The ideal gas law relates pressure P, volume V and temperature T of an ideal gas are by equation PV = nkBT, where n is the number of molecules of the gas, and kB is Bolztmann's constant. We can also write this as PV = NRT, where N denotes the number of moles of gas, and R is the gas constant. The ideal gas law is obeyed fairly well provided the density of the gas is not too high and these gases that obey it are called ideal gas. If the density is too high, the gas may condense to form a liquid, and the law fails in this case.
Ideal Gas Law
The constant volume gas thermometer is made of a bulb that is filled with a known fixed amount of a dilute gas attached to a mercury manometer. Now the bulb is brought in close contact with the source. The manometer attached to the bulb allows it to measure the exact pressure. The manometer contains a halfway filled mercury column and it is connected with a flexible tube to another partially filled column of mercury which is called the reservoir. Now the height of the mercury column is measured at the known temperature. Throughout this temperature measurement this height is fixed while the mercury in the reservoir is moved up or down.
This explains the gas thermometer. Now let us see how it works? This is where the ideal gas law comes in existence. The fundamental measurements of gas are done with pressure, P, volume, V and temperature, T. Also the SI Units of temperature is Kelvin; and T = t + 273.15 where t is in oC. The term pressure, volume and temperature of a gas are all interrelated and this interrelation was first explored by Robert Boyle. Boyle’s law states that when the temperature of a gas is kept constant, PV = Constant.
This explains the gas thermometer. Now let us see how it works? This is where the ideal gas law comes in existence. The fundamental measurements of gas are done with pressure, P, volume, V and temperature, T. Also the SI Units of temperature is Kelvin; and T = t + 273.15 where t is in oC. The term pressure, volume and temperature of a gas are all interrelated and this interrelation was first explored by Robert Boyle. Boyle’s law states that when the sample gas temperature held constant, PV = Constant.
PV=Constant
French scientist Jacques Charles now came up with his law later named as Charles law. He discovered when the pressure of a gas is kept constant, the volume is related to the temperature by following equations
V/T = Constant.
These laws were later combined to yield one universal gas law known as the ideal gas law.
PV/T=constant
And the constant proportionality factor in this equation is the Universal Gas Constant, R ie, constant = nR.
where, n gives the number of moles of the gas in the sample
Absolute Zero Temperature
Absolute zero is the defined temperature at which a thermodynamics system has the lowest energy. It corresponds to −273.15°C on Celsius scale and to −459.67°F on the Fahrenheit scale. The notion that there is an ultimately lowest temperature was suggested by the behaviour of gases at a very low pressure. It was observed that gases seem to contract indefinitely as the temperature decreases. Also an ideal gas at constant pressure would reach zero volume at what is now called the absolute zero of temperature. Real gas in actual condenses to liquid or solid at some temperature higher than absolute zero that is why the ideal gas is only an assumption to real gas behavior.
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Significance of Absolute Zero
On extrapolating this graph, what we see is that between Pressure and temperature is shown below for different gases.
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Now irrespective of the nature of gas we see that, the graphs always intercept the x axis at a point we call the absolute zero. This point represents the beginning of the Kelvin scale i.e Zero K. In the Celsius scale, it corresponds to -273.5 oC. This is possibly the coldest temperature. As a molecule gets colder, it’s energy, movements and vibrations decrease in amplitude. As we keep cooling it, at a point the atom will reach a state of minimum internal energy where the atom has lost all its energy.Thus we cay that absolute zero is a state at which the enthalpy and entropy reach their minimum value and it the lowest limit on temperature scale.
FAQs on Ideal Gas Law And Absolute Zero
1. What is the Ideal Gas Law and its formula as per the CBSE Class 11 syllabus?
The Ideal Gas Law is an equation of state for a hypothetical ideal gas. It serves as a good approximation of the behaviour of many gases under various conditions. The law combines Boyle's Law, Charles's Law, and Avogadro's Law. The formula is expressed as:
PV = nRT
Where:
- P is the pressure of the gas.
- V is the volume of the gas.
- n is the number of moles of the gas.
- R is the universal gas constant (8.314 J/mol·K).
- T is the absolute temperature of the gas in Kelvin.
2. What exactly is Absolute Zero?
Absolute Zero is the lowest possible temperature where nothing could be colder and no heat energy remains in a substance. It is defined as 0 Kelvin on the Kelvin scale, which is equivalent to -273.15° Celsius. At this theoretical temperature, all classical motion of particles ceases, and they are at their minimum possible energy state.
3. How does the Ideal Gas Law help in understanding the concept of Absolute Zero?
The Ideal Gas Law, particularly through its foundation in Charles's Law (V ∝ T), provides a theoretical basis for Absolute Zero. Charles's Law states that for a fixed amount of gas at constant pressure, the volume is directly proportional to the absolute temperature. If you plot a graph of volume versus temperature for an ideal gas and extrapolate the line backwards, it predicts that the gas would have zero volume at a temperature of -273.15°C. This theoretical point of zero volume is defined as Absolute Zero (0 K).
4. Why can a real gas never actually reach Absolute Zero or behave perfectly like an ideal gas?
A real gas deviates from ideal gas behaviour and cannot reach Absolute Zero for two main reasons:
- Intermolecular Forces: Real gas molecules have weak attractive forces (like van der Waals forces) between them. As the gas cools and particles slow down, these forces become significant and cause the gas to condense into a liquid or solid well before reaching Absolute Zero.
- Finite Molecular Volume: The Ideal Gas Law assumes gas particles have zero volume. In reality, atoms and molecules occupy a finite space. Therefore, a real gas can never be compressed to zero volume.
5. According to the kinetic theory of gases, what happens to the kinetic energy of gas molecules at Absolute Zero?
According to the kinetic theory of gases, temperature is a direct measure of the average kinetic energy of the gas molecules. Therefore, at the theoretical temperature of Absolute Zero (0 K), the average kinetic energy of the molecules would also be zero. This implies that all translational, rotational, and vibrational motion of the molecules would completely cease.
6. What are the fundamental assumptions of the Ideal Gas Law?
The Ideal Gas Law is based on a set of key assumptions about the behaviour of gas particles. These are:
- The gas consists of a large number of identical particles that are in random, constant motion.
- The volume occupied by the gas particles themselves is negligible compared to the volume of the container.
- There are no intermolecular forces of attraction or repulsion between the particles.
- Collisions between particles and with the walls of the container are perfectly elastic, meaning kinetic energy is conserved.
7. In which chapter of the CBSE Class 11 Physics NCERT textbook is the Ideal Gas Law covered?
The Ideal Gas Law and the concept of Absolute Zero are primarily covered in Chapter 13, titled "Kinetic Theory", of the CBSE Class 11 Physics NCERT textbook for the 2025-26 session. The foundational concepts are also linked to topics in Chapter 11, "Thermal Properties of Matter".
