In basic thermodynamics, warming a mixture of ammonium chloride and sodium hydroxide in a test tube is an example of which type of system, based on exchange of mass and energy with the surroundings?

Introduction / Context:
In thermodynamics, systems are classified according to how they interact with their surroundings. The most basic classification distinguishes open systems, closed systems, and isolated systems, depending on whether mass and energy can cross the system boundary. Many exam questions use simple laboratory situations, such as heating a substance in a test tube, to test whether students can correctly identify the type of system involved. This question considers warming ammonium chloride with sodium hydroxide in a test tube and asks what type of system this represents.

Given Data / Assumptions:

• The experiment involves a test tube containing a mixture of solid ammonium chloride and solid or aqueous sodium hydroxide.
• The mixture is warmed, and a gas (ammonia) is evolved during the reaction.
• The test tube is open to the air at the top and is heated by an external flame or hot source.
• We assume no special sealing or pressure control; it is an ordinary school laboratory setup.

Concept / Approach:
An open system is one that can exchange both energy and mass with its surroundings. A closed system can exchange energy, usually as heat or work, but not mass. An isobaric process occurs at constant pressure, and an isothermal process occurs at constant temperature, but these terms describe conditions rather than whether mass crosses the boundary. In the described experiment, the test tube receives energy as heat from the flame and releases gas (ammonia) into the surrounding air, representing a transfer of mass out of the system. Water vapour and other gases may also escape. Thus, both mass and energy cross the system boundary, so it is best classified as an open system.

Step-by-Step Solution:
Step 1: Identify the system as the contents of the test tube, including the reacting solids or solutions. Step 2: Observe that heat is supplied from outside, so energy clearly flows into the system. Step 3: Note that the reaction of ammonium chloride with sodium hydroxide produces ammonia gas, which escapes from the open end of the test tube. Step 4: Recognise that the escape of ammonia gas means that mass is leaving the system and entering the surroundings. Step 5: Apply the definition that an open system exchanges both mass and energy with its surroundings and match this to the situation. Step 6: Conclude that the process represents an open system and select that option.

Verification / Alternative check:
Textbook examples of open systems often include boiling water in an open container or chemical reactions in open vessels that release gas to the atmosphere. These are directly comparable to heating a mixture in an open test tube, where vapours and gases can freely leave. In contrast, a closed system might be a sealed container that allows energy transfer through its walls but does not permit mass escape. The described experiment does not maintain constant temperature or constant pressure in a controlled way, so it is not primarily an isothermal or isobaric system. These comparisons confirm that the open system label is the most appropriate.

Why Other Options Are Wrong:
A closed system cannot exchange mass with its surroundings, but in this experiment, gas clearly escapes from the open test tube, so mass transfer occurs, making the closed system description incorrect. An isobaric system is defined by constant pressure, but the test tube experiment has no active pressure control and does not highlight this feature. An isothermal system is maintained at constant temperature, which would require careful heat exchange control, not simple warming over a flame. Therefore, the isobaric and isothermal labels do not correctly characterise the main thermodynamic nature of the system described.

Common Pitfalls:
Students sometimes focus only on the presence of heat transfer and conclude that any heated system must be closed, forgetting to consider mass exchange. Another pitfall is to confuse system type with process conditions, mixing up terms like isothermal, isobaric, open, and closed without checking their exact definitions. To avoid these mistakes, always ask two questions: can mass cross the system boundary, and can energy cross the system boundary? If both can, the system is open; if only energy can, the system is closed. This simple decision rule helps in classifying many everyday thermodynamic examples correctly.

Final Answer:
Warming ammonium chloride with sodium hydroxide in an open test tube is an example of an open system that can exchange both mass and energy with surroundings.