Let's break down the statement from your NCERT textbook regarding thermometers and the concepts of absolute zero and the triple point. This will help clarify how different substances behave under temperature changes and what these terms mean in the context of thermodynamics.
Understanding Thermometers and Expansion Properties
First, let's talk about why liquid-in-glass thermometers behave differently compared to gas thermometers. Liquid-in-glass thermometers, like those using mercury or colored alcohol, rely on the principle of thermal expansion. When the temperature rises, the liquid expands and moves up the glass tube, indicating a higher temperature. However, different liquids expand at different rates when heated. This means that the readings can vary based on the specific liquid used, which is what the textbook is referring to when it mentions "differing expansion properties."
Gas Thermometers and Consistency
On the other hand, gas thermometers operate based on the behavior of gases. At low densities, all gases exhibit similar expansion properties, meaning that regardless of the type of gas used, the change in volume with temperature remains consistent. This is due to the ideal gas law, which states that the pressure and volume of a gas are directly related to its temperature. Therefore, a gas thermometer will give the same reading whether it uses oxygen, nitrogen, or any other gas, as long as the conditions are similar.
Defining Absolute Zero
Now, let’s delve into absolute zero. This is the theoretical temperature at which a system has minimal thermal energy, and it is defined as 0 Kelvin (K), which is equivalent to -273.15 degrees Celsius (°C). At this temperature, the motion of atoms theoretically comes to a complete stop, and it represents the lower limit of the thermodynamic temperature scale. In practical terms, reaching absolute zero is impossible, but scientists can get very close to it in controlled environments.
The Triple Point Explained
The triple point is another important concept in thermodynamics. It refers to the specific temperature and pressure at which a substance can exist simultaneously in three phases: solid, liquid, and gas. For water, this occurs at exactly 0.01 degrees Celsius and a pressure of 611.657 pascals. The triple point is significant because it provides a precise reference point for calibrating thermometers and defining temperature scales.
Summary of Key Concepts
- Liquid-in-glass thermometers: Their readings vary based on the liquid used due to differing expansion properties.
- Gas thermometers: Provide consistent readings regardless of the gas used at low densities.
- Absolute zero: Theoretical temperature of 0 K, where atomic motion stops.
- Triple point: The unique conditions where a substance can exist in all three phases simultaneously.
Understanding these concepts helps us appreciate how temperature measurement works and the fundamental principles of thermodynamics. If you have any more questions about these topics or need further clarification, feel free to ask!
