）、CO₂ 和 NaCl。在 Year 7 WJEC 阶段，主要要求书写文字方程式而非配平的符号方程式。文字方程式用反应物和生成物的名称加上箭头（ → ）表示反应方向。学生常见错误包括下标写错（例如把水写成 HO₂ 而不是 H₂O），或用等号代替箭头。大小写也容易混淆：“CO”是一氧化碳，而“Co”是钴元素。始终留意化合物中元素的顺序：MgO 表示氧化镁，不能写成 OMg。题目可能会让你根据给定反应写出文字方程式，务必检查反应物和生成物名称是否准确。
10. Practical Skills and Laboratory Safety | 实验技能与实验室安全
WJEC Year 7 chemistry places a strong emphasis on practical skills. You must be able to use a Bunsen burner correctly: the safety flame (yellow) is visible but cooler, and the blue heating flame is hotter and difficult to see. When heating a substance in a test tube, hold it with a test‑tube holder, point it away from yourself and others, and move it gently in the flame to avoid bumping. Never smell a chemical directly – always waft the vapour towards your nose with your hand. Recognise hazard symbols for corrosive, flammable, toxic and oxidising substances and know that you should wear safety goggles and tie back long hair. A classic mistake during filtration is to pour the mixture too quickly, causing the filter paper to break; instead, pour slowly down a glass rod. In evaporation, do not heat the evaporating dish to dryness at full flame – stop heating when crystals just start to form and leave to cool for larger crystals.
WJEC Year 7 化学非常重视实验技能。你必须能正确使用本生灯：黄色安全焰可见但温度较低，蓝色加热焰温度高且几乎不可见。用试管加热时，要用试管夹夹持，管口不要对着自己和他人，并在火焰中轻轻移动以防暴沸。切勿直接嗅闻化学试剂——应用手扇闻。识别腐蚀性、易燃、有毒和氧化性等危险标志，并知道必须佩戴护目镜、扎好长发。过滤操作中的典型错误是倒入混合物过快导致滤纸破裂；正确方法是沿玻璃棒缓慢倾注。蒸发时不要用大火将蒸发皿直接加热至干——应在晶体刚开始析出时停止加热，然后静置冷却以获得较大晶体。掌握这些操作细节能帮助你在实验考查中避免扣分。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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