CAIE化学0620 酸碱盐 强弱酸 pH 盐的制备
CAIE IGCSE 化学 0620 把酸碱内容集中在主题 7 Acids, bases and salts,分成 7.1 酸碱特性、7.2 氧化物、7.3 盐的制取三节。2026 到 2028 年的考试沿用同一份考纲,6 月与 11 月各有一场考试,印度另有 3 月系列。所有考生都要考三份卷子,核心卷考生可拿 C 到 G,拓展卷考生可拿 A* 到 G。很多学生以为酸碱部分背下颜色变化就够了,真正拉开差距的是盐的制取路线与离子方程式。下面按考点顺序把酸碱盐拆开讲,并对照强弱酸与四条制盐路线的差别。
Topic 7 of Cambridge IGCSE Chemistry 0620 covers the characteristic properties of acids and bases, oxides, and the preparation of salts. The same syllabus serves the 2026, 2027 and 2028 exam series, with papers in June and November. This guide compares strong and weak acids, then works through the four routes for making a salt and the checks examiners look for.
酸的三种典型反应与产物
酸的典型反应有三类,每一类给出不同的产物。酸与活泼金属反应生成盐和氢气,镁放进稀盐酸里会持续冒气泡,氢气可用点燃听爆鸣声的方法检验。酸与碱反应生成盐和水,这一类就是中和反应。酸与碳酸盐反应生成盐、水和二氧化碳,把气体通进石灰水会让石灰水变浑浊。三条路线里盐总在产物一侧,所以先写对盐,再配平氢和氧。描述实验时要说清用的是哪一种酸,只写加入酸拿不到方法分。
An acid shows three characteristic reactions, and each one produces a different set of products. With a reactive metal it gives a salt and hydrogen, which is why magnesium fizzes steadily in dilute hydrochloric acid; with a base it gives a salt and water, the reaction known as neutralisation; with a carbonate it gives a salt, water and carbon dioxide, and that gas turns limewater milky. Method marks in Papers 3 and 4 are lost when a description says only that acid was added, so name the acid and say how the end point was recognised.
碱、可溶性碱与铵盐的反应
碱是金属的氧化物或金属的氢氧化物,其中能溶于水的那些叫可溶性碱,英文写作 alkali。碱与酸反应生成盐和水,这条规律与中和反应是同一回事。碱还能与铵盐反应,放出氨气,氨气有刺激性气味,用湿润的红色石蕊试纸靠近会变蓝。实验室常见的可溶性碱有氢氧化钠溶液和氨水,氢氧化钠属于强碱,氨水属于弱碱。写离子方程式时记住,酸的水溶液提供氢离子,可溶性碱的水溶液提供氢氧根离子,这两个离子出现在所有中和反应里。
A base is a metal oxide or a metal hydroxide, and the soluble ones are called alkalis. Bases react with acids to give a salt and water, and they also react with ammonium salts to release ammonia, which turns damp red litmus paper blue. Sodium hydroxide solution is a strong alkali while ammonia solution is weak, and in aqueous solution acids supply hydrogen ions while alkalis supply hydroxide ions.
三种指示剂在酸和碱里的颜色
指示剂的颜色是最容易被考到又最容易写不完整的部分。石蕊在酸中变红,在碱中变蓝,中性时呈紫色。百里酚酞在酸中无色,在碱中变蓝。甲基橙在酸中变红,在碱中变黄。答题时必须写出两种颜色,只写指示剂会变色拿不到分。通用指示剂给出的是连续的颜色变化,从红色经过黄绿到紫色,用它可以在半个单位内粗略判断 pH 区间。
Indicator colours are easy marks and are just as easily lost. Litmus is red in acid and blue in alkali, thymolphthalein is colourless in acid and blue in alkali, and methyl orange is red in acid and yellow in alkali. Answers must give both colours, because a statement that the indicator simply changes colour scores nothing; universal indicator instead shows a continuous scale from red through yellow and green to violet.
pH 标度与氢离子浓度
pH 标度通常写作 1 到 14,小于 7 是酸性,等于 7 是中性,大于 7 是碱性。数值越小,溶液里氢离子的浓度越高。通用指示剂纸配比色卡,可以估出近似数值,考试常要求写出读数并说明判断依据。读数时把试纸放在白色背景上比对,颜色落在两档之间就取中间值,不要虚构精确到小数位的结论。拓展部分还要求把 pH 与氢离子浓度联系起来,同一浓度的强酸与弱酸 pH 不同,差别在电离程度而不在溶解了多少酸。同一瓶溶液在不同温度下读数会有轻微差别,题目给出室温就按室温处理,不要自行加温。
The pH scale runs from below 1 to 14, with values under 7 acidic, 7 neutral and above 7 alkaline; a lower reading means a higher concentration of hydrogen ions. Universal indicator paper with a colour chart gives an approximate value, and Paper 4 often asks for that reading plus the reasoning behind it. A strong acid and a weak acid of the same concentration do not share a pH, because the difference lies in how far each one dissociates rather than in how much was dissolved.
中和反应的离子方程式
中和反应的核心是一行离子方程式,氢离子加氢氧根离子生成水。写出这一行之后,酸碱反应的产物就能自己推出来,酸里的阴离子与碱里的金属阳离子组成盐。拓展题会要求离子方程式与化学方程式都写出来,后者还要配平并标注状态符号。常见的错误是两个地方,一是把氢离子的下标写错,二是忘了写状态符号 aq 与 l。检查时数一遍两边的氢和氧,再核对电荷是否相等。
Neutralisation reduces to one ionic equation: hydrogen ions plus hydroxide ions give water. Once that line is written, the salt can be worked out from the anion of the acid and the metal cation of the base. Marks are commonly lost by omitting the state symbols or by mis-copying the charge on the hydrogen ion, so check that hydrogen, oxygen and charge all balance.
强酸与弱酸:完全电离与部分电离
强酸在水溶液里完全电离,盐酸、硫酸和硝酸是考纲点名的三种。弱酸只部分电离,乙酸是标准例子,它的电离方程式要用可逆箭头,产物与反应物同时存在。同样的浓度下,弱酸的氢离子浓度更低,pH 更高,与金属反应也更慢。很多学生把强弱与浓稀混在一起,这是最典型的失分点,浓与稀说的是溶解了多少酸,强与弱说的是电离了多少。判断题目时先看这两个词出现在哪个位置,再决定用哪一套说法回答。
Hydrochloric, sulfuric and nitric acid are the strong acids named in the syllabus, and each is completely dissociated in aqueous solution. Ethanoic acid is the standard weak acid: it dissociates only partially, so its equation carries a reversible arrow and both molecules and ions are present. At the same concentration a weak acid has the higher pH and reacts more slowly, and confusing strong or weak with concentrated or dilute is the single most common error in this topic.
酸性氧化物、碱性氧化物与两性氧化物
氧化物的分类看它跟酸、碱分别怎么反应。二氧化碳与二氧化硫来自非金属元素,属于酸性氧化物,能跟碱反应生成盐和水。氧化铜与氧化钙来自金属元素,属于碱性氧化物,能跟酸反应。氧化铝与氧化锌既能跟酸反应又能跟碱反应,这一类叫两性氧化物,两个例子都要记牢。判断题常把氧化物分类与金属非金属属性放在一起,记住金属性对应碱性,非金属性对应酸性,两性氧化物是少数例外。
Oxides are classified by how they behave with acids and alkalis. Carbon dioxide and sulfur dioxide are acidic oxides formed from non-metals, while copper(II) oxide and calcium oxide are basic oxides; aluminium oxide and zinc oxide are the two amphoteric oxides that react with both. In Paper 4 a single mark often depends on saying that an amphoteric oxide reacts with acids and with bases.
可溶盐的制取:过量固体法
用过量不溶物制可溶盐是这套题里分值最高的一条路线,操作顺序不能乱。先把稀酸温热,再加入过量金属、过量不溶碱或过量不溶碳酸盐,直到固体不再溶解,剩余的固体说明酸已经用完。接着过滤,把多余固体去掉,留下滤液。把滤液加热蒸发,水分减少到开始出现晶体时停止加热,然后冷却结晶。最后用滤纸吸干晶体表面的残留液体。很多学生把蒸发写到干,拿到的就是粉末而不是晶体,这一步最容易丢分。
The excess-solid route is the highest-scoring method for a soluble salt, and the order of the steps carries the marks. Warm the dilute acid, add the metal, insoluble base or insoluble carbonate in excess, and stop when solid remains; filter off the excess, evaporate the filtrate until crystals begin to form at the point of crystallisation, cool, then dry the crystals between filter papers. Evaporating to dryness gives a powder rather than crystals, which is why examiners expect the word crystals.
第四条路线:钠盐、钾盐与铵盐的滴定法
钠盐、钾盐和铵盐全部可溶,没有不溶固体可以过量加入,所以要用滴定法。用移液管量取固定体积的碱液放进锥形瓶,滴入几滴指示剂,再用滴定管逐滴加入酸,直到指示剂刚好变色。记录读数时要平视液面,重复滴定直到三次结果相差不超过 0.1 立方厘米,取相符结果的平均值。这一条路线同样要经过蒸发与结晶,区别只在前一步用滴定而不是过量固体。酸碱滴定也是实践卷的重点,操作顺序与读数规范都是给分点。滴定管读数要读到小数点后两位,最后一位是估读值。
Sodium, potassium and ammonium salts are all soluble, so there is no insoluble solid to add in excess and the titration route is used instead. Measure a fixed volume of alkali into a conical flask with a pipette, add a few drops of indicator, then run acid in from a burette until the colour just changes; repeat until titres agree within 0.1 cm3 and take the mean of the concordant results. The same evaporation and crystallisation steps follow, and the technique itself is examined in Paper 5 and Paper 6.
不溶盐的制取:沉淀法
不溶盐不能靠酸与固体的反应拿到,要把两种可溶盐的溶液混合,让离子结合成不溶物析出。混合后出现沉淀,过滤把沉淀与溶液分开,再用蒸馏水冲洗沉淀,把可溶杂质带走,最后干燥。选原料时要保证两种溶液各自可溶,同时生成的产物不溶,例如硝酸银溶液与氯化钠溶液可以得到氯化银沉淀。写方程式时把沉淀标上状态符号 s,这一步常被漏掉。
An insoluble salt is made by precipitation rather than by reacting an acid with a solid. Mix two solutions of soluble salts so the chosen ions combine into an insoluble product, filter, wash the residue with distilled water to remove soluble impurities, then dry it. Silver chloride from silver nitrate and sodium chloride solution is the standard example, and the state symbol for that precipitate must be shown as s.
溶解性规则与水合晶体
四条制盐路线怎么选,取决于溶解性规则。钠盐、钾盐和铵盐都溶于水,硝酸盐也都溶于水。氯化物一般可溶,氯化银与氯化铅是例外。硫酸盐一般可溶,硫酸钡、硫酸钙与硫酸铅是例外。碳酸盐与氢氧化物一般难溶,可溶的只有钠、钾、铵三种,氢氧化钙属于微溶。水合物是晶体中化学结合了水的物质,无水物是其中不含水的物质,硫酸铜晶体带五个结晶水,受热会由蓝变白。
The choice of route follows the solubility rules. All sodium, potassium and ammonium salts are soluble, and so are nitrates; chlorides are soluble except silver and lead, and sulfates are soluble except barium, calcium and lead. Carbonates and hydroxides are insoluble apart from those of sodium, potassium and ammonium, and a hydrated substance contains water chemically combined in its crystals, as in copper(II) sulfate crystals that turn from blue to white on heating.
三份卷子的结构、评分目标与失分点
0620 的三份卷子分工固定,核心考生考选择题卷与理论卷,拓展考生考对应的高一档版本,两者都要再加一份实践卷。选择题卷 45 分钟 40 分,理论卷 1 小时 15 分钟 80 分,实践卷 40 分,权重分别约为三成、五成与两成。按 CAIE 考纲 2026 到 2028 版的说明,评分目标里知识与理解占一半,信息处理与解题占三成,实验技能占两成,酸碱盐的内容主要落在前两项。阅卷时反复出现的失分是操作步骤顺序颠倒,以及方程式不配平、不标状态。写答案前先看分值与空行数,分值往往提示需要几个要点。遇到描述实验步骤的题目,按操作发生的先后顺序写,不要按记忆里的目录顺序写。
All candidates take three components. Core candidates sit Paper 1, a 45-minute multiple-choice paper worth about 30 per cent, plus Paper 3 and one practical paper, while Extended candidates sit Papers 2 and 4. According to the 0620 syllabus for 2026 to 2028, knowledge with understanding carries half of the qualification, handling information and problem-solving about 30 per cent and experimental skills 20 per cent, and the salts topic is examined mainly through the first two. Marks are worked out against the space provided, so count the points needed before writing.
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