Back to Chemistry syllabus

Preparation and Testing of Common Gases

CSCA Preparation and Testing of Common Gases study guide organized around the publicly available CSCA syllabus. Practice Chemistry questions on aicsca.com.

Before planning this topic, check the CSCA Exam Guide 2026 for exam dates, registration, fees, and subject requirements.

Syllabus Alignment

This study guide is organized around the publicly available CSCA syllabus for international undergraduate applicants.

Who It Is For

International students preparing for CSCA Math, Physics, Chemistry, or Chinese exams.

Related Practice

CSCA Practice · Go to questions

Past papers and worked video solutions · Timed mock exams · All subject video lessons

Practice by Topic

Jump from this tutorial to filtered practice questions for the same knowledge point.

Tutorial Content

Preparation and Testing of Common Gases

Core Concepts and Logical Framework

The preparation and testing of common gases is a core skill in chemical experimentation. It follows a clear logical process: **Selecting the reaction principle → Designing the generation apparatus → Performing purification and drying → Choosing the collection method → Implementing verification tests**. Mastering this process enables a systematic approach to solving various gas preparation problems. The CSCA exam often tests the understanding of preparation principles, apparatus selection, and testing methods for specific gases (e.g., $\ce{O2}$, $\ce{H2}$, $\ce{CO2}$).

Core Preparation Principles and Apparatus

Laboratory gas preparation is typically based on reactions between solids and liquids (or liquids and liquids) at room temperature or with heating. Apparatus selection depends on the state of reactants and reaction conditions.

1. **Solid-Heated Type**: Reactants are solids and require heating.

* **Apparatus Diagram**: Similar to the setup for oxygen preparation.

* **Typical Gases**: $\ce{O2}$ ($2\ce{KClO3} \xrightarrow[\ce{MnO2}]{\Delta} 2\ce{KCl} + 3\ce{O2}\uparrow$ or $2\ce{KMnO4} \xrightarrow{\Delta} \ce{K2MnO4} + \ce{MnO2} + \ce{O2}\uparrow$).

* **Precautions**: The test tube mouth should be slightly tilted downward to prevent condensed water from flowing back and cracking the tube.

2. **Solid-Liquid, Non-Heated Type**: Lumpy or granular solids react with liquids at room temperature.

* **Apparatus Diagram**: Uses a Kipp's apparatus or its simple version (conical flask/test tube with a long-stem funnel and two-hole stopper).

* **Typical Gases**: $\ce{H2}$ ($\ce{Zn} + \ce{H2SO4} (dilute) = \ce{ZnSO4} + \ce{H2}\uparrow$), $\ce{CO2}$ ($\ce{CaCO3} + 2\ce{HCl} = \ce{CaCl2} + \ce{H2O} + \ce{CO2}\uparrow$).

* **Advantage**: Allows control over starting and stopping the reaction.

3. **Solid-Liquid or Liquid-Liquid, Heated Type**: Reactants include liquids and require heating.

* **Apparatus Diagram**: Uses a round-bottom flask, separatory funnel, alcohol lamp, etc.

* **Typical Gases**: $\ce{Cl2}$ ($\ce{MnO2} + 4\ce{HCl} (conc.) \xrightarrow{\Delta} \ce{MnCl2} + \ce{Cl2}\uparrow + 2\ce{H2O}$), $\ce{HCl}$, etc.

Gas Collection Methods

The method is chosen based on the gas's physical properties (density, water solubility) and chemical properties (whether it reacts with water or air).

1. **Water Displacement (Over Water)**: Suitable for gases that are **insoluble or slightly soluble in water** and **do not react with water**.

* **Advantage**: Higher purity.

* **Gases**: $\ce{O2}$, $\ce{H2}$, $\ce{CH4}$, etc.

2. **Upward Displacement of Air**: Suitable for gases **denser than air** (molar mass > 29) and **non-reactive with air components**.

* **Apparatus**: Delivery tube extends to the bottom of the gas jar.

* **Gases**: $\ce{CO2}$, $\ce{Cl2}$, $\ce{HCl}$, $\ce{SO2}$, etc.

3. **Downward Displacement of Air**: Suitable for gases **less dense than air** (molar mass < 29) and **non-reactive with air components**.

* **Apparatus**: Delivery tube extends to the bottom of the gas jar.

* **Gases**: $\ce{H2}$, $\ce{CH4}$, $\ce{NH3}$, etc.

Gas Purification and Drying

If the generated gas contains impurities (e.g., acid mist, water vapor), it needs purification before drying.

* **Purification (Removal of Impurities)**: Choose a reagent that absorbs the impurity without reacting with the target gas. For example, $\ce{CO2}$ containing $\ce{HCl}$ is passed through saturated $\ce{NaHCO3}$ solution to remove $\ce{HCl}$.

* **Drying**: Uses a desiccant to absorb water vapor. The principle for choosing a desiccant is: it must not react with the target gas.

* **Concentrated $\ce{H2SO4}$**: Acidic desiccant. Can dry **acidic or neutral** gases like $\ce{O2}$, $\ce{H2}$, $\ce{CO2}$, $\ce{Cl2}$, $\ce{HCl}$, $\ce{SO2}$, etc. Cannot dry $\ce{NH3}$ (reacts) or $\ce{H2S}$ (oxidizes it).

* **Soda Lime** (mixture of $\ce{CaO}$ and $\ce{NaOH}$): Alkaline desiccant. Can dry **alkaline** gases like $\ce{NH3}$. Cannot dry acidic gases like $\ce{CO2}$, $\ce{SO2}$, $\ce{HCl}$.

* **Anhydrous $\ce{CaCl2}$**: Neutral desiccant, widely applicable, but cannot dry $\ce{NH3}$ (forms complexes).

Gas Testing Methods

Tests must be **characteristic**.

1. **$\ce{O2}$**: Insert a glowing splint into the gas jar. The splint **rekindles**.

2. **$\ce{H2}$**: Pure hydrogen burns in air with a **pale blue** flame. Holding a dry, cold beaker over the flame produces **water droplets** on its surface. Impure hydrogen may explode upon ignition.

3. **$\ce{CO2}$**:

* Bubble through limewater ($\ce{Ca(OH)2}$). The limewater turns **milky/cloudy** (forms $\ce{CaCO3}$ precipitate).

* Insert a burning splint into the jar. The splint is **extinguished** (this method is not unique, as gases like $\ce{N2}$ also extinguish splints).

4. **$\ce{NH3}$**:

* Bring a moist red litmus paper near the gas. The paper turns **blue**.

* Has a pungent odor.

Typical Example Problems

**Example 1**: In the laboratory, dry and pure carbon dioxide gas is prepared using marble (mainly $\ce{CaCO3}$) and dilute hydrochloric acid. Please answer:

(1) Which type of generation apparatus should be used? Why?

(2) What impurities might be present in the generated gas? How to remove them?

(3) Which desiccant should be used to dry $\ce{CO2}$?

(4) Which method should be used to collect $\ce{CO2}$? How to test for a full jar?

**Solution**:

(1) Use a **solid-liquid, non-heated** generation apparatus (e.g., Kipp's apparatus or its simple version). Because the reactants $\ce{CaCO3}$ (solid) and dilute hydrochloric acid (liquid) react at room temperature to produce $\ce{CO2}$.

(2) Possible impurities: **$\ce{HCl}$ gas** volatilized from the hydrochloric acid, and water vapor produced in the reaction. To remove $\ce{HCl}$ gas: Pass the gas through **saturated $\ce{NaHCO3}$ solution** ($\ce{NaHCO3} + \ce{HCl} = \ce{NaCl} + \ce{H2O} + \ce{CO2}\uparrow$). Note: $\ce{NaOH}$ or $\ce{Na2CO3}$ solutions cannot be used as they absorb $\ce{CO2}$.

(3) Desiccant: Can use **concentrated sulfuric acid** or **anhydrous calcium chloride**. Because $\ce{CO2}$ is an acidic gas and does not react with concentrated $\ce{H2SO4}$; it also does not react with anhydrous $\ce{CaCl2}$. Alkaline desiccants like soda lime cannot be used.

(4) Collection method: **Upward displacement of air**, because $\ce{CO2}$ is denser than air and soluble in water.

Test for a full jar: Place a **burning splint** at the mouth of the gas jar. If the splint is **extinguished**, it indicates the jar is full of $\ce{CO2}$.

**Example 2**: Which of the following statements regarding gas preparation, collection, and testing is incorrect? ( )

A. When collecting $\ce{O2}$ over water, collection begins only when bubbles emerge from the delivery tube continuously and uniformly.

B. When collecting $\ce{CO2}$ by upward displacement of air, a burning splint is inserted into the jar to test for fullness.

C. When preparing $\ce{H2}$ from zinc granules and dilute sulfuric acid, concentrated sulfuric acid can be used to dry the hydrogen.

D. $\ce{NH3}$ can be tested using moist red litmus paper.

**Solution**:

A is correct. Initially, the air inside the apparatus is expelled, so the gas is impure. Wait until the air is purged (indicated by continuous, uniform bubbles) before collecting.

B is incorrect. The test for a full jar should be performed at the **mouth of the gas jar**. Inserting a burning splint into the jar tests the gas's properties, not whether it's full.

C is correct. $\ce{H2}$ is a neutral gas and does not react with concentrated $\ce{H2SO4}$, so it can be dried with concentrated $\ce{H2SO4}$.

D is correct. $\ce{NH3}$ dissolves in water to form an alkaline solution, turning moist red litmus paper blue.

**Answer: B**

Precautions and Common Mistakes

1. **Confusing Apparatus with Reaction Conditions**: For example, using a solid-heated apparatus for $\ce{O2}$ from heated $\ce{KClO3}$, but using a solid-liquid, non-heated apparatus for $\ce{O2}$ from $\ce{H2O2}$ and $\ce{MnO2}$ at room temperature.

2. **Incorrect Collection Method Selection**: Ignoring gas solubility or density. For example, $\ce{CO2}$ is soluble in water, so water displacement is not suitable; $\ce{NH3}$ is less dense than air and very soluble, so downward displacement of air should be used.

3. **Incorrect Desiccant Selection**: Ignoring chemical reactions between the desiccant and the gas. For example, using concentrated $\ce{H2SO4}$ to dry $\ce{NH3}$, or using soda lime to dry $\ce{CO2}$.

4. **Incorrect Position for Fullness Test**: For example, inserting the glowing splint for $\ce{O2}$ fullness into the jar (should be at the mouth), or inserting the splint for $\ce{CO2}$ fullness into the jar (should be at the mouth).

5. **Confusing Testing with Fullness Test**: Testing determines "what gas it is," while a fullness test determines "if the jar is full." Their operations and purposes are different.

Mastering the systematic thinking behind gas preparation and testing is the foundation for understanding more complex experimental processes.