Intermolecular Forces, States of Matter, Diffusion

Intermolecular forces

Cohesive and adhesive forces

Cohesive force refers to the force of attraction between like molecules e.g. water-water molecules while adhesive force refers to the force of attraction between unlike molecules e.g. water-glass molecules. 

Whether a liquid collects as a spherical drop or spreads on the surface where it is placed depends on the relationship between cohesive and adhesive forces. If the adhesive force is greater than the cohesive force, the liquid spreads e.g. oil on water, or water on glass (water is said to wet glass). If the cohesive force is greater, then the liquid collects as a spherical ball e.g. mercury on glass (mercury does not wet glass) or water on arrowroot leaf.

NOTE: Water with a lower cohesive force for example hot, fresh or soapy is better at cleaning as it wets clothes better.

Estimating the diameter (size) of an oil

Imagine an oil drop is a balloon filled with a small cluster of balls, each ball representing an oil molecule. 

When five such drops are placed in a waterbath, the oil spreads out in a thin layer on the surface of water. 

The oil spreads on the surface of water because the adhesive force (force of attraction between non-identical molecules) between the water and oil molecules is greater than the cohesive force (force of attraction between identical molecules)  between the oil molecules. The adhesive force reduces the water surface tension. The oil floats on water because it is less dense than water. Assuming that;

1. Zero loss of oil molecules during the transfer process occurs hence volume (say V) of the oil remains constant 
2. The oil forms a perfect circular patch (say radius r) and 
3. the oil patch is one molecule thick (say d), 

then, the volume of the five drops of oil is equal to the volume of the circular oil patch (essentially a cylinder that is one molecule thick), that is;

                                                                                    (i)

                                                                                        (ii)

The thickness (diameter) of the molecule represents the size of the molecule

Now, the area of the circular patch A=πr². Equation (i) may therefore be expressed as:

                                                                                      (iii)

Surface tension

Surface tension is the tendency of the surface of a liquid to behave like a stretched skin. Factors affecting surface tension of a liquid for example water include: 

1. Temperature; when the temperature rises, the speed of the molecules increases leading to a reduction in cohesive force hence a reduction in surface tension. 
2. Impurities for example oil where the higher adhesive force between water an oil molecules reduce cohesive force between the water molecules.
3. Concentration; For solutions, the surface tension depends on the concentration of the solution. Surface tension increases if the solute is very soluble, for example salt. Salty water (hard water) has a higher surface tension compared to fresh water. If the solute is not very soluble e.g. soap or phenol, surface tension reduces. 

NOTE: Mosquitoes larvae hang on the surface tension of stagnant water. One way of controlling the breeding of mosquitoes is reducing the surface tension of water for example by allowing it to flow freely. The resulting turbulence reduces the surface tension thus disrupting the breeding habitat.

Capillary action

Water rises up an open glass capillary tube dipped in water because the adhesive force between water and glass molecules is greater than the cohesive force between the water molecules (the tube is open so as to maintain equal pressure inside and outside the capillary tube). 

It is also for this reason that the meniscus curves upwards.

Hot water rise in a capillary tube is lower than cold water. This is because heat reduces the  density of water and consequently its surface tension. The lower the surface tension, the lower the capillary rise. A narrower capillary tube on the other hand produces a higher rise of water level compared to a wider tube. This is due to increased adhesive force resulting from increased relative surface area inside the tube hence more water  contact.

The cohesive force between mercury molecules is greater than the adhesive force between mercury and glass molecules. For this reason, mercury level in an open capillary tube dipped in merccury drops. It is also for this reason that the meniscus of mercury in glass curves downwards.

Increasing the temperature of mercury increases the kinetic energy of the molecules thereby reducing the surface tension. The depression of hot mercury down a capillary tube is therefore less than that of cold mercury. If capillary tubes of different diameters are used, the depression of mercury in the wider tube will be lower than in the narrower tube.

It is important to note that the behaviou of mercury in a capillary tube is opposite that of water. Colder water rises higher up the capillary tube while colder mercury has a more reduced rise (more depression). A narrower bore leads to a higher rise of water while a narrower bore leads to a lower rise (more depression) of mercury.

States of matter

Matter refers to anything that has mass and occupies space. Matter exists in three main states:

1. Solids
2. Liquids
3. Gases

Particles/molecules in solids are closely packed and in a regular manner. 

The intermolecular force is very strong and consequently the molecules do not move about but rather vibrate within their fixed positions. As such, solids do not change shape to match the containers they are placed in. A heart-shaped solid for example will still be heart-shaped even when placed in a rectangular box. 

When solids are heated, the kinetic energy of the molecules increases leading to an increase in the speed of the vibratory motion.  The intermolecular force weakens and after a critical temperature is reached, the temperature attains a constant value in spite of heat addition. The molecules then start drifting apart and the solid transforms into a liquid (liquefy or melt). 

Liquid molecules are under a weaker intermolecular force and are therefore further apart and free enough to move about. 

Additionally, liquids do not have definite shapes and usually take the shape of the container they are placed in.

As soon as the liquid is formed, evaporation starts. The rate of evaporation increases as the temperature of the liquid increases. Evaporation only involves the surface molecules and therefore increases with the surface area as well as the nature of the air above it, increasing when windy and when less humid. As the temperature of the liquid increases, the kinetic energy of the molecules increases too. At some point the change in temperature ceases, the liquid starts boiling and the intermolecular force weakens so much so that the liquid starts breaking apart, molecules drift further apart and the liquid tuns into a gas. The temperature at which a liquid starts boiling (the boiling point) increases with;

• Pressure. Water for instance has a higher boiling point at sea level where the atmospheric pressure is high compared to the top of a mountain where the pressure is low. 
• Impurities in the liquid raise the boiling point 

The intermolecular force between gas molecules is very weak and for this reason the intermolecular distances are very large

The molecules are very mobile as well. Gases do not have definite shapes and take up the shape of the containers they are in. Unlike solids and liquids, gases are compressible and therefore gas volume is not fixed.

NOTE 1: Sublimation – This refers to the process where a solid transforms directly to gas, and gas to solid. Materials that undergo sublimation do not have a liquid phase.

NOTE 2: Besides the traditional solids, liquids and gases states of matter, other states of matter have been found to exist. These include

• Plasma: Occurs naturally and consists of highly charged particles with very high kinetic energy. Most stars exist in plasma state.
• Bose-Einstein condensates: This is a manmade state of matter. It is basically a super atom made of atoms at extremely low temperatures that they are virtually immobile.

Kinetic theory of matter and Brownian motion

According to the kinetic theory, matter is made up of particles that either in constant translatory motion (liquids and gases) or vibratory motion (solids). The particles therefore possess kinetic energy. When liquids and solids are heated, the kinetic energy of their molecules increases causing them to drift further apart. This eventually leads to the change of state of the liquids/solids.  

Brownian motion is the random movement of fluid particles on account of kinetic of kinetic theory. To study Brownian motion in liquids, a beaker is filled with water and fine powder (for example pollen grain) sprinkled on the surface of water. 

When the pollen grain particles are observed through a hand lens (to magnify to size of the grain particles), they are observed to move about randomly, colliding with each other and with the walls of the container. The reason for this is that as the water molecules undergo Brownian motion (move randomly), they  collide with the minute pole grain particles making them to also start moving randomly as well. If  the water is heated, the kinetic energy of the water molecules increases leading to increased collisions with the pollen grains. The pollen grains would therefore be observed to move faster.  

Diffusion

Diffusion is the movement of particles from a region of high concentration to a region of low concentration on account of concentration gradient. If for instance a drop of green ink is added to water, the ink gradually spreads out. The water eventually attains a uniform green colour as the diffusing particles take up spaces between its molecules. 

Diffusion is more pronounced in liquids and gases (fluids) for the reason that   their molecules are energetic enough to undergo translational random motion (Brownian motion). There are various factors that affect the rate of diffusion in fluids:

1. Concentration gradient; the higher the concentration difference between the solvent (fluid at low concentration) and the solute (fluid at higher concentration), the higher the rate of diffusion. A more concentrated green ink would change the colour of the entire water faster than a less concentrated one.
2. Temperature; When the temperature of the solvent or the solute increases, the rate of diffusion also increases. An increase in temperature increases the random movement of both the solute and solvents hence the increased rate of diffusion.
3. Mass of the solute particles; Heavy particles move more slowly leading to low rate of diffusion.  
4. Density of the solvent; A denser solvent slows down the rate of diffusion. 

Examples of diffusion in daily life;

• Smell of perfumes
• When a soda is opened, CO₂ diffuses out
• Using teabags – tea diffuses in hot water
• Sugar can dissolve in tea without stirring
• Milk mixes with water without stirring.

Examples

KCSE 2021

(1) Figure 4 shows two capillary tubes X and Y of different diameters dipped in mercury.

Complete the diagram to show the meniscus in Y

The level of mercury in the wider tube is higher than in the narrower tube

(2) In an experiment, a drop of black ink is introduced at the bottom of a container filled with water. It is observed that the water gradually turns black. State the effect on the observation when the experiment is carried out using water at a lower temperature. (1 mark)

If temperature is reduced, the rate of diffusion reduces hence the water turns black more slowly.

KCSE 2020

(1) ) Define cohesive forces. (1 mark)

Cohesive force is the force of attraction between identical (or similar) molecules

(2) Figure 1 shows a capillary tube dipped in mercury in a beaker.

State with a reason the observation that would be made on the level of mercury in the capillary tube if the temperature of mercury is increased. (2 marks) 

Heat reduces the cohesive force. The depression of mercury in the capillary tube would therefore reduce (mercury level rises). 

(3) State the meaning of the term matter (1 mark)

Matter refers to anything that has mass and occupies space

(4) It’s observed that when a liquid is heated its volume increases. Explain this observation using the kinetic theory of matter (3marks)

According to the kinetic theory, matter is made up of particles that either in constant translatory motion (liquids and gases) or vibratory motion (solids). The particles therefore possess kinetic energy. For a liquid, an increase in temperature leads to an increase in the kinetic energy of the molecules. The molecules consequently drift further apart leading to an increase in volume. 

(5) Figure 15 shows a setup used to study Brownian motion in liquids.

(a) State the function of the hand lens. (1 mark)

To magnify the small grains that make up pollen grain particles.

(b)  State what is observed on the pollen grains. (1 mark) 

They move about randomly, colliding with each other in the process 

(c) Explain the observation made in (ii).   (2 marks)

Liquid molecules undergo Brownian motion which means that they move about randomly colliding with each other. As the water molecules in Figure 15 move about, they randomly collide with the minute pole grain particles making them to also start moving randomly.

(d) State and explain what would be observed on the pollen grains if the water is heated. (3 marks)

In accordance with kinetic theory, an increase in temperature leads to an increase in kinetic energy of the molecules hence increased motion. The pollen grain would therefore be observed to move faster. 

KCSE 2019

(1)  State two ways of reducing surface tension of a liquid. (2 marks

• heating (increasing the temperature)
• adding detergent

KCSE 2018

(1) Describe how the knowledge of the oil drop experiment may be used to estimate the area of oil spillage from a ship in the sea assuming the surface of the water is not distorted. (3 marks)

• Obtain a sample of the same oil and water.
• Measure the volume of the oil sample (say V_s)
• Pour it on the surface of the water sample and allow it to spread to a thin layer (assumed to be one molecule thick).
• Determine the area of the oil layer (say A_s)
• Determine the size (d) of the oil molecule using the formula

• Obtain a sample of the same oil and water.
• Measure the volume of the oil sample (say V_s)
• Pour it on the surface of the water sample and allow it to spread to a thin layer (assumed to be one molecule thick).
• Determine the area of the oil layer (say A_s)
• Determine the size (d) of the oil molecule using the formula

(2) It is observed that a drop of milk carefully put into a cup of water tums the water white after some time. State the reason for this observation. (1 mark).

Milk turns water white (without stirring) due to diffusion. The milk particles move and occupy the spaces between the water particles.

KCSE 2017

(1) In order to determine the size of an oil molecule, a student performed an experiment using five oil drops to make a circular patch of the oil on the surface of water in a waterbath. State two assumptions made by the student during the calculations.                               (2 marks)

• zero loss of oil molecules during the transfer process, hence volume (say V) of the oil remains constant 
• the oil forms a perfect circular patch (say radius r) and 
• the oil patch is one molecule thick

(2) It is observed that when 20 cm³ of alcohol is mixed with 20 cm³ of water, the volume of the mixture is 39 cm³. State a reason why the volume of the mixture is not 40 cm³. (1 mark)

Water has larger molecules hence larger inter-molecular spacing compared to alcohol. After mixing, some of the alcohol molecules occupy the spaces between the water molecules. The volume of the mixture is thus less than the total volume of the water and the alcohol before mixing.

(3)  In an experiment to determine the size of an oil molecule, oil is placed on the surface of water after sprinkling lycopodium powder on it.

(a) State two reasons why oil is used. (2 marks)

• Oil being less dense floats on water (no mixing)
• Oil easily spreads on water due to the high adhesive force and therefore capable of forming a very thin layer (mono-layer) of oil.

(b) State the function of the lycopodium powder. (1 mark)

• Allows for a clear view of the oil boundary
• Prevents oil from mixing with water

(c) State any two assumptions that are made in this experiment. (2 marks)

• Patch is monolayer (one molecule in diameter)
• The oil drop has a spherical shape (for ease of calculating its volume)
• The oil patch is perfectly circular
• No loss of oil during transfer

(d) Explain why the oil spreads on the surface of water. (2 marks)

Oil is less dense hence does not sink. The greater adhesive force reduces the surface tension of water.

(2) The following data was obtained from an experiment to determine the size of a palm oil molecule.

— Volume of 100 drops of palm oil = 15.0mm³ 

— Area of a patch from one drop of oil = 8.0 X l0⁴mm²

Determine the size of a palm oil molecule. (3 marks)

Solution

The oil patch is cylindrical, one molecule thick (d) hence;

But;

Hence;

Practice Questions

KCSE 2016

(1) When a drop of an organic acid of known volume is dropped on the surface of water in a large trough, it spreads to form a large circular patch, State one assumption made when the size of the molecule of the acid is estimated by determining the area of the patch. (I mark)

(2)  Figure 2(a) and 2(b) show capillary tubes inserted in water and mercury respectively. 

It is observed that in water the meniscus in the capillary tube is higher than the meniscus in the beaker, while in mercury the meniscus in the capillary tube is lower than the meniscus in the beaker. Explain these observations. (3 marks)

(3) State why it is easier to separate water into drops than to separate a solid into smaller pieces.

KCSE 2015

(1) Figure 2 (a) shows the initial reading of a burette used to measure the volume of oil. After 50 drops of the oil were run out, the final reading was as shown in Figure 2 (b) 

Determine the volume of one drop of oil. (2 marks)

(2) Figure 3 shows the arrangement of molecules in the three states of matter

(a) Name the process represented by the arrow. (1 mark)

(b) State the reason for the arrangement of molecules in state 3. (l mark)

KCSE 2014

(1) In a smoke cell experiment to demonstrate Brownian motion, smoke particles are seen moving randomly. State the cause of the randomness. (1 mark)

(2) Figure 6 (a) and 6(b) show capillary tubes inserted in water and mercury respectively.

It is observed that in water the meniscus in the capillary tube is higher than the meniscus in the beaker, while in mercury the meniscus in the capillary tube is lower than the meniscus in the beaker. Explain these observations. (2 marks)

(3) A drop of blue ink is introduced at the bottom of a beaker containing water. It is observed that after some time, all the water in the beaker turns blue. Name the process that takes place. (1 mark)

KCSE 2012

(1) A bottle containing a smelling gas is opened at the front of a classroom. State the reason why the gas is detected throughout the room. (1 mark)

(2) State six environmental hazards that may occur when oil spills over a large surface area of the sea. (2 marks)

KCSE 2011

(1) State the reason why it is easier to separate water into drops than to separate a solid into smaller pieces. (1 mark

KCSE 2010

(1)  When a drop of oleic acid of known volume is dropped on the surface of water in a large trough, it spreads out to form a large circular patch. State one assumption made when the size of the molecule of oleic acid is estimated by determining the area of the patch.     (1 mark)

Dr. Margaret W. Chege