Unit 7: Thermal Properties of Matter

Introduction to Thermal Properties of Matter

When we feel the warmth of the sun, use a thermometer to check our body temperature, or observe ice melting into water, we're experiencing the thermal properties of matter. This chapter explores the particle structure of matter, temperature measurement, heat transfer, and the behavior of materials at different temperatures.

Key Concepts Covered

• Particle structure of solids, liquids, and gases
• Plasma as the fourth state of matter
• Temperature and its measurement
• Heat as energy in transit
• Internal energy of substances
• Thermometers and thermometric properties
• Temperature scales (Celsius, Fahrenheit, Kelvin)
• Absolute zero and its significance
• Thermocouple thermometers
• Sensitivity, range, and linearity of thermometers

Important Definitions

Key Formulas

Detailed Chapter Content

1. Particle Structure of Solids, Liquids and Gases

According to the kinetic molecular theory, matter is composed of very small particles called molecules, which are always in motion. Their motion may be vibrational, rotational, or linear.

A mutual force of attraction called intermolecular force exists between the molecules. This force depends on the distance between molecules and decreases as the distance increases.

Molecules possess kinetic energy due to their motion and potential energy due to intermolecular forces. When a substance is heated, its temperature increases and the molecular motion becomes more vigorous, increasing the kinetic energy. Thus, the temperature of a substance depends on the average kinetic energy of its molecules.

2. States of Matter

In general, matter exists in three states: solids, liquids, and gases.

3. Plasma as the Fourth State of Matter

Plasma is a gas in which most of the atoms are ionized, containing positive ions and electrons. These particles move freely within the volume of the gas.

Because of the presence of these charged particles, plasma is a conducting state of matter, allowing electric current to pass through it. Since the gas in the plasma state shows properties very different from those of an ordinary gas, plasma is known as the fourth state of matter.

Examples of plasma include:

• The Sun and most other stars, which are in the plasma state
• Plasma TVs and gas discharge tubes, which produce plasma when electric current passes through them
• The early stage of lightning formation, known as lightning streamers, occurs due to ionized air molecules creating conducting paths through the atmosphere

4. Temperature and Heat

Temperature of a body is defined as the degree of its hotness or coldness. Temperature can be defined as a physical quantity which determines the direction of flow of thermal energy.

Heat is the energy which is transferred from one object to another due to the difference of temperature between the two bodies. Heat always flows from a body at higher temperature to one at lower temperature.

The sum of kinetic and potential energies of the molecules of an object is called its internal energy.

5. Thermometers and Thermometric Properties

A thermometer is an instrument used for the exact measurement of the hotness or temperature of a substance. Thermometers use some property of a substance, which changes appreciably with the change of temperature.

A thermometric property is a physical property of a substance that changes with temperature and is used in the construction of thermometers. Some thermometric properties are:

• It is a good conductor of heat
• It gives quick response to temperature changes
• It has uniform thermal expansion
• It has high boiling point
• It has low freezing point
• It has large expansivity (low specific heat capacity)
• It does not wet glass
• It does not vapourize
• It is visible

6. Liquid-in-Glass Thermometer

Liquids expand on heating. So, expansion in the volume of a liquid can be used for the measurement of temperature. This is known as a liquid-in-glass thermometer.

Mercury is commonly used in thermometers. It is opaque and easily seen due to its silvery colour. The thermometer is made of glass with a bulb at one end filled with mercury.

When the temperature rises, the mercury expands and moves up through the narrow capillary tube in the form of a mercury thread. The position of the end of the thread reads the temperature.

Alcohol can also be used, but it must be coloured to make it visible.

7. Fixed Points in Temperature Measurement

For the measurement of temperature, two reference temperatures called fixed points are required:

• Upper fixed point: This is the steam point, slightly above the boiling point of water at standard atmospheric pressure
• Lower fixed point: This is the melting point of pure ice, also known as the ice point

8. Temperature Scales

The three main types of temperature scales are:

9. Absolute Zero and Kelvin Scale

Absolute zero is the temperature at which molecular motion ceases, and the average kinetic energy of particles becomes zero. On the Kelvin scale, it is defined as 0 K, which is equivalent to \(-273.15^\circ C\) (rounded to \(-273^\circ C\) for calculations). It is the lowest possible temperature in the universe.

There are no negative numbers on the Kelvin scale because temperature cannot be lower than absolute zero.

10. Thermocouple Thermometer

Thermocouple thermometer is based on the flow of electric current between two junctions of two wires of different materials due to difference of temperatures at the junctions.

This type of thermometer consists of two wires of different materials, such as copper and iron. Their ends are joined together to form two junctions.

If the two junctions are at different temperatures, a small current flows across them. This current is due to the potential difference produced across the two junctions, as the wires have different resistance to the flow of current.

The greater the temperature difference, the greater the potential difference. If one junction is kept at a fixed lower temperature (e.g., in an ice bath at 0°C), the temperature of the other junction can be measured using a millivolt meter with a calibrated scale.

It is useful for very high temperatures and rapidly changing temperatures, as there is only a small mass of metal (the junction) to heat up.

11. Sensitivity, Range and Linearity of Thermometers

A thermometer is evaluated by its three key characteristics that are sensitivity, range and linearity. They help determine the suitability of the thermometer for specific use ensuring accurate and reliable measurement of temperature.

12. Improving Thermometer Parameters

A liquid-in-glass thermometer has a narrow and uniform capillary tube having a small bulb filled with mercury or alcohol at its lower end. The thin wall of the glass bulb allows quick conduction through glass to the liquid from a hot object whose temperature is to be measured.

Sensitivity: Mercury being metal is a good conductor and hence responds quickly to the change in temperature. The small amount of liquid also responds more quickly to a change in temperature. For greater accuracy, alcohol can be used as its expansivity is six times more than mercury, but it has range limitation to higher temperature measurements due to its low boiling point (78°C).

Linearity: The uniformity of the narrow tube or bore ensures even expansion of the liquid required to make the linear measuring scale.

Range: The choice of mercury allows to use it over a long-range temperature due to its low freezing point and high boiling point.

Daily Life Applications

Comparison Tables

States of Matter

Temperature Scales

Sample Problems