Edexcel化学 酸碱中和 强弱酸 制盐 指示剂
酸、碱、中和、制盐这几个词在 Edexcel GCSE Chemistry (1CH0) 里主要落在 Topic 3 Chemical changes，滴定相关内容在 Topic 2 也有涉及。这一篇用对比的方式把最容易混的几组概念摆在一起：酸和碱基的分工、强酸和浓酸的区别、金属与碳酸盐两条产气路线、可溶盐与不溶盐两种制备手法。Paper 1 考 Topic 1 到 Topic 5，Paper 2 考 Topic 1 加上 Topic 6 到 Topic 9，所以酸碱部分在 Paper 1 里是主力，Paper 2 的跨主题题还会再抽到一次。文中口径以 Edexcel 官网公布的 1CH0 spec 为准，这套 spec 从 2016 年秋季开始施教，2018 年首次考试。两卷各 1 小时 45 分钟、各 100 分、各占 50%。
Acids and alkalis sit inside Topic 3 Chemical changes of the Edexcel GCSE Chemistry (1CH0) specification, with titration content appearing in Topic 2. Both papers last 1 hour 45 minutes and carry 100 marks, and the split is Topics 1 to 5 for Paper 1 against Topic 1 plus Topics 6 to 9 for Paper 2, so acid chemistry is examined in June of the second year and can return in synoptic questions. The comparisons below are the ones that decide marks in practice rather than the definitions themselves. Specification references follow the Edexcel 1CH0 document, first taught from 2016 with first exams in 2018.
酸、碱与碱基：三个词各指什么
酸在水溶液里会电离出氢离子 H+，这是 Edexcel 定义酸的方式，所以干燥的氯化氢气体本身不显酸性，溶于水之后才有酸的性质。碱基是能和酸反应生成盐和水的物质，金属氧化物、氢氧化物、碳酸盐都算。碱是能溶于水的碱基，氢氧化钠、氢氧化钾、氨水是实验室里最常见的三个。三个词的关系是包含关系：所有碱都是碱基，但只有可溶的碱基才叫碱。碳酸铜不溶于水，它是碱基而不是碱，写成碱会扣分。pH 小于 7 是酸性，等于 7 是中性，大于 7 是碱性，这是 1CH0 spec 里 pH 标尺的基本约定。很多学生以为凡是含 OH 的都算碱，其实氢氧化铜和氢氧化铁都不溶，只能叫碱基。
An acid is defined by what it releases in water, hydrogen ions, so dry hydrogen chloride gas is not acidic until it dissolves. A base is anything that reacts with an acid to give a salt and water, which covers metal oxides, hydroxides and carbonates, while an alkali is simply a base that dissolves in water and the three laboratory examples are sodium hydroxide, potassium hydroxide and aqueous ammonia. Copper(II) carbonate is a base but not an alkali, and writing alkali there costs a mark. The pH scale sets the working vocabulary, with values below 7 acidic, 7 neutral and above 7 alkaline according to the Edexcel specification.
强酸与弱酸：看电离程度不看 pH
强酸在水里几乎完全电离，盐酸放进去就是 H+ 和 Cl-，硫酸的第一步电离也接近完全。弱酸只有一小部分分子电离，乙酸溶于水后大部分仍以 CH3COOH 分子存在，只有少量变成 H+ 与 CH3COO-。判断强弱看的是电离程度，不是 pH 数值本身。浓度同为 0.1 mol/dm3 时，盐酸的 pH 更低，和镁条反应更快，导电性也更好，因为溶液里自由移动的离子更多。Edexcel 的 spec 要求用部分电离和完全电离来解释强弱，只写 pH 小所以是强酸拿不到解释分。常见的错误是把强弱和浓度混为一谈，看到一个 pH 1 的溶液就说它是浓酸，其实浓度很高的乙酸 pH 也可能落在 2 到 3 之间。
Strong acids ionise almost completely in water, so hydrochloric acid exists as H+ and Cl- with very little intact HCl left. Ethanoic acid behaves differently because only a small fraction of its molecules ionise, leaving most of the acid present as CH3COOH molecules. The Edexcel specification asks for that ionisation contrast rather than a pH value as the explanation, and the practical consequences are lower pH, faster reaction with magnesium and better conductivity for the strong acid at the same concentration.
浓溶液与强酸：两个独立的维度
浓度说的是单位体积溶液里溶质有多少，强度说的是溶质电离了多少，两个描述互不替代。取 0.001 mol/dm3 的盐酸，它是稀的，但依然是强酸，因为溶解的部分全部电离。取 10 mol/dm3 的乙酸，它是浓的，但依然是弱酸，大部分分子没有电离。浓的弱酸 pH 可能比稀的强酸更低，导电性与反应速率却不一定更高，这两项更依赖自由离子的浓度。题目里出现浓盐酸和稀盐酸，问的通常是反应速率和现象，出现强酸和弱酸，问的是电离与平衡。阅卷时看到把 concentrated 直接当成 strong 的答案一般不给分，因为两者考的是不同的概念。
Concentration counts how much solute sits in a given volume, while strength describes how far that solute has ionised; the two descriptions answer different questions and cannot substitute for each other. A 0.001 mol/dm3 hydrochloric acid solution is dilute yet still strong, because every dissolved molecule has split into ions. A 10 mol/dm3 ethanoic acid solution is concentrated yet still weak, since most molecules remain unionised. Rate of reaction and conductivity follow the concentration of free ions rather than the label on the bottle, which is why an examiner will not accept concentrated as a synonym for strong.
pH 标尺与四种指示剂的适用场合
pH 标尺按 Edexcel 的约定从 0 到 14，每降 1 个单位，H+ 浓度大约涨 10 倍。广泛指示剂给出的是大致范围，酸性红到橙，中性绿，碱性蓝到紫，适合快速判断但不适合记录精确数值。石蕊只能区分酸性和碱性，酸性变红、碱性变蓝，中性时颜色不变，多用在气体检验和粗略判断上。甲基橙在酸里红、在碱里黄，酚酞在酸里无色、在碱里粉红，这两种适合需要看清突变的场合。pH 探头配数据记录仪能连续读出数值，Core Practical 3.6 画曲线用的就是它，读数精度到 0.1。选指示剂的标准是终点附近颜色变化明显，而不是颜色本身好看。
The pH scale runs from 0 to 14 in the Edexcel specification, and a step of one unit corresponds to roughly a tenfold change in hydrogen ion concentration. Universal indicator gives a broad colour range from red through green to purple, which suits a quick check but not a recorded value. Litmus only separates acidic from alkaline solutions, methyl orange turns from red to yellow and phenolphthalein from colourless to pink, so both are used where a sharp visible change matters. A pH probe with a data logger reads continuously to 0.1 and is the instrument behind the Core Practical 3.6 curve.
中和反应：H+ 与 OH- 结合成水
中和反应的通式是酸加碱基生成盐和水，氢氧化钠和盐酸反应得到氯化钠和水。离子层面上真正发生变化的是 H+ 加 OH- 生成 H2O，钠离子和氯离子全程没有参与，写离子方程式时要删掉。中和通常放热，用保温杯测温度会看到上升，上升幅度取决于酸的浓度与体积。生活里的对应场景不少：胃酸过多吃抗酸药，成分多为碳酸钙或氢氧化镁；土壤偏酸撒石灰，用的是氧化钙或氢氧化钙；酸性废水排放前用碱调 pH。pH 到 7 只说明 H+ 与 OH- 数量相当，溶液里的盐还在。很多学生以为只有碱能中和酸，其实碳酸盐、金属氧化物同样能中和，只是产物里多了二氧化碳。
Neutralisation follows one pattern, acid plus base giving salt and water; at the ion level the only real change is H+ plus OH- forming H2O, so sodium and chloride ions are spectators and disappear from the ionic equation. The reaction releases heat, which is why a temperature rise appears when the same volumes and concentrations are mixed in an insulated cup. Everyday examples are easy to name, from calcium carbonate antacids for indigestion to lime spread on acidic soil and alkali dosing of acidic effluent. A pH of 7 marks equal amounts of H+ and OH- rather than the absence of salt.
酸加金属与酸加碳酸盐：两条产气路线
酸和金属反应生成盐和氢气，前提是这个金属在活动性顺序里排在氢前面，镁、锌、铁都可以，铜、银、金不行。现象是持续冒泡、金属逐渐溶解、试管壁温热，检验气体用点燃的木条，听到爆鸣声说明是氢气。酸和碳酸盐反应生成盐、水和二氧化碳，现象同样是冒泡，但气体通入石灰水会变浑浊。两条路线都冒泡，靠气体检验区分，不能只看气泡多少判断。碳酸盐那一路还会生成水，中和的意味更完整。铜不跟稀盐酸反应，所以制备硫酸铜不能用铜加酸这条路。很多学生以为所有金属都能和酸反应，把铜片放进盐酸里等半天没有现象，就是活动性顺序没记牢。
Metals above hydrogen in the reactivity series, such as magnesium, zinc and iron, displace hydrogen from a dilute acid and give a salt plus hydrogen gas. The test is a lighted splint, and the squeaky pop confirms hydrogen, whereas copper, silver and gold produce nothing at all. Carbonates take a different route to salt plus water plus carbon dioxide, and that gas turns limewater milky. Both routes fizz, so the gas test rather than the amount of bubbling decides which reaction is happening. Because copper sits below hydrogen, copper(II) sulfate has to be made from copper(II) oxide instead of the metal.
三条制盐路线怎么选
制盐的三条常用路线分别是金属加酸、金属氧化物或氢氧化物加酸、碳酸盐加酸，终点都是盐，中间产物不同。选哪一条取决于金属的活动性和目标盐的可溶性。制备硫酸铜只能用氧化铜或碳酸铜，因为铜不与稀硫酸反应。制备氯化镁可以用镁条，也可以用氧化镁，前者要控制酸的量以免残留金属，后者可以直接加过量再过滤。可溶的碱基或碳酸盐加过量之后过滤掉多余固体，滤液里只剩盐，这一步是拿到纯晶体的关键。碳酸盐路线会放出二氧化碳，冒泡停止通常意味着酸已经用完。阅卷时看到只写一条路线而不说选择理由的答案，解释分基本拿不到。
The three routes to a salt are metal plus acid, metal oxide or hydroxide plus acid, and carbonate plus acid, and they differ in what else appears alongside the salt. Choosing between them depends on the reactivity of the metal and on whether the target salt dissolves. Copper(II) sulfate has to come from copper(II) oxide or copper(II) carbonate, because copper does not react with dilute sulfuric acid at all. Adding an insoluble base or carbonate in excess and then filtering leaves a solution containing only the salt, which is the step that makes pure crystals possible. An answer that names a route without justifying it rarely earns the explanation marks.
可溶盐与不溶盐：结晶与沉淀两套手法
可溶盐的制备走过量固体加酸、过滤、结晶三段：先把稀硫酸温热，分批加入氧化铜直到不再溶解，滤掉多余的黑色固体，把滤液蒸发到表面出现晶膜，冷却后析出蓝色晶体，用滤纸压干。不溶盐的制备走沉淀路线：把含目标离子的两种溶液混合，硫酸钠溶液加氯化钡溶液就得到硫酸钡沉淀和氯化钠溶液。沉淀经过滤、用蒸馏水洗涤、干燥，得到的是粉末而不是晶体。两条路线的分界在于目标盐是否溶于水，硫酸钡和氯化银不溶，用沉淀法；硫酸铜和氯化钠可溶，用结晶法。洗涤这一步常被省略，残留的氯化钠会让纯字不成立。常见的错误是在沉淀路线里也去蒸发滤液，结果什么也得不到。
A soluble salt is made in stages: warm the acid, add the insoluble base in excess until no more dissolves, filter off what is left, then evaporate the filtrate to the point where crystals begin to form and cool it. An insoluble salt such as barium sulfate is made by precipitation instead, mixing sodium sulfate solution with barium chloride solution so the product drops out as a solid while sodium chloride stays dissolved. Filtration, washing with distilled water and drying give a powder rather than crystals, and skipping the wash leaves soluble impurities behind. The dividing question is simply whether the target salt dissolves in water. Evaporating the filtrate on a precipitation route is a common slip that yields nothing.
Core Practical 3.6：pH 随加入量变化的曲线
Core Practical 3.6 的做法是向固定体积的稀盐酸里分次加入粉末状氢氧化钙或氧化钙，每次加完搅拌、等固体溶解后测 pH，把 pH 对加入量作图。曲线起点在 pH 1 到 2，前段缓慢上升，接近等量点时几乎垂直跳升，越过之后逐渐平缓到 pH 11 到 12 附近。陡升的中点是 H+ 与 OH- 数量相等的位置，pH 约等于 7。测量要用 pH 探头，广泛指示剂只能给出颜色区间，画不出这段陡升。加粉末太快会直接跳过陡升段，读到的两个点一个在酸性一个在碱性，曲线中间就断了。这条曲线的形状和滴定曲线一致，只是横轴从体积换成了质量。很多学生以为曲线陡升的位置才是反应开始的位置，其实反应从第一次加入就开始了。
Core Practical 3.6 adds powdered calcium hydroxide or calcium oxide in portions to a fixed volume of dilute hydrochloric acid, stirring and measuring pH after each addition before plotting pH against the mass added. The curve starts between pH 1 and 2, climbs slowly, then rises almost vertically near the point where the amounts of H+ and OH- match, and flattens towards pH 11 or 12 once the base is in excess. The middle of that vertical section is the end point, close to pH 7. A pH probe is needed because universal indicator only gives a colour band and cannot resolve the jump, while adding powder too quickly skips it entirely and leaves a gap in the curve. The shape repeats the titration curve, with mass on the horizontal axis instead of volume.
盐的命名与化学式配平
盐的名字前半段来自金属或铵根，后半段来自酸：盐酸给氯化物，硫酸给硫酸盐，硝酸给硝酸盐，磷酸给磷酸盐，乙酸给乙酸盐。写化学式靠电荷配平，镁离子带两个正电，氯离子带一个负电，所以氯化镁写成 MgCl2。硫酸根带两个负电，铝离子带三个正电，硫酸铝要写成 Al2(SO4)3，括号不能省。硝酸钙里钙是 Ca2+、硝酸根是 NO3-，比例是 1 比 2，写成 Ca(NO3)2。配平过的方程式里还要标状态符号，(s)、(l)、(g)、(aq) 四个标记在 1CH0 的评分方案里都给分。常见的错误是只配平原子数不配平电荷，离子方程式的两边电荷必须相等。
Salt names are built from the metal or ammonium ion first and the parent acid second: hydrochloric acid gives chlorides, sulfuric acid sulfates, nitric acid nitrates, phosphoric acid phosphates and ethanoic acid ethanoates. Formulas follow charge balance rather than counting atoms by eye, so magnesium with two positive charges and chloride with one negative charge gives MgCl2. Sulfate carries two negative charges against three on aluminium, which makes aluminium sulfate Al2(SO4)3 with the brackets doing real work. Calcium nitrate needs one Ca2+ for every two nitrate ions, hence Ca(NO3)2. In ionic equations the charges on both sides must balance as well as the atoms, and that is where most lost marks sit.
答卷上的高频失分点
现象和结论要分开写，冒泡是现象，生成二氧化碳是结论，只写后者拿不到观察分。解释题要落到粒子或离子层面，写因为酸和碱反应不算解释，写 H+ 与 OH- 结合成水分子才算。单位与精度也有要求，温度记到 0.5 摄氏度或 0.1 摄氏度，质量记到 0.01 克，pH 记到 0.1，全篇统一。离子方程式只在题目要求或 Higher 卷的相关小问里出现，写的时候先配平原子再配平电荷。中和的定义范围要放宽到碱基，把中和等同于酸加碱会漏掉碳酸盐和金属氧化物。阅卷时最常见的一处失分是把无色写成透明，酚酞在酸里是无色而不是透明，这两个词在化学描述里不是一回事。
Observations and conclusions belong in separate sentences, since fizzing is what you see while carbon dioxide is what you infer, and only the first earns observation marks. Explanations have to reach the particle or ion level, so stating that acids react with alkalis explains nothing while naming the combination of H+ and OH- into water does. Recording precision matters too, with temperature to 0.5 or 0.1 degrees Celsius, mass to 0.01 g and pH to 0.1, kept consistent through the paper. Ionic equations appear where the question asks for them, and charge balance comes after atom balance. Examiners regularly see colourless written as transparent, which is not the same statement for phenolphthalein sitting in acid.
从 GCSE 到 A-Level：质子转移这条主线
GCSE 阶段的酸定义停在水溶液中产生 H+，中和写成 H+ 加 OH- 生成水，够用且不涉及平衡。A-Level 换成质子转移的说法：酸是质子给予体，碱是质子接受体。共轭酸碱对随之出现，H+ 在水里实际以 H3O+ 存在，不再孤立存在。强弱在 A-Level 用 Ka 和 pKa 量化，pH 由 -log[H+] 算出，弱酸的 pH 要用平衡近似求。两条线并不矛盾，GCSE 的离子视角正是 A-Level 的起点。提前在 GCSE 答卷上写共轭酸碱对不会加分，评分方案里没有这一项。想为 A-Level 做准备，把质子转移这条主线先理顺，比提前背 Ka 公式有用。
At GCSE the acid definition stops at hydrogen ions in aqueous solution and neutralisation is written as H+ plus OH- giving water, which is enough because no equilibrium is involved. A-Level replaces that with proton transfer, an acid donating a proton and a base accepting one, which brings conjugate pairs with it and treats H+ in water as H3O+. Strength becomes quantitative through Ka and pKa, and pH follows from the negative logarithm of the hydrogen ion concentration, with weak acids needing an equilibrium approximation. The two views are consistent, since the ion picture at GCSE is the starting point for the A-Level treatment. Writing conjugate pairs in a GCSE answer earns nothing because the mark scheme has no such point.
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