Problem 143 - Chemistry A Molecular Approach

Question

A volatile liquid (one that easily evaporates) is put into a jar, and the jar is then sealed. Does the mass of the sealed jar and its contents change upon the vaporization of the liquid?

Step-by-Step Solution

Answer

The mass of the sealed jar and its contents does not change upon the vaporization of the liquid.

Step 1: Understand the Conservation of Mass

The principle of conservation of mass states that mass cannot be created or destroyed in a closed system through chemical reactions or physical transformations. Since the jar is sealed, it constitutes a closed system. Therefore, no matter can enter or leave the jar.

Step 2: Consider Physical Change

When the liquid inside the jar vaporizes, it changes from a liquid to a gaseous state. This is a physical change, not a chemical reaction. The mass of the liquid remains the same before and after vaporization.

Step 3: Assess the System as a Whole

Since the jar was sealed before the liquid vaporized, the mass of the gas after vaporization is still contained within the jar. The total mass of the jar and its contents—both the remaining liquid and the vaporized gas—remains constant.

Key Concepts

Closed System

Imagine you have a container that is completely sealed off from the outside environment. No matter what happens inside, nothing gets in or out. This is the essence of a closed system in the context of physics and chemistry. The volatile liquid in the jar forms such a system with its surroundings once sealed. Because the container prevents material from escaping or entering, it’s a perfect example of this concept.

While a closed system can experience a wide range of processes internally, the total mass of the system remains unchanged. The conservation of mass principle, which is fundamental in science, dictates that within such a system, mass is conserved—meaning it cannot be created or destroyed. This principle holds the key to understanding our exercise on the sealed jar with the volatile liquid, as it ensures that despite any changes the liquid undergoes, the mass we measure before and after will be the same.

Physical Change

Now, let’s discuss the physical transformation our liquid undergoes. A physical change is a type of change where the form of matter is altered but one substance does not transform into another. Unlike chemical changes, a physical change does not affect the composition of the substance. Examples of physical change include melting, freezing, and, pertinent to our exercise, vaporization.

In the case of the sealed jar, the volatile liquid vaporizes. This means it transitions from a liquid state to a gas. But crucially, despite this change of state, the molecular composition of the substance remains the same. No new substance is created, and no mass is lost or gained in the process, adhering to the conservation of mass within the closed system of our jar.

Vaporization

Drilling down more specifically, vaporization is a fascinating process where a substance transitions from its liquid to its gaseous phase. This can occur in two main ways: evaporation and boiling. When liquid in our sealed jar becomes gas, it doesn't disappear or lose mass; it simply spreads out to fill the available space.

The process does not involve a change in the mass of a substance but in its density and volume. Think of it like a dancer who spreads their arms wide versus standing with their arms crossed. The dancer's mass doesn’t change; they're just taking up more space. Similarly, when the liquid vaporizes inside the sealed jar, no mass is lost—the molecules are just further apart, now filling the jar as a gas instead of a liquid.