Boyle's Law

Experiment 9 

Theory

Properties of gases and Gas laws

Gaseous state is a state of matter in which the substance does not have any specific shape or volume. It adopts the form and size of its container. The fundamental macroscopic properties of gases are pressure, volume, temperature and mass of the gas. These can be explained by kinetic theory by considering their molecular composition and motion.  Careful, scientific observation has determined that these variables are related to one another, and the values of these properties determine the state of the gas. 

These relationships among pressure, temperature and volume of a gas lead to Gas laws. Boyle's Law tells us that the volume of gas increases as the pressure decreases. Charles' Law tells us that the volume of gas increases as the temperature increases and Avogadro's Law tells us that the volume of gas increases as the amount of gas increases. The ideal gas law is the combination of the three simple gas laws.

Boyle’s Law

According to Boyle’s Law, the pressure (P) of a given mass of gas is inversely proportional to its volume (V), provided that the temperature of the gas remains constant.

For an enclosed gas, at constant temperature (T);

or,         

ie;     

The quill tube is helpful in verifying Boyle’s law.

Since the volume of gas inside the tube(V) = cross sectional area of the tube(a) × length of air column(l),

So,  

Learning outcomes

• Students learn about the properties of gases and their relationships.
• Students understand Boyles law and its applications.
• Students understand the relation between pressure and volume of a given mass of the gas.

Materials Required

• Quill tube
• Stand
• Metre scale

Procedure

• Arrange the quill tube horizontally on a stand.
• The length of the air column is measured using a metre scale.
• Vertical heights at the two ends of mercury thread from the table are also measured using a metre scale.
• The difference between them gives the vertical height ‘h’ of the mercury thread. Here h=0. So pressure inside the tube is also H, which is the atmospheric pressure.ie; 76 cm of Hg
• The quill tube is then placed in a slanting position with the open end upwards.
• The length of the air column is measured and the vertical height, h of Mercury is noted. Now the pressure inside the tube, P=H + h.
• Quill tube is then placed in different positions, such as: vertical position with open end upwards and with open end downwards, slanting position with open end downwards and measure its corresponding length of the air column l and vertical height h. 
• Now P×l  is calculated in each case.

Observations

Calculations

• Calculate P × l in each case.
• Plot a graph between Pressure and reciprocal of length, taking pressure (P) along X axis and reciprocal length (1/l) along Y axis.

Result

From the tabular column, it is found that; P × l is a constant.

The graph between reciprocal of length and pressure is a straight line, which shows that pressure of the given mass of gas is inversely proportional to length of air column.

It indicates the inverse proportionality of pressure and volume of a given mass of gas, hence verifies Boyle’s law.

Precautions :

1. Air in tube A must be pure and dry.

2. The wooden board must be set vertical

3. Position of mercury levels must be noted using set squares

4. Atmospheric pressure must be taken in the beginning and at the end of the Its mean must be used in calculation.

5. Graph must be plotted carefully. 

Sources of error :

1. The air in tube A may not be pure and dry.

2. Base may not be broad and heavy to keep the, apparatus

Viva-Voce [Boyle's Law]

Question. 1. State  Boyle’s Law ?

Answer. Read theory of Experiment  9.

Question. 2. Why do we use the mercury  in Boyle’s law apparatus ?

Answer. Pressure difference (P) is in terms of difference of mercury levels in tube A and B. This difference added algebraically to atmospheric pressure gives the pressure of air in tube A in cm of Hg.

Question. 3. Does Boyle’s law hold good under all conditions ?

Answer. No. Boyle’s law is only hold good when temperature is high and pressure is low.

Question. 4. Why is it necessary to make  the Boyle’s law apparatus vertical ?

Answer. If it is not vertical, the reading of mercury column  will not be accurate.

Question. 5. For which gas  have you verified Boyle’s law ?

Answer. For air.

Question. 6. What is atmospheric pressure ?

Answer. It is the pressure on earth surface due to air column  Pa = 1.013 x 10-5  Nm-2.

Question. 7. What is nature of graph  between pressure and volume for a gas  at constant temperature ?

Answer. Hyperbola.

Question. 8. What is nature of graph  for P vs. 1/V for a gas  at constant temperature ?

Answer. Straight line.

Question. 9. How the pressure depend upon radius  of tube A and B?

Answer. Pressure does  not depend upon the radius (Area) of tube.

Question. 10. What is ideal gas  equation ?

Answer. PV = nRT

Physics is one of the most important subjects in Class 12. As the CBSE exam approaches, students get busy preparing for different subjects. But an essential part of the CBSE exam is the practical exams which consist of 30 marks.

Students must know all the experiments along with theorems, laws, and numerical to understand all the concepts of 12th standard physics in a detailed way. Two experiments (8 + 8 marks) are asked from each section in the practical exam. The experiment records and activities consist of 6 marks, the project has 3 marks and viva on the experiment consist of 5 marks.

The Physics Practicals For Class 12 CBSE is given here so that students can understand the experiments in a better way. Students are suggested to study the theory and law behind the experiment properly before performing the experiment.Also Go through the viva voce questions and answers for each and every experiment which are provided on the website .