Question

Network Covalent Atomic Solids Carbon tetrachloride displays a triple point at 249.0 \(\mathrm{K}\) and a melting point (at 1 \(\mathrm{atm}\) ) of 250.3 \(\mathrm{K}\) . Which state of carbon tetrachloride is more dense, the solid or the liquid? Explain.

Short answer

The solid state of carbon tetrachloride is more dense than the liquid state.

Step-by-step solution

Understand Triple Point

The triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.

Understand the Relationship Between Temperature and Density During Phase Changes

In general, as a pure substance goes from solid to liquid at its melting point, its density decreases because the molecules begin to move faster and get further apart, thereby occupying more space. However, some substances like water, behave unusually, with the solid (ice) being less dense than the liquid.

Analyze Carbon Tetrachloride's States

Since carbon tetrachloride displays a triple point at 249.0 K and the melting point at 1 atm is slightly higher at 250.3 K, the solid and liquid are at equilibrium at the triple point. For most substances, the solid is more dense than the liquid and would sink in the liquid phase which means that liquid carbon tetrachloride is less dense than its solid form.

Key concepts

Understanding the Triple Point

The triple point is a unique condition where three phases of a substance, specifically gas, liquid, and solid, reach a distinct balance of temperature and pressure, allowing them to coexist in a stable thermodynamic equilibrium. This is an essential thermodynamic concept because it represents the exact state at which a material's properties are definable and predictable across all three phases.

For carbon tetrachloride, the triple point occurs at 249.0 K, illustrating a precise moment where solid, liquid, and gaseous carbon tetrachloride exist at once. The triple point is not just a fascinating scientific phenomenon; it's also a practical reference used for calibrating thermometers and for detailed scientific modeling of substances.

Deciphering Phase Changes

A phase change is a transformation from one state of matter to another, such as from solid to liquid or from liquid to gas. Phase changes are part and parcel of the physical behavior of materials and significantly impact their density.

Consider a typical phase change: as a substance melts, its solid structure breaks down, and the particles that were once held in a rigid pattern gain freedom to move. This transition often results in an increase in volume and a corresponding decrease in density. Nonetheless, the change in density during phase transitions is not universal; some materials, like water, exhibit anomalous behavior where the solid form is less dense than the liquid form.

With carbon tetrachloride, as we approach the melting point at a constant pressure (1 atm), we can anticipate a change in state from solid to liquid. For most substances under normal conditions, this would denote a decrease in density from the solid to the liquid state.

Exploring Thermodynamic Equilibrium

Thermodynamic equilibrium is a state wherein a system's macroscopic properties, such as temperature, pressure, and volume, remain constant over time. Within this stable context, there's no net change in the distribution of matter and energy. When a substance is at its triple point, it's in thermodynamic equilibrium across all three phases: solid, liquid, and gaseous.

For carbon tetrachloride, the triple point serves as the condition under which solid and liquid forms can coexist in a state of balance. In such equilibrium, there's no net flow of energy or matter between the phases, and the rates of freezing and melting or condensation and vaporization are equal. The consequence of this equilibrium is the absence of a spontaneous phase change, as long as the external conditions remain constant.