Problem 78 - Chemistry The Science in Context

Question

Solid helium cannot be converted directly into the vapor phase. Does the phase diagram of helium have a triple point?

Step-by-Step Solution

Answer

Answer: No, helium does not have a triple point.

Step 1: Understand phase diagrams

A phase diagram represents the state of a substance (solid, liquid, or gas) as a function of temperature and pressure. Generally, the diagram has three main regions or phases: solid (low temperature and high pressure), liquid (medium temperature and pressure), and gas (high temperature and low pressure). The regions in which these phases can coexist in equilibrium can be marked on the phase diagram.

Step 2: Understand the phase changes

Generally, a substance undergoes phase changes in the following ways: solid to liquid, liquid to gas, and solid to gas. However, in the case of helium, the exercise states that it cannot be directly converted from solid to vapor (gas). This restricts one of the usual phase changes in helium.

Step 3: Analyze the phase diagram of helium

The phase diagram of helium outlines the phases at various temperatures and pressures. The diagram shows that helium transitions from solid to liquid and liquid to gas but not directly from solid to gas.

Step 4: Determine if the helium phase diagram has a triple point

As helium cannot be converted directly from a solid to a gas (as given in the exercise), there is no single point on the phase diagram where the solid, liquid, and gas phases of helium coexist in equilibrium. Therefore, helium does not have a triple point.

Key Concepts

Solid to Liquid Phase Transition

Understanding the transition from solid to liquid, also known as melting, is essential for interpreting phase diagrams. During this process, a solid absorbs heat at a constant temperature, known as the melting point, until its entire structure breaks down into a liquid state. The conditions under which this occurs are succinctly represented on a phase diagram where pressure and temperature axes meet.

For instance, ice turning into water at 0 degrees Celsius and 1 atmosphere pressure is a classic example of this phase change. However, the melting point can change with varying pressures. In substances like helium, adjustments in pressure have a significant role in the melting process due to its unique properties as a noble gas with weak atomic forces.

Liquid to Gas Phase Transition

The transition from liquid to gas, commonly known as evaporation or boiling, is another crucial phase change depicted on phase diagrams. This happens when a liquid's molecules gain enough energy, often through heat absorption, to overcome intermolecular forces and enter a gas phase.

The liquid-to-gas transition point, also known as the boiling point, depends on the ambient pressure. At higher altitudes, where pressure is lower, water boils at temperatures lower than 100 degrees Celsius—illustrating how pressure influences phase transitions. For substances like helium, the phase diagram indicates the specific conditions under which a direct transition from liquid to gas is possible.

Triple Point

The triple point in a phase diagram is a singular, defining condition where all three phases of a substance coexist in equilibrium—solid, liquid, and gas. At this precise temperature and pressure, substances can be seen to switch between the phases spontaneously and are in a unique state of balance.

For water, the triple point occurs at exactly 0.01 degrees Celsius and 0.006 atmospheres pressure. Since helium cannot transition directly from solid to gas under normal phase change conditions, it lacks a triple point on its phase diagram. This absence of a triple point is due to the fact that helium's solid phase does not coexist with its gas phase at any pressure or temperature combination, which is an exceptional characteristic among the elements.