IGCSE化学 酸碱 指示剂 强酸弱酸 盐的制法
IGCSE 化学 0620 里，酸碱这一块内容不算多，出题面却很宽：指示剂变色、金属与碳酸盐反应的现象、氧化物分类、盐的制法、溶解度规则，五六年里几乎每张卷都能见到。Cambridge 官方 syllabus 把它们归在 Topic 7 Acids, bases and salts（代码 0620，适用于 2026、2027 与 2028 年考试）。Paper 3 与 Paper 4 各 80 分、占 50%，这一章的分值通常落在 8 到 12 分之间，偶尔与化学计量或定性分析拼成一道大题。很多学生以为背下几个化学方程式就够了，其实阅卷时扣得最多的是现象描述不完整与条件写错。下面按卷面顺序把酸、碱、指示剂、氧化物与盐的制法拆开讲，每个知识点都给出考场上真正会被检验的细节。
Acids, bases and salts make up Topic 7 of Cambridge IGCSE Chemistry 0620, the syllabus used for examination in 2026, 2027 and 2028; the topic looks small on paper yet it reaches into metals, oxides, salt preparation, solubility rules and qualitative analysis. On Paper 3 and Paper 4, worth 80 marks and 50% each, a question built around this topic usually carries 8 to 12 marks, and it is often stitched together with mole calculations or gas tests. Marks are lost less on the equation itself than on incomplete observation sentences and a missing condition.
三条考试路线与卷面位置
0620 的考生走两条路线：Core 考 Paper 1 与 Paper 3，成绩上限是 C；Extended 考 Paper 2 与 Paper 4，最高可以拿 A*。Paper 1 与 Paper 2 都是 45 分钟 40 道四选一，各占 30%，每道题只考一个点，指示剂颜色或氧化物分类都可能单独成题。Paper 3 与 Paper 4 都是 1 小时 15 分钟 80 分，各占 50%，平均每分不到一分钟。剩下的一卷是实验技能，Paper 5 是 1 小时 15 分钟的实验考，Paper 6 是 1 小时 40 分的笔试替考，两卷各占 20%，由学校报考时选定。酸碱内容在大题里通常带情境，4 到 6 分一题，要求同时写出现象、方程式与理由。阅卷时这一步最容易失分：学生把选择题的习惯带进大题，只写结论不写现象。
Candidates sit either the Core route of Paper 1 and Paper 3 or the Extended route of Paper 2 and Paper 4, and the route sets the ceiling, with Core stopping at grade C while Extended can reach A*. Both multiple-choice papers last 45 minutes for 40 marks and 30% of the grade, and both theory papers last 1 hour 15 minutes for 80 marks and 50%, which leaves under a minute per mark; the practical element is a single paper worth 20%, either the 1 hour 15 minute laboratory test or the 1 hour written Alternative to Practical. In the theory papers an acids question arrives inside a context and carries 4 to 6 marks, so a bare conclusion earns nothing without the observation and the equation behind it.
酸的三类反应与现象写法
酸的第一个通性是跟金属反应生成盐和氢气，稀盐酸加镁条会看到气泡与金属逐渐溶解，方程式写成 Mg + 2HCl -> MgCl2 + H2。第二个通性是跟碱反应生成盐和水，这就是中和反应，现象往往不明显，温度计会显示放热。第三个通性是跟碳酸盐反应生成盐、水和二氧化碳，把气体通进石灰水会变浑浊。很多学生以为所有金属都能这样反应，其实铜与稀盐酸不反应，钾、钠、钙与水反应更剧烈，考纲只要求用镁、锌、铁这类金属做例子。写现象时要把气体名称、溶液颜色变化与固体溶解情况分开写，光写冒气泡拿不到全部分。阅卷时错得最多的是把碳酸盐反应的气体写成氢气，看到气泡就下结论。
Three reactions define an acid: with a metal it gives a salt and hydrogen, with a base it gives a salt and water, and with a carbonate it gives a salt, water and carbon dioxide, and only the last one turns limewater milky. Copper does not react with dilute hydrochloric acid at all, while potassium, sodium and calcium react far more violently with water than the syllabus examples of magnesium, zinc and iron. Write the observation as separate pieces of evidence, because a bubble with an unlabelled gas is worth half marks, and a carbonate reaction reported as hydrogen is the commonest loss on this question.
碱的定义与铵盐放氨
碱是金属的氧化物或氢氧化物，能溶于水的碱才叫碱溶液，比如氢氧化钠、氢氧化钾与氨水。碱跟酸反应生成盐和水，跟铵盐反应会放出氨气，加热时更明显。检验氨气用湿润的红色石蕊试纸，试纸变蓝，也可以用蘸了浓盐酸的玻璃棒靠近，看到白烟。铵盐加碱的方程式可以写成 NH4Cl + NaOH -> NaCl + H2O + NH3。常见的错误是学生把生成的气体写成水蒸气，或者漏写加热条件，因为常温下这个反应很慢。阅卷时这类题按现象、气体名称与方程式分点给分，缺一项就少一分。
Bases are the oxides or hydroxides of metals, and an alkali is simply a base that dissolves in water, which is why sodium hydroxide, potassium hydroxide and ammonia solution behave so similarly. Bases neutralise acids and also displace ammonia from ammonium salts, a reaction that needs heating before it runs at a useful rate; the gas turns damp red litmus blue, and a glass rod dipped in concentrated hydrochloric acid gives white smoke in its presence. Students lose the mark by naming water vapour instead of ammonia, or by leaving out the heat condition and then writing an observation that does not match their equation.
三种指示剂的变色与 pH 读数
考纲点名的指示剂只有三种：石蕊、百里酚酞与甲基橙，这三种在不同酸碱条件下的颜色都要背。石蕊在酸性溶液里变红、在中性或碱性溶液里变蓝；甲基橙在酸性里红、碱性里黄，橙色是中间色；百里酚酞在酸性里无色、碱性里变蓝，这个口径跟其他考试局常用的无色酚酞不一样，属于 CAIE 特有的写法。用广泛 pH 试纸可以比较氢离子浓度，读数从 0 到 14，酸性小于 7，中性等于 7，碱性大于 7。很多学生以为指示剂能测出具体 pH，其实它们只给出酸碱性与大致范围，要数值只能用 pH 试纸或 pH 计。滴定实验里最常用甲基橙，因为两头颜色差别大，中间色不好判断，所以终点要取第一次出现稳定橙色的读数。
The syllabus names three indicators only, litmus, thymolphthalein and methyl orange, and every colour change has to be memorised precisely, with thymolphthalein colourless in acid and blue in alkali rather than the phenolphthalein used by other boards. Universal indicator paper compares hydrogen ion concentration on a scale from 0 to 14, where a reading below 7 is acidic, 7 is neutral and above 7 is alkaline; indicators show whether a solution is acidic or alkaline, not a numerical pH. In a titration methyl orange is preferred because its two end colours are far apart and the intermediate orange is a usable end point.
氢离子 氢氧根离子与中和离子方程式
水溶液里的酸都含 H+ 离子，碱溶液都含 OH- 离子，这是酸碱通性的来源。中和的离子方程式只有一条：H+ + OH- -> H2O，写成离子形式时钠离子与氯离子属于旁观离子，不该出现在式子里。盐酸与氢氧化钠反应的完整方程式是 HCl + NaOH -> NaCl + H2O，两者要分清，题目问离子方程式时写成完整式是常见错误。酸与碱的浓度不影响中和的离子方程式，浓硫酸与稀硫酸中和氢氧化钠时核心反应相同，区别在于稀释放热与生成盐的溶解情况。这一步最容易混的是把 H2O 写成 H2O2，或者忘了给离子标电荷。阅卷时离子方程式按电荷守恒与配平各占分，写错电荷这一分就没了。
Every aqueous acid contains hydrogen ions and every aqueous alkali contains hydroxide ions, and the neutralisation equation H+ + OH- -> H2O is the reason the same four reactions keep appearing on the papers. Sodium and chloride are spectator ions in hydrochloric acid plus sodium hydroxide, so they stay out of the ionic equation although they belong in the full one, and students who write the full equation when the question asks for the ionic version hand back a mark. Concentration changes the rate and the heat released but not the core reaction, and the two mistakes that cost most are H2O2 in place of H2O and an ion written without its charge.
强酸与弱酸的电离程度
强酸在水里完全电离，盐酸的方程式写成 HCl(aq) -> H+(aq) + Cl-(aq)，箭头是单向的。弱酸只部分电离，乙酸写成 CH3COOH(aq) <=> H+(aq) + CH3COO-(aq)，用可逆符号表示。浓度相同的两种酸，强酸的 pH 更低、导电性更强、与金属反应更快，这几条经常一起考。很多学生以为酸性强弱跟浓度是一回事，其实浓的乙酸溶液 pH 仍可能比稀盐酸高。判断的时候要看电离程度，而不是溶液里酸分子的总量，这部分属于 Supplement 内容，Paper 2 与 Paper 4 才会考。阅卷时最常见的是把可逆符号写成单箭头，或者用稀释来解释导电性差异。
A strong acid is completely dissociated in water, as HCl(aq) -> H+(aq) + Cl-(aq) shows, while a weak acid such as ethanoic acid is only partially dissociated, which is why its equation carries a reversible sign. For two solutions of the same concentration the strong acid has the lower pH, conducts better and reacts faster with a metal; acid strength and concentration are different ideas, and a concentrated weak acid can still sit at a higher pH than a dilute strong one. Answers are marked on the extent of dissociation, so quoting dilution to explain a conductivity difference misses the point entirely.
酸性氧化物与碱性氧化物的分类
氧化物可以先按金属与非金属分，再按它跟酸碱的反应分：非金属氧化物多为酸性氧化物，二氧化硫与二氧化碳都能跟碱反应生成盐和水；金属氧化物多为碱性氧化物，氧化铜与氧化钙跟酸反应生成盐和水。氧化钙遇水生成氢氧化钙会放出大量热，这个反应常用在自热餐盒与土壤改良里。考纲点名的例子就这几个，选择题却喜欢把它们混在一起考，比如问哪一种氧化物既不属于酸性也不属于碱性。判断时先看组成元素，再看它与谁反应，两步走比死记例子可靠。阅卷时错得最多的是把二氧化碳归成碱性，因为学生没想清楚二氧化碳溶于水显酸性的这一步。
Oxides are first grouped by whether the element joined to oxygen is a metal or a non-metal, then sorted by what they react with; sulphur dioxide and carbon dioxide are acidic because they neutralise alkalis, while copper(II) oxide and calcium oxide are basic because they neutralise acids. Calcium oxide reacting with water is strongly exothermic, which is why it appears in self-heating food packs and in soil treatment. The classification that catches students out most often is carbon dioxide, which is acidic because it forms an acid in water, not basic.
两性氧化物与铝锌的特殊性
两性氧化物既能跟酸反应又能跟碱反应，考纲只点名氧化铝与氧化锌两种。氧化铝加盐酸生成氯化铝和水，加氢氧化钠则生成偏铝酸钠和水，两个方向都能得到盐。锌表面的氧化锌也有同样性质，所以锌既能溶于酸也能溶于强碱。判断的一个小技巧是看元素在周期表里的位置，处在金属与非金属交界附近的元素，氧化物往往两性。很多学生以为两性就是既能溶于水又能溶于酸，其实溶解性跟两性没有关系，氧化铝不溶于水。Paper 4 常在这里出 2 到 3 分的解释题，要求写清与酸和与碱分别生成了什么。
An amphoteric oxide reacts with acids and with bases, and the syllabus names only two examples, aluminium oxide and zinc oxide; aluminium oxide gives a salt and water with hydrochloric acid, and a different salt and water with sodium hydroxide, so both directions end in a salt. The same behaviour explains why zinc dissolves in strong alkali as well as in acid, and the pattern is easiest to remember for elements sitting near the metal and non-metal boundary of the Periodic Table. Amphoteric does not mean soluble, since aluminium oxide does not dissolve in water, and Paper 4 usually asks for 2 or 3 marks comparing the two products.
可溶盐的四种制法与选路逻辑
可溶盐有四条制法路线，选哪一条由原料决定。碱加酸要走滴定，因为碱和酸都是溶液，没有多余的固体可以过滤掉，所以用指示剂确定恰好中和的那一点，再把溶液蒸发结晶。金属加酸适用于锌、镁、铁这类比氢活泼的金属，做法是加过量金属直到不再冒泡，然后过滤掉剩余金属。不溶的碱与不溶的碳酸盐是第三条和第四条路，同样靠加过量固体再过滤，好处是不需要指示剂，现象就能判断终点。四条路线最后一步都是蒸发浓缩、冷却结晶、过滤、干燥，区别只在怎么控制恰好完全反应。常见的错误是滴定实验里用碳酸钙做原料，它根本不溶，溶液里没法确定终点。
A soluble salt is made by one of four routes and the choice follows from the starting materials: an alkali plus an acid needs a titration, because both are solutions and there is nothing to filter off, so an indicator marks the exact neutral point before evaporation. A metal plus an acid suits reactive metals such as zinc, magnesium or iron, where excess metal is added until fizzing stops and the leftover solid is filtered out. An insoluble base or an insoluble carbonate works the same way without an indicator, and every route ends with evaporation, cooling, crystallisation, filtration and drying; using calcium carbonate in a titration is a common error because the solid never dissolves.
溶解度规则与例外名单
盐能不能溶，决定了它用哪条路线做。钠盐、钾盐、铵盐与所有硝酸盐都可溶；氯化物可溶，只有氯化铅与氯化银例外；硫酸盐可溶，但硫酸钡、硫酸钙与硫酸铅例外；碳酸盐难溶，钠、钾、铵的碳酸盐例外；氢氧化物难溶，钠、钾、铵与氢氧化钙例外，氢氧化钙只是微溶。这些例外正是考点，题目常问某两种溶液混合会不会有沉淀，答题时要把生成的盐拿去跟例外名单对照。很多学生以为颜色也能帮忙判断，其实多数盐溶液是无色的，只有含铜离子、铁离子与亚铁离子的溶液有颜色。阅卷时这一步最容易失分的是只写沉淀名称不写颜色，而颜色往往单独占一分。
The solubility rules decide which preparation route is even possible, and the exceptions carry the marks: sodium, potassium, ammonium and nitrate salts all dissolve, chlorides dissolve except lead and silver, sulphates dissolve except barium, calcium and lead, carbonates are insoluble apart from the sodium, potassium and ammonium ones, and hydroxides are insoluble apart from those plus partially soluble calcium hydroxide. A precipitation question is answered by naming the new salt and checking it against that list; colour is a separate matter, because most salt solutions are colourless and only copper, iron(II) and iron(III) solutions are coloured. Answers routinely lose a mark for naming a precipitate without its colour.
沉淀法与结晶水的检验
不溶盐用沉淀法做，把两种可溶盐溶液混合，生成的难溶盐直接沉下来，再过滤、洗涤、干燥。硫酸钡的做法是硫酸钠加氯化钡，方程式写成 Na2SO4 + BaCl2 -> BaSO4 + 2NaCl，这个白色沉淀也用来检验硫酸根离子。洗涤这一步常被忽略，目的是把附着在沉淀表面的可溶盐洗掉，题目问为什么洗涤，要的就是这一句。结晶水是晶体里以固定比例结合的水，五水硫酸铜写作 CuSO4·5H2O，蓝色晶体加热会变成白色的无水硫酸铜，加水又变回蓝色，这一对现象常用来检验水。含结晶水的物质叫水合物，不含水的叫无水物，加热失去结晶水是化学变化而不是物理变化。阅卷时学生会把白色与无色混用，无水硫酸铜是白色固体，它的溶液才是蓝色。
Insoluble salts are made by precipitation: two soluble solutions are mixed, the new insoluble salt drops out, and the solid is filtered, washed and dried, as in Na2SO4 + BaCl2 -> BaSO4 + 2NaCl, where the white precipitate is also the standard sulfate test. Washing removes soluble salts clinging to the surface of the precipitate, and that single sentence is what the question is after when it asks why the solid is washed. Water of crystallisation is bound in fixed proportion, as in CuSO4·5H2O, where heating the blue crystals gives white anhydrous copper(II) sulphate and adding water restores the blue, a pair of observations commonly used to test for water.
Paper 5 与 Paper 6 的读数口径与排期
Paper 5 与 Paper 6 各 40 分、占 20%，全部考实验技能，酸碱相关的题几乎每年都有：用滴定管做中和滴定、按给定方法制盐、用 pH 试纸与指示剂比较酸碱性。滴定读数要读到小数点后两位，视线与凹液面最低点齐平，两次读数相差不超过 0.05 cm3 才叫一致。数据表格要把单位写在表头，不能每个数字后面都带单位；画图要用铅笔与直尺，异常点标出来但不连进平滑曲线。错得最多的是把自变量与因变量写反，或者控制变量只写一个。三周复习可以这样排：第一周过知识点与方程式，第二周做 Paper 3 与 Paper 4 的真题并整理现象描述，第三周专攻 Paper 5 与 Paper 6 的方法题。误差分析要写具体，写 human error 一分不给，写滴定管读数时的视差才给分。
Papers 5 and 6 carry 40 marks and 20% each and test practical skills only, and acidic and alkaline solutions appear almost every year through titration, salt preparation and pH comparison. In a titration the burette is read to two decimal places with the eye level with the bottom of the meniscus, and two readings count as concordant when they differ by no more than 0.05 cm3; tables take units in the heading rather than after every number, and graphs are drawn with pencil and ruler with anomalous points marked but left out of the line of best fit. Error analysis earns nothing unless it names a specific cause such as parallax when reading the burette, and mixing up the independent and dependent variables is the single most common loss on these papers.
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