Unit 14: States of Matter and Phase Changes — Short Questions

When a liquid evaporates the energy is primarily come from the surrounding environment and the remaining from the liquid itself. Energy is provided from outside in the form of heat which increases rate of evaporation.

Condensation is not an endothermic process but it is an exothermic process. According to the kinetic particle theory, in a gas, particles move away rapidly and are far apart. As gas loses energy, particle motion slows down, and attractive forces pull the particles closer together, forming a liquid, during condensation.

Naphthalene is an organic aromatic compound having a harsh smell. Naphthalene balls are placed among silky clothes which disappear after sometimes due to process of sublimation. This means they change directly from solid to gas (naphthalene vapour) without turning into liquid.

Temperature remains constant during a phase change like melting or boiling, because the heat energy added is used to overcome the attractive forces between the particles, not to increase the kinetic energy. Since temperature is a measure of kinetic energy so the temperature remains constant. The heat provided is called latent heat.

Gases can be compressed easily but liquids cannot be compressed. According to kinetic theory, the particles in a liquid are quite close to one another They are also moving in all possible directions. So in general, liquid cannot be compressed and are generally incompressible.

Boiling point of a liquid is affected by changing external pressure. So boiling point of water can be changed in laboratory by applying change in external pressure. If external pressure is increased on the liquid, it boils at higher temperature.
Example: Use of pressure cooker boils the water at higher temperature. Low external pressure makes the water to boil at low temperature.

Wet clothes take longer time to dry during winter. This is because, the air is cooler and is more humid. Therefore, rate of evaporation is decreased and clothes require relatively longer time to dry.

The physical state of a solid substance can be changed by simple heating. Heat increases the kinetic energy of the particles and they start vibrating at a higher frequency at a particular temperature. Their vibrational motion becomes so fast that it overcomes the cohesive forces. As a result, the solid starts melting and particles lose their mean position, solid collapses and turn into liquid.

Diffusion is a process of movement of molecules from an area of higher concentration to lower concentration. It is most commonly associated to gases because gas molecules are free to move easily. But diffusion is not just limited to gases. Solids and liquids can also diffuse. In solids, diffusion is very much low due to restricted motion of its molecules. Liquids diffuse faster than solids but slower than gases.

During night, the temperature of earth becomes very low. It causes the air near the surface of earth to get cool. When the air's temperature drops to its dew point, the water vapour in the air condenses into tiny droplets on the surface forming dew in the early morning.

It was suggested that physical properties of gases such as their ability to compress or expand, or diffuse could be understood by assuming that these gases consist of particles which are continuously moving randomly. This idea then led to formulation of kinetic particle theory for gases.

The outcomes of kinetic particle theory are:
• Kinetic particle theory explains all the laws which govern the behaviour of gases.
• It explains the composition of liquid and solid states of matter.
• This theory also explains the interconversion of all the three states.

Postulates of kinetic particle theory of gases:
• According to kinetic particle theory gases are composed of particles which are in a continuous state of random motion in all the possible directions.
• The pressure exerted by a gas is due to the collisions of its particles with the walls of the container

According to kinetic theory, the particles of a liquid do not have any fixed position and shape. Owing to the presence of inter particle forces, however, a liquid has a fixed volume and it keeps its level as well.

According to kinetic particle theory, the inter-particle forces in the solid substances are so strong that they keep their particles arranged in a fixed position. These particles possess vibrational motion only. These restricted movements force solid substances to have a fixed shape and a fixed volume.

Internal energy Definition: The internal energy of a substance is the total energy it contains. It includes the kinetic energy of its particles and the potential energy due to bonding between them. Heat increases the internal energy of a system.

Melting Point The temperature at which a solid changes its state to become a liquid is called its melting point.
Example: Ice melts at 0°C

Effect of heat after melting: Further heating the solid after it has started melting does not increase its temperature. Instead, all the heat energy provided at this moment is utilized to convert the solid into its liquid.

Evaporation The molecules of a liquid keep on coming out from the surface of a liquid at all temperatures and such an escape of molecules from surface is called evaporation.
It is a surface phenomenon because the molecules of surface escape out of the liquid.

Boiling point The temperature of a liquid at which vapour pressure becomes equal to the atmospheric or external pressure is called its boiling point.
Example: Boiling point of water is 100°C.

At the boiling point, the heat provided to the liquid is used to convert it into gaseous form and during this its temperature remains constant. Because the heat provided is used only to break the forces of attraction between its particles.

Condensation (Conversion of a gas into its liquid): When a gas is cooled, the kinetic energy of its molecules decreases, as a result the molecules come closer with a significant force of attraction between them. At a suitable lower temperature, the increased attractions bring the molecules so close that they are changed into a liquid form. This is called condensation.

Heating and cooling curves Interconversion of physical states can also be understood with the help of a graph drawn between the internal energy and the temperature of a system. Such a graph is also called a heating or cooling curve.

At boiling point, the kinetic energy (temperature) remains constant. While the potential energy increases as molecules move apart to form gas.

The boiling point of water in Karachi is higher than Murree because the atmospheric pressure at Murree is lower than that in Karachi, so we find that the boiling point has decreased at lower atmospheric pressure.

Sublimation The direct conversion of a solid to vapours without melting is called sublimation.
Example: Solid carbon dioxide which is also called dry ice, changes directly to gaseous carbon dioxide at room temperature without first melting to liquid state.

The energy needed for sublimation comes from within the substance and the remaining absorbed from the surrounding.This energy is sufficient to overcome the attractive forces of the neighbouring molecules which then escape into the vapour phase.

Deposition The process reverse to sublimation is called deposition where a gas changes directly to a solid without going into the liquid state.
Example: Formation of frost in winter season is an example of deposition.

When a solid air freshener is exposed to the atmosphere or heated in the air, the air freshener particles gain enough energy to overcome the attractive forces and spread in the nearby atmosphere in the form of sweet smelling vapours. So in this way they disappear automatically after some days.

Sublimation printing When the process of sublimation is used to print a design into a material or fabric, it is called sublimation printing.
The process involves the printing that transfers a design into a fabric using ink and heat.

Sublimation is used in daily life in:
• Air fresheners
• Sublimation printing on T-shirts, ceramics, wood or metals.

According to Kinetic Theory, the pressure exerted by the gas in a container is caused by the collisions of its molecules with the walls of the container.
The pressure changes directly with the number of molecules colliding with the wall per unit of time.

The pressure – volume relationship is called Boyle's law. It states that "the volume of a given mass of a gas is inversely proportional to its pressure at constant temperature."
Mathematically
V ∝ 1/P
(V is the volume of gas and P is its absolute pressure)
V = k 1/P
PV = k = constant
P₁V₁ = P₂V₂

Charles' law
Statement: The law states that, the volume of a given mass of a gas varies directly with temperature when pressure is kept constant.
Mathematical form:
V ∝ T (P and mass of gas are kept constant)
V ∝ T
V = kT
V/T = k = constant

Avogadro's law
Statement: This law states that, equal volumes of different gases must contain an equal number of molecules if the temperature and pressure are kept constant.
Mathematical form
V ∝ n (at constant temperature and pressure)
V/n = k = constant

Diffusion Molecules present in gases are in a constant state of random motion. Due to this molecular motion the gas particles spread out and intermix from an area of high concentration to an area of low concentration. This property of gases is called diffusion.
Example: When a bottle of body perfume is opened in one corner of a room its sweet smell slowly spreads throughout the room after sometime due to the process of diffusion.

Effect of temperature on diffusion: Rate of diffusion increases with the increase in temperature as the particles have more kinetic energy and hence they move faster. This eventually leads to rapid mixing and spreading

Rates of diffusion play a crucial role in the delivery and movement of drugs throughout the body. This process of diffusion ensures that essential substances reach their target locations quickly which eventually helps in the treatment of diseases.

See Q.13 from theory.

See Q.14 from theory.

Evaporation is a continuous process which occurs at all temperature. Evaporation does not need input of energy from outside because it required a smaller amount of energy which is produced from inside of the liquid due to continuous collisions of molecules.

A heating curve is a graph that shows the temperature of a substance changes with time. The same graph when studied in reverse order show cooling curve. This occurs because of same phase changes of a substance occur in heating and cooling.

Ice melts from pressure in the hands of snowball because interconversion of physical states of matter may not be brought about by changing the temperature only. Sometimes their interconversion may be affected more conveniently by changing both the temperature and pressure at the same time or by varying the pressure only at constant temperature.

Phase change are related to changes in temperature and intermolecular forces. When we add or remove heat, it affects the movement and intermolecular forces of molecules. So, phase change occurs.
Example: When we heat a pure substance in solid state, heating overcomes its intermolecular forces causing solids to melt or liquid to vapourize. Similarly, when cool a pure substance, molecules slow down and arrange their pattern of arrangements like gas to liquid or liquid to solid (condensation, freezing).

See Q.21 from theory.

See Q.30 from theory.

See Q.10 from theory