General Properties of Gases

The Gaseous State

In gases, the intermolecular force of attraction is so weak and the intermolecular space is so large that the particles become completely free to move randomly in the entire available space.

General Properties of Solids

1.  Shape and Volume: a gas has neither a definite shape nor a definite volume. It takes the shape and volume of the vessel in which it is kept. It occupies all the available space in the vessel because the particles can freely move throughout the space.

2.  Density: the density of gases is lower than those of solids and liquids.

3.  Melting and boiling points: at normal atmospheric pressure, the melting and boiling points of a gas are below room temperature. They are lower than those of solids and liquids.

4.  Compressibility: the compressibility of a gas is very high. The intermolecular space in a gas being large, the particles can be made to come much closer together under pressure. That is why a large amount of air can be pumped into the small volume of a cycle tube or a football bladder. The property of compressibility of gases is made use of in storing fuel gases in cylinders. Butane is called a petroleum gas as it is obtained from petroleum refineries. Butane has been found to be an efficient domestic fuel. It is liquefied under pressure, packed in cylinders and made available for usage as LPG (Liquefied Petroleum Gas). Oxygen supplied to hospitals in cylinders is another example of compressed gas. So is Compressed Natural Gas (CNG), the fuel which is now increasingly being used in vehicles.

Effect of heating and cooling

Effect of heating and cooling: A gas can be made to expand or contract on heating and cooling. On heating, the energy of the gas particles is increased. As a result, the gas particles move faster and also go farther away from each other. This results in the expansion of the gas. On cooling, the energy of the gas particles is decreased and so the movement of the particles is solved down. The particles also come much closer to each other, leading to the increase in the intermolecular force of attraction. This causes the gas to contract.

Diffusion of gases

Diffusion of gases: gas particles move with high speeds and the intermolecular spaces in gases are very large. These properties allow a gas to diffuse easily into another. Thus, gases mix together spontaneously, despite their differences in densities. When a bottle of perfume is open in the corner of a room, the fragrance of the perfume spreads in air and covers the entire space of the room. Also the smell of the food being cooked in the kitchen reaches our nostrils, even if we are not physically present in the room. This is because the particles constituting the smell mix with the particles of air, spread out and reach our nostrils. Other examples include virus on sneezing, dissolution of oxygen and carbon dioxide in sea water etc.

Condensation of gases

Condensation of gases: The conversion of gas into liquid is called condensation. It can be carried out by either increasing the pressure on the gas or decreasing its temperature. By doing this, the intermolecular distance is decreased and force of attraction is increased thus changing into liquid state.

·   Dry Ice: carbon dioxide gas at very low temperature and under high pressure can be transformed into a solid which appears like ice. The solid so obtained is called dry ice. On reducing the pressure on the surface of the solid, it gets directly converted into vapour without changing its liquid state. So, dry ice is a sublimable substance.

·   Condensation of water vapour: it takes place on a cold surface. By taking ice in a glass container. Ice absorbs heat from the air and starts to melt. The moisture present in air is thus cooled and finally gets condensed on the outer walls of the container as water droplets.

Pressure exerted by gas

Pressure exerted by gas: the particles of gas are continuously moving in all possible directions with different velocities. In this process, they collide with one another and with walls of the container. In doing so, they exert some pressure on the walls of the container. The force applied by the gas particles per unit area on the walls of the vessel is called the ‘pressure of the gas.’ The pressure of air is called atmospheric pressure. The pressure of air at sea level is one atmosphere. The unit of pressure is Pascal (Pa). I atmosphere = 1.01325 X 105 Pa.

Interconvertibility of the States of Matter

Interconvertibility of the States of Matter:

All the three states of matter are interconvertible i.e. matter can be changed from one state to another.For example: ordinary water. When water is cooled at 0°C, it is transformed into ice (solid). Heated to 100°C, it begins to boil and goes into the gaseous state (vapours). On cooling, water vapour gradually passes into the liquid state and finally to the solid state (ice).

Explanation: 

At normal pressure (1 atm) the boiling point of water is 100°C (373.15 K). During boiling, the temperature of a liquid does not rise despite heating. Instead, the supplied heat energy goes to raise the kinetic energy of the particles. When the particles acquire enough energy, they overcome the attractive forces of other particles and go into vapour state. As the heat energy received by the liquid remains hidden inside the bulk of liquid, it is called the latent heat of vaporization.

Freezing

Freezing is the process when a liquid turns into a solid. Freezing occurs when heat is lost from an object, which causes the molecules to slow down and form tighter bonds. One example of freezing is when water turns into ice. Freezing is the opposite of melting, and two steps away from evaporation.

Difference between steam and boiling water

Difference between steam and boiling water

The temperature of both steam and boiling water is the same. But, steam causes more severe burns than boiling water. This is because steam contains extra heat energy in the form of latent heat of vaporization.

Boiling and boiling point

Boiling and Boiling Point: A boiling point is the temperature at which “boiling” occurs. Whereas boiling is a phase change from liquid to gaseous state. The boiling point tells us how much energy we have to add to break the intermolecular bonds between all of our molecules. So a higher boiling point means that you have more intermolecular forces to overcome. For example, water boils at 100 °C.

Atmospheric pressure influences the boiling point of water. When atmospheric pressure increases, the boiling point becomes higher, and when atmospheric pressure decreases (as it does when elevation increases), the boiling point becomes lower.

As elevation increases, atmospheric pressure decreases because air is less dense at higher altitudes.

At lower altitudes, the atmospheric pressure is lower, the vapour pressure of the liquid needs to be lower to reach boiling point. Therefore, less heat is required to make the vapour pressure equal to the atmospheric pressure. The boiling point is lower at higher altitude.

Vaporization

Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomenon.

Measurement of temperature

Measurement of temperature

There are three scales for the measurement of the temperature of a system. These are: degree Celsius (°C), degree Fahrenheit (°F) and Kelvin (K).

On Celsius scale the freezing point of water is taken as 0°C, whereas its boiling point is taken as 100°C. So, this scale is calibrated from 0 to 100°C.

On the Fahrenheit scale, the freezing point of water is 32°F and its boiling point is taken as 212°F. So, this scale is calibrated from 32 to 212°F.

°F = 9/5 (°C) + 32

On the Kelvin scale, the freezing point of water is taken as 273.15K and its boiling point is taken as 373.15K. The Kelvin scale is related to the Celsius scale as follows:

t°C = (273.15 + t)K

For convenience, we use 0°C = 273 K

Difference between Gas and Vapour

Difference between Gas and Vapour

Usually there is no difference between gas and vapour. The term ‘vapour’ is used for those gases which exist in the liquid form at room temperature. For example, the gaseous form of water is called vapour. On the other hand, hydrogen, oxygen, nitrogen, carbon dioxide etc exist as gases at room temperature. Hence, each one of these is called a ‘gas’, not a vapour.

Absorption of Heat

Absorption of Heat

It has been found that black or any dark coloured object is a better absorber of heat than a white or light coloured object. Black object’s ability to absorb more heat is utilized in designing solar cookers. During summer, white or light coloured dresses are preferred to black or dark coloured dresses as white dresses absorb less heat.

Plasma state of matter

Plasma: state of matter

Scientists are reported to have discovered a new state of matter which is called ‘plasma state’. This state does not fit into any of the hitherto known three states of matter. Hence it is called the ‘forth state of matter.’

Plasma state consists of highly ionized gas in which the particles exist in super energetic and super excited state. For example: fluorescent tubes and neon sign bulbs. Fluorescent tubes contain helium or other gases. When electric current is passed through gas, it produces glowing plasma, having a characteristic colour depending upon the nature of gas. Plasma is produced in the sun and in stars due to high temperature. It is the presence of plasma that makes them glow.

Bose Einstein Condensate

Bose – Einstein Condensate

In 1920, Indian scientist Satyendra Nath Bose on the basis of his statistical calculations gave the concept of fifth state of matter. Einstein too, predicted the possibility of such a state. Later, three American scientist succeeded in obtaining this state by supercooling a gas of extremely low density. The process is called Bose – Einstein condensation, and this state of matter is called Bose – Einstein condensate (BEC).