📚 AQA AS Chemistry Unit 2 Mark Scheme June 2019 | AQA AS 化学 Unit 2 评分标准（2019年6月）
If you are working through the AQA AS Chemistry Unit 2 paper from June 2019, the mark scheme is a vital tool. It shows exactly how marks are awarded, which equations are expected, and which words must appear for a 'correct' answer.
如果你正在刷 AQA AS 化学 Unit 2 2019 年 6 月的真题，那么评分标准就是你的“答案之书”。它清楚说明了每个得分点如何给分、期望的方程式是什么，以及哪些关键词必须出现才算“正确”。
1. Why the Mark Scheme Matters | 为什么评分标准至关重要
The June 19 mark scheme is not just a list of model answers. It contains phrases such as 'allow', 'ignore', and 'do not accept', and these phrases tell you exactly how credit is given for alternative valid answers.
2019 年 6 月的评分标准不只是一份“标准答案”列表。它包含 'allow'、'ignore' 和 'do not accept' 等短语，而这些短语决定了替代性正确答案能否得分。
'Allow' means your answer is chemically equivalent and can earn the mark.
'Allow' 表示你的答案在化学上等价，因此可以获得该分。
'Ignore' means an extra word or a relevant but unnecessary statement will not destroy an otherwise correct answer.
'Ignore' 表示多余的词或相关但不必要的表述不会使原本正确的答案失分。
'Do not accept' usually targets a common misconception, a missing key term, or an impossible chemistry statement.
'Do not accept' 通常针对常见误解、关键术语缺失或不符合化学规律的表述。
The June 2019 Unit 2 mark scheme also uses 'error carried forward', often shortened to ECF, which means a later answer can still score marks if it is based on an earlier wrong value but the method is correct.
2019 年 6 月 Unit 2 评分标准还使用“错误延续”原则，常简写为 ECF，意思是如果后续答案基于前面的错误数值，但方法正确，仍然可以获得后续步骤分。
2. Command Words: State, Calculate, Explain and Deduce | 指令词：State、Calculate、Explain 与 Deduce
Many AS chemistry students lose marks because they respond to the wrong command word. The mark scheme aligns every answer to the cognitive demand of the question.
很多 AS 化学学生在考试中失分，是因为没有正确理解指令词。评分标准会把每个答案对应到题目所要求的思维层次上。
| Command word | 指令词 | What the mark scheme expects | 评分标准期望 |
|---|---|
| State | Give a name, formula, value or fact with no explanation. One word can be enough. |
| Calculate | Show working. A correct answer with units usually gets full marks, but a wrong answer with no working gets zero. |
| Explain | Give a reason using chemistry ideas. The mark scheme often awards separate marks for cause and consequence. |
| Deduce | Use information in the question to reach a conclusion. You are not expected to recall a memorised result. |
| Suggest | Apply knowledge to an unfamiliar context. The mark scheme allows any chemically sensible answer. |
In the June 19 paper, 'explain' questions tended to carry two marks: one for a correct chemical idea and one for making the link to the observation or data given.
在 2019 年 6 月的试卷中，'explain' 类问题通常占 2 分：1 分来自正确的化学观点，另 1 分来自将该观点与题目给出的现象或数据联系起来。
3. Equations and State Symbols | 方程式与状态符号
Balanced equations are used throughout Unit 2. The mark scheme will normally allow a correct equation even if the order of reactants is different, but it will not allow unbalanced equations or incorrect formulae.
配平方程式贯穿整个 Unit 2。评分标准通常允许反应物顺序不同但正确的方程式，但不会接受未配平的方程式或错误的化学式。
State symbols can change the number of marks in a question. For example, when an equation is explicitly required, the mark scheme expects solid, aqueous, liquid and gas labels to be included.
状态符号有时会直接影响得分。例如，当题目明确要求写出化学方程式时，评分标准期望你写出固态、水溶液、液态和气态符号。
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
In the mark scheme, this equation would score the full equation mark if the formulae and balance are correct. If state symbols are omitted but were not explicitly requested, examiners usually ignore the omission; if they were requested, the mark is withheld.
在评分标准中，只要化学式和配平正确，这个方程式可以获得方程式分。如果题目没有明确要求状态符号，省略一般会被忽略；但若题目明确要求，则省略会扣分。
4. Amount of Substance and Unit Conventions | 物质的量与单位规范
Unit 2 calculations often test amount of substance, molar mass, concentration and gas volumes. The mark scheme gives method marks, so you must show enough steps for the examiner to follow.
Unit 2 的计算题经常考查物质的量、摩尔质量、浓度和气体体积。评分标准会给方法分，因此你必须写出足够多的步骤，让考官能够看清思路。
n = m / M
n = c × V ( V in dm³ )
Volumes given in cm³ must be divided by 1000 before substitution. The mark scheme awards the final unit mark only if the correct unit is written after the numerical answer.
如果题目给出的体积单位是 cm³，代入公式前必须除以 1000。评分标准要求最终答案写出正确单位，否则最后一个“答案分”可能不给。
Always write 'mol' for amount of substance, not an abbreviation or no unit.
物质的量单位要写 'mol'，不能只写数字或使用不规范的简写。
Concentration is usually in mol dm⁻³, not mol dm³ or M unless the mark scheme says so.
浓度的常用单位是 mol dm⁻³，除非评分标准特别说明，否则不要写 mol dm³ 或 M。
Give numerical answers to 3 significant figures unless the question says otherwise.
除非题目另有说明，最终数值建议保留 3 位有效数字。
In June 2019, many candidates lost one mark because they used the mole ratio from the balanced equation but then forgot to convert 25.0 cm³ into 0.0250 dm³. This is exactly the type of error the mark scheme separates: method mark correct, final answer mark lost.
在 2019 年 6 月考试中，许多考生因为已经正确使用方程式中的摩尔比例，却没有把 25.0 cm³ 换算成 0.0250 dm³，从而丢掉了最后的答案分。评分标准会这样拆分：方法分给分，答案分不给。
5. Energetics and Hess's Law | 能量学与 Hess
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