Unit 7: Thermal Properties of Matter — Long Questions

Kinetic Molecular Theory of Matter:

According to this theory,

(i) Matter is composed of very small particles called molecules which are always in motion. Their motion may be vibrational, rotational or linear.

(ii) There exists a mutual force of attraction between the molecules know as intermolecular force.

(iii) The inter molecular force depends upon the distance between the molecules. It decreases with increasing distance between them.

(iv) The molecules possess kinetic energy due to motion and potential energy due to force of attraction. When a substance is heated, its temperature rises and its molecular motion becomes more vigorous which increase the kinetic energy of the molecules. Thus, the temperature of the substance depends upon the average kinetic energy of its molecules. In general, matter exists in three states solids, liquids, and gases as shown in the figure

States of Matter

Most of the properties of the solids, liquids and gases can be explained on the basis of kinetic molecular theory of matter.

(i) Solid: In case of solids, the intermolecular forces are so strong that they keep the molecules bound. So, the molecules are held at fixed position but still they show vibrational motion about their fixed pints. This is why, the solids have a definite shape and a definite volume.

(ii) Liquid: In case of liquids, intermolecular forces so weak that it cannot hold the molecules at fixed positions and the molecules can slide over each other in random direction. A liquid, therefore, molecules it acquires the shape of the containing vessel.

(iii) Gas: Gas molecules are relatively far away from one and another. Due to which, gas neither possesses a definite volume nor a definite shape.

Plasma

The plasma is a gas in which most of the atoms are ionized containing positive ions and electrons. They are freely moving in the volume of the gas. Due to presence of positive ions and free electrons, plasma is the conduction state of matter. It allows electric current to pass through it. Since the gas in plasma state has properties which are quite different from ordinary gas, therefore, plasma is known as fourth state of matter.

Occurrence of Plasma

(i) The Sun and the most of other stars are in plasma state.

(ii) Plasma is also found in plasma TV and in gas discharge tubes when electric current passes through them.

(iii) The plasma state also occurs during the early stages of lightning formation known as lightning streamers which are the conduction paths through the atmosphere due to ionized air molecules.

Temperature and heat

Temperature - Heat is the form of energy which is transferred from one object to another due to difference of temperature between the two bodies.

Temperature of a body is defined as degree of its hotness or coldness. - Its quantity is denoted by Q.

It is denoted by T. - Its unit is Joule.

Its Sl unit is Kelvin.

Temperature and internal Energy

We know that matter is composed of molecules which are always in motion. Molecules of a solid are vibrating about their fixed positions. The molecules of a liquid are sliding one over the other and those of gases are randomly moving, The molecules possess kinetic energy on account of their motion. Potential energy is also associated with molecules because of their attractive forces.

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

When we heat a substance, its molecular motion becomes more vigorous which means an increase in its internal energy. As a result, temperature of the substance rises.

The heat energy transferred to a body increases the internal energy of its molecules due to which its temperature rises.

Remember that, it is not true to say that a substance contains heat. The substance contains internal energy. The word heat is used only when referring to the energy actually in transit from hot to cold body.

Thermometers

"The instrument used for measurement of hotness or coldness of a substance is called thermometer." Our sense of touch can tell us whether an object is hot or cold. It gives an idea about the object's temperature but we cannot measure the actual temperature of the body just by touching it.

Thermometers use some property of a substance, which changes appreciably with the change of temperature.

Basic Thermometric Properties

Some basic thermometric properties for a material suitable to construct a thermometer are the following:

i. It is a good conductor of heat.

ii. It gives quick response to temperature changes.

iii. It has uniform thermal expansion.

iv. It has high boiling pint.

v. It has low freezing point.

vi. It has large expansivity (low specific heat capacity).

vii. It does not wet glass.

viii. It does not vapourize.

ix. It is visible.

Temperature It is degree of hotness or coldness.

Measuring instrument Thermometer is used to measure temperature.

Liquid-in-Glass Thermometer

We know that liquids expand on heating. So, expansion in the volume of a liquid can be used for the measurement of temperature. This is known as liquid-in-glass thermometer. One such liquid which is commonly used in thermometers is mercury. It is made of glass, It has a bulb at one end filled with mercury.

When the temperature rises, the mercury expands and moves up through the arrow capillary tube in the form of a mercury thread. The position of the end of thread reads the temperature. Mercury is opaque and can be easily seen due to tis silvery colour. Alcohol is also a choice for the thermometric liquid, but it must be coloured to make it visible.

Construction

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. Mercury being metal is a good conductor and hence responds quickly to the change in temperature. The small quick response makes the device sensitive. Use of mercury is quite sensitive for normal measurements. 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).

The uniformity of the narrow tube or bore ensures even expansion of the liquid required to make the linear measuring scale. The choice of mercury allows to use it over a long-range temperature due to its low freezing point and high boiling point. It provides a fairly long range of measurements of temperature.

Temperature Scales

For the measurement of temperature, a scale is to be constructed which requires two reference temperature called two fixed points. One is the steam point slightly above the boiling of water at standard atmospheric pressure. This corresponds to upper fixed point of the scale. The second fixed point is the melting point of pure ice or simply ice point. It is called the lower fixed pint. Different scales of temperature have been constructed by assigning different numerical value to these fixed points. Three different scales are:

i. Celsius or Centigrade scale

ii. Fahrenheit scale

iii. Kelvin scale

i. Celsius: In Celsius or centigrade scale, the numerical values assigned to lower and upper fixed points are 0and 100. AS the difference between their values is 100, so the space between these points is divided into 100 equal parts. Each part is known as 1°C.

ii. Fahrenheit: In Fahrenheit scale, The lower fixed point is labelled as 32 and upper 212. As the difference between these two numbers in 180, so in this scale the space between these points is divided into 180 equal parts. Each part is known as 1°F. Celsius and Fahrenheit scales are generally used in ordinary life.

iii. Kelvin: There is a third scale of temperature known as kelvin scale or Absolute temperature scale. IT is used in scientific measurements. In Kelvin scale, the lower and upper fixed points are labelled as 273 and 373. As the difference between these values is 100, so the width of 1 k is the same as that of 1°C. The zero point of this scale is the temperature at which the molecules of substance cease to move. Their average kinetic energy becomes zero. This is known as absolute zero. Its value is −273.15°C. For calculations, it is simply taken whole universe. The matter does not exist below absolute zero temperature.

Conversion of Temperature from One Scale to Another:

If the temperature of a body is Tc on Celsius scale, Tf on Fahrenheit scale and Tk on kelvin scale, then these reading are related by the following formulae:

(i) Conversion of Celsius (centigrade) to Fahrenheit scale
Tf = 9/5 x Tc + 32 .......(1)

(ii) Conversion of Fahrenheit to Celsius scale:
Tc = 5/9 (Tf - 32) .......(2)

(iii) Relationship between Kelvin and Celsius scales:
Tk = Tc + 273 .......(3)

Thermocouple Thermometer

This type of thermometer consists of two wires of different materials such as copper and iron.

Construction

The 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 two wires have different resistance to the flow of current. The greater is the difference of temperatures, the greater is the potential difference or voltage produced across the junctions. If one end of the junction is kept at a fixed lower temperature, say by placing it in an ice bath at 0°C for reference, the temperature of other junction at a higher temperature can be measured using a millivolt meter by a calibrated scale on it.

Uses

This type of thermometer is particularly useful for very high temperatures and also rapidly changing temperature as there is only a small mass of metal (the junction) to heat up.

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.

Sensitivity

Sensitivity of a thermometer refers to its ability to detect small changes in the temperature of an object.

Example The minimum division on the scale of a thermometer is 1°C. The accuracy of its temperature measurement will be 1°C. On another thermometer the marks are 0.1°C. The accuracy will be up to 0.1°C and said to be more sensitive. Its measurement will be more precise than the measurement by a thermometer with an accuracy of 1°C.

Range

This refers to the span of temperature, from low to high, over which the thermometer can measure accurately. For example, a clinical thermometer designed for human body temperature has a narrow or short range, say from 35°C to 45°C. A long-range thermometer is usually used for science experiments in the laboratory with markings from −10°C to 110°C. The choice of liquid for thermometers put a lower and upper limit for the range of a thermometer. For example, Mercury freezes at −39°C and boils at 357°C. Hence, we can construct mercury in glass thermometers within this range. The marking scale depends on desired range of measurement. For extremely low temperatures, alcohol is used. Alcohol has a much lower freezing point about −112°C which increases its lower limit for the range but it has lower upper limit as it boils at 78°C.

Linearity

This refers to a direct proportional relationship between the temperature and scale reading across entire range of measurement. A good linear thermometer should measure equal increments on the scale corresponding to equal change in the temperature. It means that marking on the scale should be evenly spaced over the whole range. High linearity means more consistent and proportional scale readings over the entire range to ensure accuracy of measurement.

The zero on kelvin scale is called absolute zero. It is a theoretical temperature at which all substances have zero thermal energy. At this temperature volume of an ideal gas reduces to zero. Absolute zero is equivalent to 0 K, -450 °F, or -273 °C.

At this the molecules of a substance cease to move. Their average kinetic energy becomes zero. Absolute zero is the lowest possible temperature ever to be in the whole universe. The matter does not exist below absolute zero temperature.