Unit 5: States and Phases of Matter — Short Questions

Evaporation causes cooling because of the loss of high-energy molecules from the surface of a liquid. In any liquid, molecules are constantly moving with varying kinetic energies — some fast, some slow.

Escape from Surface Molecules with higher-than-average kinetic energy can overcome intermolecular forces and escape from the surface into the air as vapor.

Loss of Energy These escaping molecules take more energy with them. The average kinetic energy of the remaining molecules decreases.

Temperature Drop Since temperature is directly related to average kinetic energy, the temperature of the liquid drops, creating a cooling effect.

Liquids with stronger intermolecular forces tend to have lower rates of evaporation at a given temperature because more energy is required for their molecules to escape into the vapor phase.

Evaporation Process For a molecule to evaporate, it must have enough kinetic energy to overcome the intermolecular attractions holding it in the liquid.

Stronger forces makes harder to escape. In liquids like water, which have strong hydrogen bonds, molecules are tightly held together. Only a few molecules at any moment have enough energy to break free.

Weaker forces makes easier to escape. In liquids like acetone (with weaker dipole-dipole or dispersion forces), more molecules can escape at the same temperature.

After taking a bath, your skin is covered with a thin layer of water. When you stand under a fan, you feel cool because of evaporation, enhanced by moving air.

Evaporation of Water Water molecules on your skin begin to evaporate into the air. Evaporation requires energy, which is taken from your body heat.

Loss of Heat Energy The most energetic (fast-moving) water molecules escape first, reducing the average kinetic energy (and thus temperature) of your skin.

Role of the Fan The fan blows away the humid air near your skin and replaces it with drier air, which speeds up evaporation.

Result Faster evaporation means more heat is removed from your skin, making you feel cooler.

When a liquid is placed in a closed vessel at constant temperature, its molecules start to evaporate. Some of these vapors collide with the surface and condense back into the liquid. Over time, the rate of evaporation becomes equal to the rate of condensation — this is called dynamic equilibrium. At this point, the amount of liquid and vapor remains constant, and vapor pressure becomes stable.

The boiling point of water depends on atmospheric pressure. Lahore is at a lower altitude with higher atmospheric pressure, so water boils at 100°C. In contrast, Murree Hills are at a higher altitude where atmospheric pressure is lower, so water boils at a lower temperature. Hence, boiling point decreases with altitude (98°C at Murree Hills).

1. Floating of Ice: Ice is less dense than liquid water, so it floats. This forms an insulating layer on lakes and rivers in winter, preventing the water below from freezing, which helps aquatic life survive in cold climates.

2. Weathering of Rocks: When water enters cracks in rocks and freezes, it expands due to lower density. This exerts pressure on the rock, causing it to break apart over time — a process called frost weathering.

Boiling occurs when the vapor pressure of a liquid equals the external pressure. When external pressure increases, the liquid needs to reach a higher temperature to make its vapor pressure match the new external pressure. Hence, boiling point increases with increasing external pressure.

Special liquid crystal devices can be used to diagnose the tumors and infections in the human body. The most common application of liquid crystal technology is liquid crystal displays (LCDs). Examples of liquid crystal screens are oscillograph and TV, laptops, cell phone displays. Liquid crystals are used in TV displays, computer screens, calculators and watches.

Crystalline solids
Definition: The solids which have definite regular and three dimensional geometric shapes are called crystalline solids.
Example: Diamond, sodium chloride, ice, etc.

Amorphous solids
Amorphous solids, contrary to the crystalline solids, do not possess regular three dimensional geometrical shapes.
Example: The amorphous solids are glass, wood amorphous sulphur (plastic sulphur), charcoal, coal, coke, etc.

Order of increasing boiling points
Butane < Propanone < Propanol

Explanation
Butane has only weak London dispersion forces, so it has the lowest boiling point.
Propanone has dipole-dipole interactions due to its polar carbonyl group, which are stronger than London forces.
Propanol has hydrogen bonding due to the – OH group, which is the strongest intermolecular force among the three, so it has the highest boiling point.

Water molecules form strong hydrogen bonds with each other due to the attraction between the partially positive hydrogen and the partially negative oxygen atoms. At the surface, molecules experience unbalanced inward forces, pulling them closer together. This creates a tightly held surface layer, leading to high surface tension. Hydrogen bonding is the key force behind this cohesion among water molecules.

(i) NH₃ (Ammonia): Hydrogen bonding due to N−H bonds and high electronegativity of nitrogen.

(ii) Ar (Argon): London dispersion forces, it's a non-polar noble gas.

(iii) CH₃COCH₃ (Propanone): Dipole-dipole interactions, due to the polar carbonyl group.

(iv) CH₃OH (Methanol): Hydrogen bonding, due to the presence of an −OH group.

CH₄ (Boiling point 109 K) It is a non-polar molecule with only London dispersion forces, which are weak, resulting in the lowest boiling point despite its low molar mass.

NH₃ (Boiling point 240 K) Ammonia is a polar molecule and forms hydrogen bonds due to the presence of N−H bonds. These are stronger than dispersion forces, so NH₃ has a higher boiling point than CH₄.

H₂O (Boiling point 373 K): Water has strong hydrogen bonding due to highly electronegative oxygen and two O−H bonds, allowing extensive hydrogen bonding. This results in the highest boiling point, even though its molar mass is only slightly more than CH₄ and NH₃.

The boiling point increases with the strength of intermolecular forces:
London forces < Dipole-dipole < Hydrogen bonding.

The rate of diffusion in liquids is lower than in gases because the intermolecular forces in liquids are stronger. This restricts the free movement of molecules. Also, the molecules in liquids are closely packed, leaving little space for fast diffusion like in gases.

When pressure on a gas is doubled at constant temperature, its volume becomes half. This is explained by Boyle's Law (V ∝ 1/P). Gases are highly compressible due to the large empty spaces between their molecules.

According to kinetic molecular theory, molecules of a liquid are in constant random motion. They possess kinetic energy which increases with temperature. Their motion allows them to diffuse and flow, though slower than gases due to stronger forces.

Liquids can flow because their molecules are not fixed in place and have the ability to move past each other. Although they are closely packed, the intermolecular forces are not strong enough to completely lock them, allowing fluid motion.

When temperature is increased, the kinetic energy of liquid molecules increases, causing slight expansion. However, this expansion is very small compared to gases, as liquid molecules are already close together and face strong intermolecular forces.

Non-polar molecules exhibit instantaneous dipole-induced dipole forces due to temporary shifts in electron clouds, creating momentary dipoles that induce dipoles in neighboring molecules.

With increasing molecular mass, the size and polarizability of atoms increase, which strengthens id-id forces, resulting in higher boiling points down the group.

Iodine has a much higher molecular mass and size compared to fluorine, so it experiences stronger id-id forces, making it a solid, whereas fluorine remains a gas due to weaker forces.

These are forces of attraction between the positive end of one polar molecule and the negative end of another. They occur in molecules like HCl and CHCl₃, where permanent partial charges are present.

Molecules with larger or linear surface areas (like n-pentane) have stronger id-id forces and higher boiling points, while branched molecules have smaller surface areas and weaker forces.

Hexane has a greater molecular mass and longer chain length, resulting in stronger instantaneous dipole-induced dipole forces compared to ethane, thus a higher boiling point.

Boiling point is the temperature at which the vapour pressure of a liquid equals the atmospheric pressure. At this point, bubbles form and rise continuously from within the liquid.

Water has strong hydrogen bonding, which means more energy is needed to overcome these forces, leading to a higher boiling point than alcohols.

On Mount Everest, atmospheric pressure is lower, so water reaches its vapour pressure at a lower temperature, thus boiling at only 69°C.

Hydrogen bonding requires 1. A hydrogen atom covalently bonded to F, O, or N,
2. The electronegative atom must have a lone pair to attract the partially positive hydrogen.

Hydrogen bonding is stronger because it involves highly polarized covalent bonds. The strong attraction between δ+ hydrogen and δ− electronegative atoms leads to significant intermolecular force.

Water has two hydrogen atoms and two lone pairs on oxygen, enabling it to form two hydrogen bonds per molecule. In contrast, NH₃ has one lone pair and HF has only one H atom, limiting their bonding capacity.

Due to extensive hydrogen bonding among water molecules, a large amount of energy is required to break the bonds. This makes the boiling point of water (100°C) unusually high compared to other group 16 hydrides.

In ice, water molecules form a tetrahedral structure with empty spaces due to hydrogen bonding. This increases the volume and decreases density, causing ice to float on water.

Hydrogen bonding in water absorbs and stores more heat energy without a large temperature rise, giving it a high specific heat capacity, which stabilizes temperature in environments.

Water molecules at the surface are strongly attracted to those beneath due to hydrogen bonding. This causes a stretched surface film, responsible for high surface tension.

Viscosity is a liquid's resistance to flow. Water's strong hydrogen bonds create internal friction, increasing viscosity, compared to non-hydrogen bonding liquids like hexane.

HF forms strong hydrogen bonds due to fluorine's high electronegativity. Although it can form only one bond per molecule, the strength of this bond raises its boiling point above ammonia.

Surface tension is the force that acts along the surface of a liquid, making it behave like a stretched elastic sheet. It occurs due to unbalanced inward forces on surface molecules caused by intermolecular attractions.

Water droplets form spheres because surface tension tries to minimize the surface area. A sphere has the smallest surface area for a given volume, hence it's the most energy-efficient shape.

As temperature increases, the kinetic energy of molecules increases, reducing the intermolecular attraction. This causes surface tension to decrease with rising temperature.

Water has strong hydrogen bonds between its molecules, leading to high intermolecular forces. Alcohol and acetone have weaker forces, hence lower surface tension.

Viscosity is the resistance of a liquid to flow. It arises due to friction between adjacent layers of the liquid, where outer slower layers resist the movement of faster inner ones.

With increasing temperature, molecular motion becomes faster, weakening intermolecular attractions. This results in lower viscosity, making the liquid flow more easily.

Honey has stronger intermolecular forces and a more complex molecular structure compared to water. This leads to greater resistance to flow, resulting in higher viscosity.

Glycerine can form hydrogen bonds due to its hydroxyl groups, whereas hexane is non-polar and lacks such interactions. Stronger intermolecular forces in glycerine cause higher viscosity.

Evaporation is the process where high-energy molecules escape from a liquid's surface. This leaves behind low-energy molecules, lowering the liquid's temperature and causing a cooling effect.

Earthen pots are porous, allowing water to slowly evaporate through them. The energy for evaporation comes from the water itself, lowering its temperature and keeping it cool.

Vapour pressure is the pressure exerted by vapor molecules in equilibrium with their liquid at a given temperature. It occurs in a closed container where evaporation and condensation rates become equal.

Vapour pressure depends only on temperature and intermolecular forces. As long as equilibrium is reached, the quantity or surface area of liquid doesn't affect vapour pressure.

When surface area increases, more vapour molecules collide with the liquid surface, increasing the rate of condensation to match the evaporation rate more quickly.

Petrol has weaker intermolecular forces and more volatile components compared to kerosene, resulting in a higher vapour pressure.

In a pressure cooker, vapour is trapped and cannot escape, raising internal pressure. This increases the boiling point of water, allowing food to cook faster.

Acetone evaporates faster due to weaker intermolecular forces. Its rapid evaporation absorbs more heat from the skin, causing a greater cooling effect.

As temperature increases, molecular kinetic energy rises, increasing evaporation and vapour pressure until equilibrium is achieved at a higher level.

The amount of heat absorbed by one mole of a solid to convert it into liquid at its melting point under 1 atm pressure. For water: ΔHf = 4.6 kJ/mol.

The amount of heat required to convert one mole of a liquid into vapor at its boiling point under 1 atm pressure. For water: ΔHv = 40.6 kJ/mol.

Because water has stronger hydrogen bonding, which requires more energy to overcome compared to the weaker forces in NH₃ (21.7 kJ/mol) and HCl (15.6 kJ/mol).

Particles gain energy, move farther apart, and change their phase — from solid to liquid during fusion, and from liquid to gas during vaporization.

Because their particles are tightly packed due to strong interparticle forces, leaving no significant space for compression.

They only vibrate about their mean positions; no translational or rotational motion occurs.

Temperature slightly increases vibrational motion, but expansion is negligible due to strong forces keeping the particles fixed.

Due to strong intermolecular or interatomic forces (ionic, covalent, metallic, etc.) that require high energy to break.

Only vibrational kinetic energy, since particles cannot translate or rotate.

Because the particles are already closely packed, and the available space is minimal due to tight arrangements (maximum ~74% space occupied).

Crystalline solids have a regular, definite geometric shape and long-range order of particles, while amorphous solids lack such regularity and typically appear as lumps or powders.

A crystal lattice is the regular, three-dimensional arrangement of ions, atoms, or molecules in space that forms the structure of crystalline solids.

Crystalline solids have

• Definite geometric shape
• Sharp melting points
• Cleavage planes
• Growth through crystallization from a saturated solution
• A characteristic habit (shape) during growth.

Crystalline solids break along specific planes known as cleavage planes, and these planes have a characteristic angle relative to each other.

Crystals grow by the slow evaporation of a solvent or by seeding from a saturated solution, where particles arrange in a specific, orderly fashion.

The habit of a crystal refers to the typical shape it takes when it grows under specific conditions. The shape can change if the conditions, like the presence of impurities, change.

Amorphous solids do not have a regular, repeating pattern of particles. They lack the long-range order characteristic of crystalline solids.

No, amorphous solids melt over a wide range of temperatures rather than at a specific melting point.

Crystalline solids are anisotropic, meaning their properties depend on the direction of measurement. Amorphous solids are isotropic, meaning their properties are the same in all directions.

Yes, amorphous solids like glass can be molded and blown into various shapes due to their lack of a definite structure.

Liquid crystals are substances that are neither fully liquid nor fully solid. Their molecules can move like liquids but have a restricted range of motion like solids.

The molecules in liquid crystals are typically rigid, rod-like, and linear, with their length being four to eight times greater than their diameter.

Liquid crystals flow like liquids, but their motion is limited. They can spin around their long axis but cannot rotate end over end.

In liquid crystals, the molecules tend to align parallel to each other along their long axis, similar to logs stacked in a pile of firewood.

Liquid crystals flow like liquids and show viscosity similar to liquids, as they can move around and flow to some extent.

The main application of liquid crystals is in electro-optic devices, such as Liquid Crystal Displays (LCDs), which modulate light electrically.

The viscosity of liquid crystals is similar to liquids. They exhibit fluid-like behavior, although their motion is more restricted than in typical liquids.

Liquid crystals show optical properties similar to crystals, as they can reflect or transmit light in a specific manner, making them suitable for display technologies.

Liquid crystals can be used in diagnostic devices to detect tumors because tumors often have a different temperature than surrounding tissues. The temperature difference makes the tumor detectable using cholesteric liquid crystals.

Liquid crystal temperature sensors are used to identify faulty connections on circuit boards and in thermometers to detect temperature changes.

Liquid crystals are commonly used in electronic displays, such as those in TVs, computer screens, calculators, and watches.

Liquid crystals are gaining increasing importance in industrial and scientific applications, especially in electro-optic devices, where they are used to control light and display information.

Calculation of Relative Molecular Mass (Mr) of a Gas

General gas equation can be employed to calculate the relative molecular mass of a gas whose P, T, V and mass in grams are known. This is achieved by rearranging equation.

Putting n = m/M in equation.

Where m = mass in g and M = molar mass of the gas.

PV = mRT/M

Now rearranging equation

M = mRT/PV

At molecular level, the molar mass becomes relative molecular mass of a compound, therefore, we can write:

Mr = mRT/PV

Drugs with higher viscosity are susceptible to oxidation due to complex mechanism. They have reduced diffusion and dissolution effect. That's why they are not easily oxidized.