Problem 60 - General Chemistry: Principles and Modern Applications

Question

Why is the triple point of water (ice-liquid-vapor) a better fixed point for establishing a thermometric scale than either the melting point of ice or the boiling point of water?

Step-by-Step Solution

Answer

The triple point of water is a better fixed point for establishing a thermometric scale than the melting point of ice or the boiling point of water because at the triple point, all three phases of water co-exist in equilibrium. This state is well-defined and can be accurately reproduced in a controlled laboratory setting, making it more reliable and accurate. In contrast, the melting and boiling points can be influenced by impurities or atmospheric changes.

Step 1: Understand the Concepts

A triple point of a substance is the temperature and pressure at which the substance can co-exist in three phases - solid, liquid, and gas. In the case of water, this is 0.01 degrees Celsius at standard pressure. Thermometric scales are established at fixed points, which are physical situations where a simple change can occur.

Step 2: Compare with other Points

Both the melting point of ice and the boiling point of water have been traditionally used as reference points in many thermometric scales. However, these points can be influenced by impurities or atmospheric changes, leading to slight inaccuracies.

Step 3: Explain the advantage of the Triple Point

The triple point of water, however, represents a unique state where all three phases co-exist in equilibrium. The conditions for this state are well-defined and can be reproduced accurately in a controlled laboratory setting, making it more reliable and accurate as a fixed point for establishing a thermometric scale.

Key Concepts

Thermometric Scales

Thermometric scales are the systems we use to measure temperature in a consistent way. These scales use fixed reference points to ensure that measurements are standardized across different contexts. Some common thermometric scales include Celsius, Fahrenheit, and Kelvin. Each scale has a specific way of setting its zero point and an increment that is defined by a known physical change.

• The Celsius scale uses the freezing and boiling points of water as its benchmarks.
• Fahrenheit also uses the same points, but at different values, with 32°F for ice melting and 212°F for water boiling.
• The Kelvin scale is a thermodynamic scale that begins at absolute zero, the coldest possible temperature.

To determine a thermometric scale, precise fixed points are crucial. This ensures consistency across different devices and scientific experiments. By using well-defined physical changes, these scales provide a reliable framework to measure temperature accurately.

Melting Point of Ice

The melting point of ice serves as a fundamental reference point in understanding temperature. It is the temperature at which ice turns into water under normal atmospheric pressure, which is precisely 0 degrees Celsius. This point is significant because it represents a simple physical transformation from solid to liquid.

One of the benefits of using the melting point of ice as a reference is its clear and observable transition. However, certain factors can affect the precision of this point as a reliable measure:

1. Impurities in water can lower its freezing point.
2. Atmospheric pressure variations can slightly alter the exact temperature at which ice melts.

These variations can lead to inconsistencies, making it less ideal as a universal fixed point compared to more stable points like the triple point of water.

Boiling Point of Water

The boiling point of water is another critical benchmark used in thermometric scales, typically defined as 100 degrees Celsius at one atmosphere of pressure. At this temperature, water transitions from a liquid to a gas.

Using the boiling point of water as a reference is intuitive because it is easy to observe and repeat in many settings. However, it is subject to several influencing factors:

• The presence of dissolved substances can raise the boiling point, known as boiling point elevation.
• Changes in atmospheric pressure, such as those experienced at higher altitudes, can lower the boiling point.

These variables allow for large and unpredictable discrepancies, which is why fixed points like the triple point of water provide a more stable and reproducible standard for thermometric scales.