📚 KS3 CIE Acids and Alkalis Mark Scheme | KS3 CIE 酸碱与评分标准
In KS3 CIE Science, the Acids and Alkalis topic regularly appears in Checkpoint tests and school examinations. This article uses the official mark scheme style to show you exactly how examiners award marks for definitions, colour changes, word equations and practical observations.
在 KS3 CIE 科学中，酸碱主题经常出现在 Checkpoint 测试和学校考试中。本文按照官方评分标准风格，说明考官如何就定义、颜色变化、文字方程式和实验观察给分。
1. How the CIE Mark Scheme Works for Acids and Alkalis | CIE 酸碱评分标准如何运作
The mark scheme awards marks for specific scientific keywords, not for general descriptions. For example, saying 'the pH goes down' may earn no mark unless the answer states 'pH becomes lower because acid is added' or 'pH moves towards 7' in neutralisation.
评分标准根据具体的科学关键词给分，而不是笼统的描述。例如，说「pH 下降」可能得不到分，除非答案说明「因为加入酸所以 pH 降低」或「中和时 pH 趋向于 7」。
CIE KS3 questions usually assess three areas: AO1 Knowledge with understanding, AO2 Handling information and problem solving, and AO3 Experimental skills and investigations. In Acids and Alkalis, AO1 covers definitions and pH values; AO2 covers selecting indicators and predicting salt names; AO3 covers colour changes and gas tests.
CIE KS3 题目通常考查三个方面：AO1 知识与理解、AO2 信息处理与问题解决、AO3 实验技能与探究。在酸碱主题中，AO1 涵盖定义和 pH 值；AO2 涵盖选择指示剂和预测盐的名称；AO3 涵盖颜色变化和气体检验。
2. Key Definitions Examiners Look For | 考官关注的核心定义
An acid is a substance that produces hydrogen ions (H⁺) in aqueous solution. An alkali is a soluble base that produces hydroxide ions (OH⁻) in aqueous solution. A base is any substance that neutralises an acid, but only soluble bases are called alkalis.
酸是在水溶液中产生氢离子 (H⁺) 的物质。碱是溶于水产生氢氧根离子 (OH⁻) 的物质。碱是能中和酸的物质，但只有可溶性碱才称为 alkali。
The pH scale measures how acidic or alkaline a solution is, from 0 to 14. A neutral solution has pH 7. Mark schemes often require the exact phrase 'below 7' or 'above 7', not just 'acid' or 'alkali'.
pH 标度用于衡量溶液的酸性或碱性，范围从 0 到 14。中性溶液 pH 为 7。评分标准通常要求写出「低于 7」或「高于 7」，而不只是「酸」或「碱」。
| Keyword | Mark-scheme definition | 中文含义 |
|---|---|---|
| Acid | Substance producing H⁺ in water; pH below 7 | 在水中产生 H⁺；pH 小于 7 |
| Alkali | Soluble base producing OH⁻ in water; pH above 7 | 可溶性碱，在水中产生 OH⁻；pH 大于 7 |
| Base | Substance that neutralises an acid | 能中和酸的物质 |
| Indicator | Dye that changes colour in acid or alkali | 在酸或碱中变色的染料 |
| Neutralisation | Acid + base → salt + water | 酸 + 碱 → 盐 + 水 |
| Salt | Product formed from neutralisation of an acid | 酸被中和后生成的产物 |
3. pH Scale and Universal Indicator Mark Points | pH 标度与通用指示剂的给分点
The pH scale is logarithmic. A solution of pH 3 is ten times more acidic than pH 4, and one hundred times more acidic than pH 5. This idea is often tested with 'how many times more acidic' questions, and the answer must include the factor of 10 per pH unit.
pH 标度是对数的。pH 3 的溶液酸性是
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