Question

Describe the meaning of the expression "collecting a gas over water."

Short answer

Collecting a gas over water is a method to collect gases from reactions using water to displace and measure the gas volume in an inverted container.

Step-by-step solution

Understanding Gas Collection

The phrase 'collecting a gas over water' refers to a method used to collect gases that are insoluble or slightly soluble in water. This is a common practice in chemistry when dealing with reactions that release gases.

Apparatus Setup

The typical setup for collecting gases over water includes a beaker or trough filled with water and an inverted graduated cylinder or gas jar. The container is filled with water and inverted into the water trough, trapping the water inside initially.

Reaction Initiation

A chemical reaction that produces a gas is started. The produced gas is directed, usually via a tube, into the inverted container. The gas displaces the water in this container by pushing it out.

Collection and Measurement

As the gas travels through the water, it collects in the inverted container. The volume of gas collected can be measured by the change in water level inside the inverted container. The assumption here is the gas is not soluble in water, maintaining its volume for accurate measurement.

Key concepts

Insoluble Gases

In chemistry, insoluble gases are gases that do not dissolve significantly in water. When gases have low solubility, their interactions with water molecules are minimal. This property makes them perfect candidates for the water displacement method.

Gases like oxygen, nitrogen, and hydrogen are considered to be insoluble or only slightly soluble in water.

• Useful for experiments where the purity of the gas is priority.
• Limited interaction with water ensures the gas remains unchanged.

This means the volume of gas collected closely matches the green signal for reactions. The minimal solubility allows the volume of gas to be almost equal to what is produced in the reaction.

Chemical Reactions

A chemical reaction is a process where substances known as reactants transform into different substances called products. During these reactions, it's common for gases to be released. For collecting gas over water, choosing the right type of reaction is crucial.

Reactions that produce insoluble gases are ideal for this method.

• Typical reactions include decomposition and displacement reactions.
• The produced gas is collected without significant losses.

Engaging in chemical reactions that release insoluble gases supports precise measurement and analysis. Many laboratory experiments rely on these reactions to measure gas volume and analyze reaction characteristics.

Gas Measurement

Measuring gas can be a challenge, especially with its tendency to diffuse or react with its surroundings. However, understanding the volume of gas produced in a reaction is vital.

The water displacement method, along with the use of graduated cylinders, helps tackle this challenge. When measured, the gas volume is the difference between the initial and the final water level.

• This measurement approach assumes the gas doesn't dissolve in water.
• Precision is enhanced through proper calibration of measuring tools.

Accurately assessing gas volumes permits researchers to derive data critical for evaluating reaction efficiencies and rates.

Water Displacement Method

This method is both simple and effective, commonly used in chemistry labs to collect gases. It involves directing gas into an inverted container filled with water, causing the water to be pushed out by the incoming gas.

Using the water displacement method gives several advantages:

• Cost-effective due to minimal equipment.
• Allows rapid setup and execution of experiments.

The displaced water indicates the amount of collected gas, providing a visual and quantitative representation. For best results, the apparatus should ensure a tight fit so no gas escapes during collection. This technique’s simplicity makes it accessible for educational purposes and demonstrations.