7.8 Variation in Physical Properties as a Tool for Measuring Temperature of a Substance

We cannot see temperature directly. We cannot pick it up, put it in a box, or measure it with a ruler. So how do we measure it? We rely on the fact that many physical properties of materials change in predictable ways when they are heated or cooled. For example, most liquids expand when heated, the pressure of a gas increases when heated, and the color of certain crystals changes with temperature. These temperature-dependent properties are the basis for all thermometers. Understanding how these properties vary with temperature is essential for understanding how thermometers work.

Definition

Thermometric Properties are the physical properties of a substance that change in a predictable, measurable way with temperature. These properties are used as the basis for measuring temperature in thermometers.

• Volume of a liquid (thermal expansion)
• Pressure of a gas at constant volume
• Volume of a gas at constant pressure
• Electrical resistance of a material
• Color of a liquid crystal
• Electromotive force (EMF) of a thermocouple

Main Idea

The main idea is that temperature can be measured by observing how a physical property changes. When a substance is heated, its particles move faster, causing changes in its physical properties. By calibrating a device to respond to one of these changes, we can create a thermometer. The choice of thermometric property depends on the temperature range, the required precision, and the application. Different thermometers use different properties, which is why there are many types of thermometers.

Core Concept

The text states: "Now we know that when the temperature of a body increases, it increases the kinetic energy of its particles. These fast-moving particles can cause variations in different physical properties (volume, pressure, change in colour, electrical resistance etc.) in a predictable way. Variations in these physical properties may be used to measure the temperature of a body. These physical properties on the basis of which a thermometer works is called its thermometric property."

The text describes several thermometric properties and the thermometers based on them:

1. Expansion of Liquids The text states: "Most liquids expand upon heating. Liquids that expand on heating uniformly can be used as thermometric materials. We use this property for liquids in glass thermometers, which use mercury or alcohol as thermometric materials."

• Property: Volume expansion of a liquid.
• Thermometer: Liquid-in-glass thermometer (mercury or alcohol).
• Mechanism: When the thermometer is heated, the liquid expands and rises up the capillary tube. The height of the liquid column indicates the temperature.

2. Variation of Volume and Pressure of a Gas The text states: "When a gas is heated, its volume as well as pressure may change. These variations in volume and pressure are used in gas thermometers to measure temperature."

• Property: Volume or pressure change of a gas.
• Thermometer: Gas thermometer (constant pressure or constant volume).
• Mechanism: For a constant-volume gas thermometer, the pressure of the gas increases with temperature. For a constant-pressure gas thermometer, the volume of the gas increases with temperature.

3. Variation in Colour of Crystals The text states: "Liquid crystals are those materials that change colour with a change in temperature. Liquid crystals are packed inside a plastic strip."

• Property: Color change of liquid crystals.
• Thermometer: Liquid crystal thermometer.
• Mechanism: The liquid crystals change color at specific temperatures. By matching the color to a calibrated scale, the temperature can be determined.

4. Resistance Thermometers (Thermistors) The text states: "It can measure temperature due to change in its resistance (change in opposition to flow of current through it) due to variation of its temperature."

• Property: Electrical resistance.
• Thermometer: Resistance thermometer (thermistor).
• Mechanism: The resistance of a material (e.g., platinum or a semiconductor) changes with temperature. By measuring the resistance, the temperature can be determined.

5. Bimetallic Strip Thermometers The text states: "It can measure temperature by variation in volume expansion of [two different metals]."

• Property: Differential thermal expansion of two different metals.
• Thermometer: Bimetallic strip thermometer.
• Mechanism: Two different metals (with different expansion rates) are bonded together. When heated, one metal expands more than the other, causing the strip to bend. The amount of bending indicates the temperature.

6. Thermocouple The text states: "It can measure temperature due to change in its emf produced due to variation in its temperature."

• Property: Electromotive force (EMF) generated at the junction of two different metals.
• Thermometer: Thermocouple thermometer.
• Mechanism: When two different metals are joined at a junction and heated, a voltage (EMF) is generated. The voltage is proportional to the temperature difference between the hot junction and the cold junction.

Table of Thermometric Properties The text includes a table (Table 7.3) summarizing the different types of thermometers and their thermometric properties:

Detailed Explanation

The variation of physical properties with temperature is the foundation of thermometry (the science of temperature measurement).

• Changes measurably with temperature.
• Changes in a predictable (ideally linear) way.
• Is reproducible.
• Is sensitive enough to detect small temperature changes.

Different properties are suitable for different temperature ranges and applications. For example, mercury thermometers are suitable for everyday temperatures but not for very low temperatures (mercury freezes at -39°C). Thermocouples can measure very high temperatures (e.g., in furnaces) and very low temperatures.

Liquid-in-Glass Thermometers Mercury and alcohol are commonly used in liquid-in-glass thermometers. Mercury has a uniform expansion rate over a wide temperature range and does not wet the glass. Alcohol is used for low-temperature measurements because it has a lower freezing point than mercury. However, alcohol wets the glass and has a less uniform expansion rate.

Gas Thermometers Gas thermometers are more accurate and have a wider range than liquid-in-glass thermometers. They are often used as standard instruments for calibration. The constant-volume gas thermometer is particularly important because the pressure of a gas is directly proportional to absolute temperature (for an ideal gas), making it a primary thermometer.

Resistance Thermometers (Thermistors) Resistance thermometers measure temperature by measuring the electrical resistance of a material. Platinum resistance thermometers are highly accurate and stable, and they are used as standard thermometers in many laboratories. Thermistors (semiconductor-based) are very sensitive and are used in electronic devices.

Thermocouples Thermocouples are rugged, inexpensive, and can measure a wide range of temperatures. They are widely used in industry (e.g., in furnaces, engines, and industrial processes). They work on the Seebeck effect, where a temperature difference between two junctions of different metals generates a voltage.

Liquid Crystal Thermometers Liquid crystal thermometers are used for applications where a visual indication of temperature is needed, such as fever thermometers (the strip changes color to indicate body temperature) and aquarium thermometers.

Bimetallic Strip Thermometers Bimetallic strips are used in thermostats (to control temperature in ovens, air conditioners, and refrigerators) and in some dial thermometers. The bending of the strip can be used to open or close an electrical circuit.

Intermediate Level

In physics, thermometric properties are the basis of all temperature measurement. A thermometric property is any physical property that changes in a known, reproducible way with temperature.

• Volume expansion: Used in liquid-in-glass thermometers and bimetallic strips.
• Pressure change at constant volume: Used in constant-volume gas thermometers.
• Volume change at constant pressure: Used in constant-pressure gas thermometers.
• Electrical resistance: Used in resistance thermometers and thermistors.
• EMF (voltage): Used in thermocouples.
• Color change: Used in liquid crystal thermometers.

The choice of thermometric property depends on the temperature range and the required accuracy. For example, a mercury thermometer is suitable for everyday temperatures but not for very high or very low temperatures. A thermocouple can measure very high temperatures (e.g., in a furnace). A resistance thermometer is very accurate and is used for calibration. A liquid crystal thermometer is convenient for medical use.

All thermometers must be calibrated. This involves establishing a scale by using fixed points (like the freezing and boiling points of water) and dividing the range between them into equal intervals.

Important Points

• - Thermometric Property: A physical property that changes with temperature.
• - Common properties: Volume expansion, pressure change, electrical resistance, EMF, color change.
• - Liquid-in-glass thermometer: Uses volume expansion of mercury or alcohol.
• - Gas thermometer: Uses pressure or volume change of a gas.
• - Resistance thermometer: Uses electrical resistance.
• - Thermocouple: Uses EMF generated at a junction of two metals.
• - Liquid crystal thermometer: Uses color change.
• - Bimetallic strip: Uses differential expansion of two metals.

Remember

• - Thermometric Property: Changes with temperature.
• - Liquid: Volume expansion.
• - Gas: Pressure or volume change.
• - Resistance: Electrical resistance change.
• - Thermocouple: EMF change.
• - Liquid crystal: Color change.
• - Bimetallic strip: Bending due to differential expansion.

Common Mistakes

• - Thinking temperature can be measured directly: Temperature is measured indirectly through a thermometric property.
• - Assuming all thermometers are the same: Different thermometers use different properties and are suitable for different temperature ranges and applications.
• - Forgetting calibration: Thermometers must be calibrated using fixed points (e.g., the freezing and boiling points of water) to be accurate.

Quick Revision

Thermometric properties are physical properties of a substance that change in a predictable way with temperature. These properties are used as the basis for thermometers. Common thermometric properties include the volume expansion of liquids (used in liquid-in-glass thermometers), the pressure or volume change of gases (used in gas thermometers), electrical resistance (used in resistance thermometers), EMF (used in thermocouples), and color change (used in liquid crystal thermometers). Different thermometers are chosen based on the temperature range and the required accuracy.

60-Second Revision

• Liquid: Volume expansion.
• Gas: Pressure or volume change.
• Resistance: Electrical resistance change.
• Thermocouple: EMF change.
• Liquid crystal: Color change.
• Bimetallic strip: Bending due to differential expansion.

Frequently Asked Questions (FAQs)

Q1. What is a thermometric property? Answer: A thermometric property is a physical property of a substance that changes in a predictable, measurable way with temperature. It is used as the basis for measuring temperature in a thermometer.

• Volume expansion of a liquid (used in liquid-in-glass thermometers)
• Pressure change of a gas at constant volume (used in gas thermometers)
• Electrical resistance (used in resistance thermometers)
• EMF (used in thermocouples)
• Color change (used in liquid crystal thermometers)
• Differential expansion of metals (used in bimetallic strips)

Q3. How does a liquid-in-glass thermometer work? Answer: It uses the volume expansion of a liquid (mercury or alcohol). When heated, the liquid expands and rises up the capillary tube. The height of the liquid column indicates the temperature.

Q4. How does a thermocouple work? Answer: A thermocouple consists of two different metals joined at a junction. When the junction is heated, it generates a small voltage (EMF) that is proportional to the temperature difference between the hot junction and the cold junction. By measuring the voltage, the temperature can be determined.

Q5. Why are different thermometers used for different applications? Answer: Different thermometers use different thermometric properties, which are suitable for different temperature ranges and precision requirements. For example, a mercury thermometer is suitable for everyday temperatures, while a thermocouple can measure very high temperatures.

Q6. Why must a thermometer be calibrated? Answer: A thermometer must be calibrated to establish a reliable temperature scale. This is done by using fixed points (like the freezing and boiling points of water) and dividing the range between them into equal intervals. Without calibration, the readings would not be accurate.

Multiple Choice Questions (MCQs)

• Q1. A physical property that changes with temperature is called a:
• A. Thermometric property
• B. Thermodynamic property
• C. Mechanical property
• D. Chemical property

• Correct Answer: A
• Explanation: The text states that physical properties on the basis of which a thermometer works are called thermometric properties.

• Q2. Which of the following is a thermometric property?
• A. Mass
• B. Volume expansion
• C. Weight
• D. Color

• Correct Answer: B
• Explanation: The text lists volume expansion as a thermometric property (used in liquid-in-glass thermometers).

• Q3. A liquid-in-glass thermometer uses:
• A. Electrical resistance
• B. Volume expansion of a liquid
• C. Color change
• D. EMF

• Correct Answer: B
• Explanation: The text states that liquid-in-glass thermometers use the expansion of liquid (mercury or alcohol).

• Q4. A thermocouple thermometer uses:
• A. Volume expansion
• B. Pressure change
• C. EMF
• D. Color change

• Correct Answer: C
• Explanation: The text states that a thermocouple measures temperature due to change in its emf.

• Q5. A resistance thermometer uses:
• A. Volume expansion
• B. Pressure change
• C. Electrical resistance
• D. EMF

• Correct Answer: C
• Explanation: The text states that resistance thermometers measure temperature due to change in resistance.

• Q6. A bimetallic strip thermometer uses:
• A. Volume expansion of a liquid
• B. Differential expansion of two different metals
• C. Pressure change of a gas
• D. Electrical resistance

• Correct Answer: B
• Explanation: The text states that a bimetallic strip thermometer measures temperature by variation in volume expansion of two different metals.

• Q7. A liquid crystal thermometer uses:
• A. Volume expansion
• B. Color change
• C. Pressure change
• D. Electrical resistance

• Correct Answer: B
• Explanation: The text states that liquid crystals change color with temperature.

• Q8. Which thermometer uses the pressure change of a gas?
• A. Liquid-in-glass thermometer
• B. Constant-volume gas thermometer
• C. Thermocouple
• D. Bimetallic strip

• Correct Answer: B
• Explanation: The text states that a constant-volume gas thermometer uses the pressure change of a fixed mass of gas.

• Q9. Which of the following is NOT a thermometric property?
• A. Volume expansion
• B. Electrical resistance
• C. Mass
• D. Color change

• Correct Answer: C
• Explanation: Mass does not change significantly with temperature. Volume expansion, electrical resistance, and color change are thermometric properties.

• Q10. The table in the text lists the thermometric property for a bimetallic strip as:
• A. Volume expansion of a gas
• B. Volume expansion of a solid
• C. Pressure change of a gas
• D. Changes in EMF

• Correct Answer: B
• Explanation: The text's table lists "Volume expansion of a solid" as the thermometric property for a bimetallic strip thermometer.

Frequently Asked Questions (FAQs)

Q1. What is the difference between a thermometric property and a thermodynamic property? Answer: A thermometric property is a physical property that changes with temperature and is used to measure temperature (e.g., volume expansion, electrical resistance). A thermodynamic property is a property of a system that is independent of the path taken to reach a state (e.g., internal energy, entropy, temperature, pressure, volume). Thermometric properties are a subset of thermodynamic properties.

Q2. Why is the constant-volume gas thermometer considered a primary thermometer? Answer: Because it relies on fundamental physical laws (the ideal gas law, which states that pressure is directly proportional to absolute temperature at constant volume). It does not need to be calibrated against another thermometer, making it a primary standard.

Q3. How does a bimetallic strip work? Answer: A bimetallic strip consists of two different metals bonded together. When heated, one metal expands more than the other, causing the strip to bend. The amount of bending is proportional to the temperature change. This bending can be used to open or close an electrical circuit (as in a thermostat) or to move a pointer.

Q4. Why is mercury used in thermometers? Answer: Mercury has a uniform expansion rate over a wide temperature range, does not wet the glass, and is opaque, making it easy to read. However, it is toxic, so its use is being phased out in many applications.

Q5. Why is alcohol used in some thermometers? Answer: Alcohol is used in thermometers because it has a lower freezing point than mercury, making it suitable for low-temperature measurements. It also expands more than mercury for the same temperature change, making it more sensitive. However, it wets the glass and has a less uniform expansion rate.

Q6. How does a liquid crystal thermometer work? Answer: Liquid crystal thermometers use liquid crystal materials that change color at specific temperatures. When the temperature changes, the liquid crystals change their molecular arrangement, which changes the wavelength of light they reflect. By matching the color to a calibrated scale, the temperature can be determined.

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