Problem 11 - Chemistry Matter and Change

Question

Explain why the container of water must be inverted when a gas is collected by displacement of water.

Step-by-Step Solution

Answer

The container is inverted to use water as a seal, prevent air contamination, and ensure gas collection without escaping.

Step 1: Understand the Collection Method

When collecting gas over water, the method is called 'displacement of water.' This involves capturing a gas by displacing the water in an inverted container (such as a graduated cylinder) that's submerged in a water-filled basin or tank.

Step 2: Recognize Water as a Sealing Agent

By inverting the container and submerging its open end below the surface of water, the water acts as a seal. This prevents the gas from escaping into the air and ensures that it goes into the container instead. Water forms a barrier that helps trap the gas efficiently.

Step 3: Consider the Role of Buoyancy

An inverted container filled with water wants to float due to buoyancy. As gas collects in the top (now bottom) part of the container, it displaces the water, causing the gas to exploit the buoyancy effect by staying within the container until intentionally released.

Step 4: Maintain a Closed System

Inverting the container creates a closed system where the gas is collected without mingling with the atmosphere. This is crucial for capturing gases that are less dense than air, as they would escape upwards if not contained properly.

Key Concepts

Displacement of Water Method

Collecting gas via the displacement of water method is a clever way to capture gases in a laboratory setting. This method involves gathering the gas in an inverted container, often a graduated cylinder, submerged in a larger container filled with water. The use of water is key because it acts as a barrier that the gas must displace to enter the container. This prevents the gas from escaping.

• The gas must have a lower density to rise and displace the water.
• The water both seals the container to trap the gas and acts as a measurable indicator of how much gas is collected.

This technique is perfect for gases that don't react with water, ensuring that the gas collection remains clean and uncontaminated.

Inverted Container

When using the displacement of water method, the container, typically a graduated cylinder, must be inverted. The open end of this container is submerged under water, helping form a perfect seal.

• The water filling the container creates a barrier that must be displaced for the gas to enter.
• Inverting the container ensures that the gas rises into it, not escaping into the open air.

This setup is essential for accurately measuring the amount of gas because the displacement easily shows how much water is replaced by the gas. In essence, the inverted container efficiently captures and contains the gas, maximizing both safety and accuracy.

Buoyancy

Buoyancy plays a crucial role in the displacement of water method. When a container is inverted and filled with water, it tends to float due to the buoyancy effect. As gas is introduced into this system, it gradually displaces the water.
The collection process utilizes the buoyancy principle in several ways:

• The gas rises to the top of the inverted container, displacing the water below.
• Since the gas is often less dense than the surrounding water, it easily moves upwards into the container.
• Buoyancy helps in keeping the measured water level consistent, showing how much gas has been collected.

Understanding buoyancy is vital to ensuring the experiment runs smoothly and yields correct readings of gas volume.

Closed System

Creating a closed system is imperative in the displacement of water method. By inverting the container and submerging its open end into the water, it effectively becomes a closed environment. This setup prevents interaction with outside air, maintaining the composition and purity of the gas collected. Having this closed system is especially important when:

• The gas being collected has a lower density than air and would otherwise rise and escape.
• Working with reactive gases or needing precise volume measurements.

This careful containment ensures that researchers can observe and measure the gas without losing any volume, leading to accurate experimental results.