化学检验

化学· 2.44–2.50（第2(h)节）· 15 分钟阅读

1. 常见气体的检验★★☆☆☆⏱ 3 min

气体检验是考试中最高频的实验类考点之一，你必须熟记每种气体的检验方法和准确的实验现象才能拿到满分。

Gas	Test Method	Observation
Hydrogen, $H_{2}$	Hold a lighted splint at the test tube mouth	Squeaky pop sound as hydrogen burns
Oxygen, $O_{2}$	Insert a glowing splint into the test tube	Glowing splint relights immediately
Carbon dioxide, $C O_{2}$	Bubble gas through limewater	Limewater turns milky/cloudy white
Ammonia, $N H_{3}$	Hold damp red litmus near the gas outlet	Damp red litmus paper turns blue
Chlorine, $C l_{2}$	Hold damp blue litmus near the gas outlet	Damp blue litmus turns red, then bleaches white

📐 例题

一名学生收集到一种无色气体，该气体可以使湿润的红色石蕊试纸变蓝，请识别该气体。

1
步骤1：将该现象与气体检验对照表匹配，考纲中唯一的碱性气体是氨气。
2
步骤2：该现象是氨气独有的特征，无需额外验证实验。
3
答案：氨气（ $N H_{3}$ ）

Exam tip:

检验氨气和氯气时，不要只写「石蕊试纸」，必须指明试纸的初始颜色以及是否为湿润状态，因为干燥的石蕊试纸遇这些气体不会变色。

2. 阳离子焰色试验★★☆☆☆⏱ 3 min

焰色试验用于识别固体或水溶液样本中的金属阳离子，你必须熟记每种列明阳离子的准确焰色才能拿到满分。

📘 定义

焰色试验

一种定性分析方法，利用金属阳离子在本生灯火焰中受热时发出的特征颜色，识别样本中存在的阳离子。

例:

Sodium ions emit a bright yellow flame when heated.

Clean a nichrome/platinum wire loop by dipping it in concentrated hydrochloric acid, then holding it in a roaring blue Bunsen flame until no colour is emitted.
Dip the clean loop into the solid or aqueous sample.
Hold the loop in the edge of the roaring blue flame and observe the flame colour.

Cation	Flame Colour
Lithium, $L i^{+}$	Red
Sodium, $N a^{+}$	Yellow
Potassium, $K^{+}$	Lilac
Calcium, $C a^{2 +}$	Orange-red
Copper(II), $C u^{2 +}$	Blue-green

📐 例题

一名学生对未知固体进行焰色试验，观察到淡紫色火焰，请识别样本中存在的阳离子。

1
步骤1：将该焰色与焰色试验结果对照表匹配。
2
步骤2：淡紫色是钾阳离子的独有焰色。
3
答案：钾离子， $K^{+}$

Exam tip:

钾的焰色不要写「紫色」：爱德思的评分标准仅接受「淡紫色（紫丁香色）」作为正确答案。

3. 阳离子的氢氧化钠检验法★★★☆☆⏱ 3 min

稀氢氧化钠（ $N a O H$ ）溶液可通过沉淀反应或释气反应检验铵根、铜(II)、铁(II)、铁(III)阳离子，你必须熟记每种阳离子对应的准确沉淀颜色。

Cation	Test Method	Observation
Ammonium, $N H_{4}^{+}$	Warm sample with dilute $N a O H$ , test any gas with damp red litmus	Damp red litmus turns blue (ammonia gas released)
Copper(II), $C u^{2 +}$	Add a few drops of dilute $N a O H$ to the sample	Blue precipitate of copper(II) hydroxide forms
Iron(II), $F e^{2 +}$	Add a few drops of dilute $N a O H$ to the sample	Green precipitate of iron(II) hydroxide forms
Iron(III), $F e^{3 +}$	Add a few drops of dilute $N a O H$ to the sample	Brown precipitate of iron(III) hydroxide forms

📐 例题

向未知溶液中加入稀氢氧化钠溶液，生成绿色沉淀，请识别溶液中存在的阳离子。

1
步骤1：将沉淀颜色与氢氧化钠检验结果对照表匹配。
2
步骤2：与氢氧化钠反应生成绿色沉淀是铁(II)阳离子的独有特征。
3
答案：亚铁离子， $F e^{2 +}$

Exam tip:

不要混淆铁离子的沉淀颜色： $F e^{2 +}$ 生成绿色沉淀， $F e^{3 +}$ 生成棕色沉淀。评分标准不接受用「橙色」或「红色」描述 $F e^{3 +}$ 的沉淀。

4. 阴离子的检验★★★☆☆⏱ 3 min

阴离子检验利用沉淀反应或气体释放反应来识别带负电的离子。你必须明确说明硝酸银和氯化钡是经过酸化的才能拿到全分，因为酸可以除去会产生假阳性结果的干扰性碳酸根离子。

Anion	Test Method	Observation
Chloride, $C l^{-}$	Add dilute nitric acid, then silver nitrate solution	White precipitate of silver chloride forms
Bromide, $B r^{-}$	Add dilute nitric acid, then silver nitrate solution	Cream precipitate of silver bromide forms
Iodide, $I^{-}$	Add dilute nitric acid, then silver nitrate solution	Yellow precipitate of silver iodide forms
Sulfate, $S O_{4}^{2 -}$	Add dilute hydrochloric acid, then barium chloride solution	White precipitate of barium sulfate forms
Carbonate, $C O_{3}^{2 -}$	Add dilute hydrochloric acid, bubble gas through limewater	Limewater turns milky (carbon dioxide released)

📐 例题

向某未知溶液中加入酸化硝酸银，生成黄色沉淀。请确定其中存在的阴离子。

1
步骤1：将沉淀颜色与卤离子检验结果表进行匹配。
2
步骤2：与酸化硝酸银反应生成黄色沉淀，证明存在碘离子。
3
答案：碘离子， $I^{-}$

Exam tip:

进行卤离子和硫酸根检验时，必须先说明所加的酸再加检验试剂，例如要写「先加稀硝酸，再加硝酸银」，不能只写「加硝酸银」。

5. 水与水的纯度检验★★☆☆☆⏱ 2 min

你需要掌握两种独立的水检验方法：一种用于确认水的存在，另一种用于确认水样是纯净的。

📘 定义

无水硫酸铜(II)

不含结晶水的白色结晶态硫酸铜(II)，与水接触后生成水合硫酸铜(II)时会变为蓝色。

例:

$C u S O_{4} (white) + 5 H_{2} O \to C u S O_{4} \cdot 5 H_{2} O (blue)$

Test for presence of water: Add a small amount of anhydrous copper(II) sulfate to the sample. If water is present, the white solid turns blue.
Test for purity of water: Measure the boiling point of the sample at 1 atm pressure. Pure water boils at exactly 100°C. Impure water will boil at a temperature higher than 100°C, over a range of temperatures. You can also measure melting point: pure water freezes at exactly 0°C.

📐 例题

某学生用无水硫酸铜(II)检验一种无色透明液体，固体变为蓝色。他测得该液体的沸点为102°C。请说明这一现象反映了该液体的什么性质。

1
步骤1：无水硫酸铜(II)变蓝证明该液体中含有水。
2
步骤2：102°C的沸点高于纯水的沸点（1标准大气压下为100°C），因此该水不纯净，含有溶解的固体杂质。
3
答案：该液体是含有杂质的非纯水。

Exam tip:

不要将水的存在检验和水的纯度检验混淆：无水硫酸铜(II)只能证明水存在，无法证明水是纯净的。

6. 常见陷阱

错误做法:

Writing 'litmus paper' without specifying colour or dampness for ammonia/chlorine tests

原因:

Dry litmus does not react with ammonia or chlorine gases, so the observation is only valid for damp litmus of the correct starting colour

正确做法:

Always write 'damp red litmus paper' for ammonia tests, and 'damp blue litmus paper' for chlorine tests

错误做法:

Writing 'purple' instead of 'lilac' for potassium flame test colour

原因:

The Edexcel mark scheme explicitly only accepts 'lilac' as the correct answer for potassium flame colour

正确做法:

Use the mnemonic 'Potassium = Lilac' to remember the exact required term

错误做法:

Omitting 'acidified' when describing silver nitrate or barium chloride tests for anions

原因:

Interfering ions like carbonate will produce false positive precipitates if the reagent is not acidified, so this is a required part of the test to score full marks

正确做法:

Always specify 'acidified silver nitrate' for halide tests, and 'acidified barium chloride' for sulfate tests

错误做法:

Confusing $F e^{2 +}$ and $F e^{3 +}$ precipitate colours with sodium hydroxide

原因:

These are commonly mixed up, and mark schemes only accept green for $F e^{2 +}$ and brown for $F e^{3 +}$

正确做法:

Use the mnemonic 'Fe2 Green, Fe3 Brown' to remember the correct precipitate colours

错误做法:

Using anhydrous copper(II) sulfate to test for pure water

原因:

Anhydrous copper(II) sulfate only confirms water is present, not that it has no dissolved impurities

正确做法:

Use boiling point or melting point measurement to confirm water purity: pure water boils at exactly 100°C and freezes at 0°C at 1 atm

7. 速查表

Test Type	Target Substance	Reagent/Method	Positive Observation
Gas	$H_{2}$	Lighted splint	Squeaky pop
Gas	$O_{2}$	Glowing splint	Splint relights
Gas	$C O_{2}$	Limewater	Turns milky
Gas	$N H_{3}$	Damp red litmus	Turns blue
Gas	$C l_{2}$	Damp blue litmus	Turns red then bleaches white
Flame test	$L i^{+}$	Heat in blue flame	Red flame
Flame test	$N a^{+}$	Heat in blue flame	Yellow flame
Flame test	$K^{+}$	Heat in blue flame	Lilac flame
Flame test	$C a^{2 +}$	Heat in blue flame	Orange-red flame
Flame test	$C u^{2 +}$	Heat in blue flame	Blue-green flame
Cation	$N H_{4}^{+}$	Warm with $N a O H$	$N H_{3}$ gas released (damp red litmus turns blue)
Cation	$C u^{2 +}$	$N a O H$ solution	Blue precipitate
Cation	$F e^{2 +}$	$N a O H$ solution	Green precipitate
Cation	$F e^{3 +}$	$N a O H$ solution	Brown precipitate
Anion	$C l^{-}$	Acidified $A g N O_{3}$	White precipitate
Anion	$B r^{-}$	Acidified $A g N O_{3}$	Cream precipitate
Anion	$I^{-}$	Acidified $A g N O_{3}$	Yellow precipitate
Anion	$S O_{4}^{2 -}$	Acidified $B a C l_{2}$	White precipitate
Anion	$C O_{3}^{2 -}$	Dilute $H C l$ + limewater	Limewater turns milky
Water	Presence of $H_{2} O$	Anhydrous $C u S O_{4}$	White solid turns blue
Water	Pure $H_{2} O$	Measure boiling point	Boils at exactly 100°C at 1 atm

8. 常见问题

卤离子检验时，我需要说明硝酸银是经酸化处理的吗？

是的！你必须明确写出「酸化硝酸银」才能拿到满分，因为酸可以除去会同样生成白色沉淀的干扰性碳酸根离子。

$K^{+}$ 离子的焰色是什么颜色？

淡紫色（紫丁香色），不是紫色。爱德思考试局的评分标准仅接受「淡紫色」作为钾的焰色试验正确答案。

如何检验水是纯净的，而不只是确认水的存在？

检验水的存在可以使用无水硫酸铜(II)（遇水变蓝）。要确认纯度则需要测量沸点：1标准大气压下纯水的沸点恰好为100°C，不纯的水沸点会高于该温度。

深入阅读

下一步

Now that you have mastered all required chemical tests for Edexcel IGCSE Chemistry, you can apply this knowledge to practical problem-solving questions where you identify unknown ionic compounds from test results. This skill is heavily tested in both Paper 1 and Paper 2, especially in 6-mark extended response questions that ask you to design an experimental procedure to identify an unknown substance. You should also practice linking test results to ionic compound formulae and balanced chemical equations for precipitation reactions to score full marks on longer questions.