CHAPTER FOUR: PARTICULATE NATURE OF MATTER – PHYSICS S1-S2 NOTES

4.1  Matter

Definition:  Matter   is any thing that occupies space and has mass.

States (Phases) of Matter

There are three states of matter namely:

1. Solid, e.g.  stone, wood etc.
2. Liquid e.g.  water, paraffin etc.
3. Gas e.g.  oxygen, nitrogen etc.

Each of these states is made up of so many tiny particles called atoms and molecules. The arrangement of these particles and the magnitude of the forces holding them makes one state different from other states.

 Note:  Examples of things which are not matter are:- Light, Sound and Heat.

4.11  Differences in the three states of matter

(a)  Solids  

1. The molecules are closely packed and are arranged in a regular pattern called lattice.
2. The forces holding the molecules are strong as such
3. The molecules are not free to move.
4. The molecules vibrate about a fixed position.
5. Solids have a definite shape and volume.
6. There is very little diffusion in solids. E.g. Naphthalene.
7. Due to the close packing of the particles, solids cannot be compressed.

(b)  Liquids

1. The particles in liquid are fairly close to each other and are in irregular pattern.
2. The forces holding the molecules are weak.
3. The molecules move randomly throughout the liquid.
4. Liquids have definite volume but no definite shape. They take the shape of the container in which they are placed and then acquire a definite volume.
5. Liquids have slow diffusion.
6. They cannot be compressed.

(c)  Gases

1. The particles in a gases are far apart from each other.
2. The forces holding the molecules are very weak.
3. The particles move randomly and at comparatively high velocities.
4. Gases have no definite shape and no definite volume. They fill the whole container they are placed in.
5. There is high rate of diffusion in gases.
6. Gases are quite easy to compress. This is because there are large spaces between the particles.

The diagrams showing a model representation of the three states of matter

Figure 4.0

Note:  The diagrams in figure 4.0 show that the molecules are of the same size for all the three states. However, is not the case. Even for the same state, particle size is different for different substances.

4.2  Kinetic Theory of Matter

 The kinetic theory of matter states that:

1. Matter is made up small particles called atoms or molecules.
2. The molecules are in a constant random motion.
3. The speed of movement of the particles increases when temperature increases.
4. The particles (molecules) in gases move faster than those of liquids and those of liquids move faster than those of solids.

4.21  Assumptions of Kinetic Theory of Gases

1.  Molecules of gases move in straight line with a very great speed or velocity until they collide with each other or with the walls of the container in they are placed. Pressure of a gas is due to collisions of the gas molecules with the walls of the container. This pressure increases as the temperature increases.

2.  The volume of the gas molecules is negligible compared to the total volume of the container; so the molecules can be taken to be points of negligible volume.

3.  The forces of attraction between the gas molecules are negligible.

4.  The average kinetic energy of the molecules is a measure of the temperature of the gas.

5.  The collisions of the molecules are perfectly elastic. (i.e. When molecules collide, there is no loss in kinetic energy).

4.22  The proof of Kinetic Theory of Matter

The kinetic theory of matter can be proved by:

1. Brownian Motion and
2. Diffusion.

Experiment  4.0  To demonstrate kinetic theory of matter using Brownian motion.

Apparatus/Requirements

A smoke cell, microscope, source of light, glass rod or convex lens, smouldering cigarette or paper.

Procedure

• Place the source of light at a small distance such that the rays are incident on to the window of the glass cell.
• Place the glass rod or converging lens between the source of light and the smoke cell.
• Adjust the object distance such that the light rays are in focus in the smoke cell thus strongly illuminating it.
• Fill the smoke cell with some smoke from a smouldering paper.
• View the smoke particles from above using a microscope as show in the figure below.

Observation

The white specks of the smoke particles are seen moving with constant random motion.

Explanation

The random motion of the smoke particles is due to the bombardment with the air molecules which are in a constant random motion. The air molecules can not be seen, only their effects can be seen.

NB:  If the smoke cell is replaced by a glass container containing water with some pollen grains suspended in it, the pollen grains will also be seen moving with a constant random motion.

 Effect of heat on Brownian motion

When the temperature of the smoke cell is increased, the smoke particles are seen moving faster.

Explanation

Due to the increase in temperature, the molecules acquire kinetic energy. The kinetic energy is directly proportional to the velocity of the particles (i.e. from K.E = ½) hence increased velocity.

4.23  Effect of Heat on Matter

(a)  Heating Solid

When a solid is heated, its particles acquire kinetic energy and vibrate more violently. The particles continue to vibrate until a point is reached when the vibration overcome the binding force (forces of attraction between the particles). The crystalline structure corrupts and the particles become mobile. At this point the solid melts or fuses (i.e. changes to liquid at a constant temperature called melting point. The process is called melting or fusion.

On further heating of the liquid, the molecules of the liquid acquire increased kinetic energy. The kinetic energy continues to increase until they overcome the forces of attraction between the molecules. At this point the liquid boils (i.e. changes to vapour at a constant temperature called the boiling point. And the process is called boiling or evaporation.

(b)  Effect of Cooling

When a gas or vapour at a higher temperature is cooled, the kinetic energy of the particles also reduces. Since the kinetic energy of the particles is directly proportional to their velocity, the average velocity (speed) of the particles gradually decreases until the forces of attraction between the particles build up and the particles come closer i.e. they condense. At this point, the gas changes to liquid at a constant temperature. The process is called condensation.

Cooling the liquid further causes more loss in kinetic energy until eventually the particles settle to form a solid at a constant temperature called freezing point. The process involved is called freezing or solidifying.

For example when ice is heated, it changes to liquid water. On heating the water, it changes to vapour. If the vapour is cooled, it condenses to liquid. On further cooling of the liquid it changes to ice.

4.3  Properties of Matter

The properties of matter are as a result of the behaviour of its molecules. The properties resulting from the behaviour of molecules include:

1. Molecular forces,
2. Diffusion,
3. Capillarity,
4. Surface tension and
5. Elasticity.

4.31  Molecular forces

A molecular force refers to the attractive force that exists between molecules. The molecules may be of the same substance or of different substances.

Types of molecular forces

There are two types of molecular forces namely:

• Cohesion and
• Adhesion.

(i)  Cohesion

Cohesion is the force of attraction between the molecules of the same kind.

E.g. The force of attraction between water molecules.

(ii)  Adhesion

 Adhesion is the force of attraction between molecules of different substances.

 E.g. The force of attraction between water molecules and glass molecules.

The magnitude of cohesion and adhesion determine:

• The shape of liquid meniscus when in contact with other substances.
• Ability to wet substances and
• The rise or fall in a capillary tube.

For example:

(i)  Water in a glass tube wets glass. This is because the adhesion is greater than cohesion. The result is that the meniscus of water curves upwards as shown in figure 4.3

(ii)  Mercury in a glass tube does not wet glass because cohesion is greater than adhesion. The result is that the meniscus of mercury curves downward. See figure 4.3 (ii)

Diagrams showing the meniscus of water and mercury

4.32  Diffusion

Diffusion is the spreading of molecules of one substance in to the molecules of another substance. Or

Diffusion is the movement of molecules from a region of high concentration to a region of low concentration.

Factors which determine the rate of diffusion depends

The rate of diffusion depends on the following factors:

1. Size of the diffusing molecules,
2. Molecular weight
3. Temperature
4. Pressure
5. Size of the pore (in the porous material) across which the molecules diffuse

The table below shows the factors which determine the rate of diffusion and there explanations.

Figure 4.1

Diffusion is extremely slow in solids, average in liquids and very fast in gases. For example a small amount of perfume or ammonium gas placed at one corner of a room spreads quickly and fills the whole room.

Experiment  4.2  To show diffusion in liquid e.g. Water

Apparatus/Requirements

A beaker, Crystal of potassium permanganate, water and a glass tube.

Procedure:  – Fill a beaker with clean water.

– Drop a fairy large crystal of potassium permanganate in the middle of

the bottom of beaker with a help of a glass tube.

– Leave the set up to stand for some time.

Observation

The crystal dissolved and spread through out the water forming a purple solution.

Explanation

The molecules of the potassium permanganate moved slowly moved in to the molecules of water to give the purple colour. We say that the molecules of potassium permanganate diffused into the molecules of water.

Experiment  4.3  To show diffusion in gases

Apparatus/Requirements

Porous pot, manometer, water, a delivery tube, hydrogen gas and air.

Procedure

• Arrange the apparatus as shown in the diagram below.

• Supply hydrogen gas into the space surrounding the porous pot and observe the water levels in the arms of the manometer.

Observation:  The level of the liquid in the left arm of the manometer falls, while that in the right arm rises.

Explanation:

The hydrogen molecules increase the pressure inside the porous pot. And the pressure acts on the water surface in the limb of the manometer thus pushing the water level downward.

When the hydrogen supply is stopped, the situation is reversed. This is because the hydrogen that diffused into the porous pot diffuses out faster than the air outside porous pot diffuses into the pot.

NB:  Brownian motion and diffusion, in liquid or in gases is a strong evidence of kinetic theory of matter. That is, it shows that matter is made up of small particles and that the particles are in a constant random motion.

4.33  Capillarity

Capillarity is the rise or fall of liquid in a capillary tube.

Capillary rise is due to adhesion being greater than cohesion. Examples of liquids that have capillary rise when a capillary tube is dipped into them are: Water, paraffin and diesel. While capillary fall is due to cohesion being greater than adhesion. Example of liquid that has a capillary fall is mercury.

The diagrams in figure 4.6 show capillary rise and fall in glass tubes of different diameters

Uses of Capillarity

It helps:  i)  Fuel e.g paraffin to rise up in wicks of stoves and lamps.

• Water to move up tree trunks to the leaves.
• Blotting paper to absorb liquids.

4.34  Surface tension

Surface tension is the force on a liquid surface that makes the liquid surface to behave as if it is covered with thin elastic membrane.

Experiment  4.4  To show surface tension in a liquid e.g. water

Apparatus/Requirements:  A glass trough, bloating paper, a needle or pin and water.

Procedure

• Fill a beaker with clean water.
• Place a filter paper or bloating paper on the surface in the trough.
• Drop a needle on the bloating paper.
• Leave the apparatus to stand for some time as shown in figure 4.7 below.

Figure 4.7

Observation

The filter paper absorbs water and sinks to the bottom of the trough.

The needle is seen floating on the water surface.

Conclusion

The surface of liquid water behaves as if it is covered with a thin elastic skin or membrane.

How to reduce or get rid of surface tension

Surface tension in liquids is reduced or got rid of by adding the following.

1. Soap solution,
2. Detergent solution,
3. Oil.

These solutions get rid of surface tension by dissolving process.

Experiment  4.5  To show the effect of detergent solution on surface tension

Repeat the experiment 4.4 above.

When the needle is floating, carefully add a detergent solution with a help of syringe to the water and observe.

Observation:   The needle quickly sinks to the bottom.

Conclusion:   Surface tension can be reduced by detergent solution.

4.35  Elasticity  

Elasticity is a property of a material that enables the material to return to its original size and shape after deformation.

Materials that have elasticity (e.g. rubber, foam, metal wires) are said to be elastic. While those which do not have this property (e.g. wood, glass, paper) are said to be inelastic.

Self-Check 4.0

SECTION A

1.  When a crystal of potassium permanganate is carefully placed at the bottom of a beaker containing water, it spreads uniformly in the water after some days. This is due to:

A. diffusion  B. capillarity

C. surface tension    D. Brownian motion

2.  Soap is used to wash clothes because it

A. increases capillarity in the clothes

B. reduces capillarity in the clothes

C. increases surface tension allowing water to penetrate the dirt easily.

D. reduces surface tension allowing water to penetrate the dirt easily.

3.  Brownian motion experiment shows that molecules of gasses are

A. Stationary  B. in motion in one direction only

C. In constant random motion D. More closely packed than molecules in liquid.

4.  When mercury is spilt on glass it forms small spherical droplets because its

A. Density is high.   B. Surface tension makes its surface elastic.

C. Molecules are small D. Cohesive force is greater than adhesive force with the glass.

5.  A needle may float on a clean water but sinks when some detergent is added to water because the detergent

A. reduces the density of water.  

B. increases adhesive force between the needle and water molecules.  

C. lowers the surface tension of water.  

D. makes water surface slippery.

6.  The particles in a solid at room temperature are

A. Close together and vibrating.  B. Close together and stationary.  

C. Far apart and moving at random.  D. Close together and moving at random.

7.  Mercury forms spherical drops when spilt on a wooden bench because it

A. is very viscous B. has a high density

C. has a high cohesive force D. has a low surface tension

8.  Water wets glass because

A. adhesive forces between water and glass molecules are greater than cohesive forces

B. adhesive forces between water and glass molecules are more than cohesive forces

C. surface tension forces between water and glass molecules are more than adhesive forces.

D. Surface tension forces are less than cohesive forces.

9.  The forces which hold the molecules in water drop together are called…

A. Surface tension  B. Adhesive

C. Cohesive D. Electrostatic forces

10.  When water spreads on a glass plate, the forces between it’s molecules and glass molecules are due to

A. Surface tension  B. Adhesion  C. Cohesion  D. Viscosity.

11.  In a Brownian motion experiment, the

A. smoke particles are seen moving about with uniform velocity.

B. motion observed is caused by the air molecules colliding with the smoke particles.

C. Size of particles are found to increase the motion.

D. smoke cell has a vacuum within it.

12.  When smoke is introduced in a smoke cell and observed under a microscope, it is observed as particles moving at random. This is mainly because the particles

A. are hot B. collide with one another

C. collide with air molecules D. collide with the walls of the smoke cell

13.  Which of the following statements is incorrect when a tin containing air tightly sealed is heated?

A. the average speed of molecules increases

B. the molecules of air hit the walls of the tin harder

C. The molecules of the air strike the walls of the tin less often.

D. The pressure inside the tin increases.

14.  When a room is filled with smoke, the smoke tends to concentrate

A. around the walls  B. close to the walls

C. close to the roof  D. midway between the roof and the floor

15.  Capillary rise in a tube dipped in water is due to

1. Surface tension   B. adhesive force being greater than cohesive force

C. high vapour pressure D. Atmospheric pressure acting on the surface of the water

16.  Which of the following statements about states of matter is/are true?

(i)  a liquid has a definite volume but no a definite shape.

(ii)  a vapour has no definite volume and no definite shape.

(iii)  a solid has a definite volume and shape.

(iv)  a gas has a definite volume and shape.

A. (i) and (iii) only  B. (i) only  C. (i), (ii) and (iii) only  D. (iii) only

17.   When viewing Brownian motion in a smoke cell the observer sees:

1. Air molecules moving in a random motion  
2. air molecules vibrating regularly

C. air molecules colliding with each other

D. Smoke particles in a random motion.

18.  Which of the below is a matter?

A. Heat B. Sound C. Air D. Light

19.  Which of the following statements below is wrong?

A. Matter is made up of atoms.  B. Solid, liquid and gas are states of matter.

C. Solids and liquids have definite shape and volume. D. Gas molecules move freely.

20.  The diagrams in figure show two capillary tubes standing in a trough of mercury and two capillary tubes standing in a trough of water. The correct order of arrangement of the tubes in order of increasing height of the liquid column is;

A. (i), (ii), (iv),(iii) B. (ii), (i), (iv), (iii)  C. (ii), (iv), (i), (iii)  D. (iii), (iv), (i), (ii)

SECTION B

21.  Describe the relationship between molecules of liquids, gases and solids in terms of:

(a)  the arrangement of the molecules through out the bulk of the material,

(b)  the separation of the molecules,

(c)  the motion of the molecules and

(d)  The forces of attraction between the molecules.

22.  (a)  (i)  What is meant by the term diffusion?

(ii)  State factors on which diffusion depends.

(b)  Describe an experiment to show diffusion in liquids.

(c) A porous pot containing air is connected to a water manometer. Explain what happens if hydrogen is let in the space surrounding the as shown in the figure.

(d)  (i)  Describe a simple experiment to show surface tension in water.

(ii)  State two factors, which affect surface tension.

23.  A pin is placed on a bloating paper, which is on the surface of water as shown in figure 4.10 below

(a)  Explain what happens after some time.

 (b) Explain what happens when some soap solution is carefully added to the water.

24.  Draw a well labelled diagram you would use to describe Brownian motion.

1. How is the motion of the smoke particles best described?
2. What accounts for the motion of the smoke particles?
3. The motion is viewed using bigger smoke particles.

   What difference in the motion would this lead to.

   Give reason for the difference.

25.  The diagram in figure 4.11 shows an arrangement for observing Brownian motion.

(a)  Explain:

(i)  The observation made.  

(ii)  What will be observed when the glass cell temperature is raised.

(b)  State one factor which determines the rate of diffusion of a gas.

26.  (a)  Distinguish between cohesion and adhesion.

(b)  Sketch diagrams to show the level of liquid in a capillary tube that is immersed in a liquid which has greater;

i)  Cohesion than adhesion

ii)  Adhesion than cohesion

27.  (a)  Define surface tension.

(b)  Describe a simple experiment to show the existence of surface tension in water.

(c)  Explain the following observations as fully as you can.

(i)  A small needle can be floated on the surface of water, but if a drop of detergent is added to the water the needle sinks.

(ii)  Damp courses are used in modern houses.

(iii)  Gases can easily be compressed but liquids cannot.

(iv)  Diffusion occurs more easily in a gas than in a liquid.

28.  (a)  State the kinetic theory of matter.

(b)  Describe experiments, in each case, to show:

(i)  Diffusion in liquids

(ii)  Diffusion in gases.

(c)  Use kinetic theory to explain the result of your experiment demonstrating the diffusion in gases.

4.4  SIZE OF A MOLECULE

Molecules are so small that we cannot measure their sizes accurately. The approximate size of a molecule can only be estimated through an experiment.

Experiment 4.6  To estimate the size of a molecule

Apparatus/Required

Cooking oil (solute), solvent, clean glass trough, water, burette and lycopodium powder.

Procedure

1. Dissolve a small volume, x cm³, of vegetable oil (solute) in a larger volume, v cm³, of petroleum ether (solvent) to form a total volume, V
   cm³, of oil-petroleum ether solution.
2. Fill a clean (grease free) trough with water.
3. Sprinkle lycopodium powder uniformly to cover the surface of the water in the trough.
4. Drop a small volume, y cm³, of the oil-petroleum ether solution from a burette on to the water surface covered with the lycopodium powder. The oil repels the powder leaving a clear oil film.
5. Measure and record the diameter, d, of the circular oil film.

Calculations

(a)  To find the volume of the oil film

 Volume of oil (solute) dissolved  =  x cm³

 Volume of petroleum ether used  =  v cm³

 Vol. of solution dropped on water  =  y cm³

 Total volume of solution, V, =  (x + y)  cm³

If the total volume, (x + y), cm³ of oil-petroleum ether solution contains x cm³ _{of oil}, then the volume, V, of oil contained in y cm³  = x y  cm³

Note that from the above expression, we can write the form for calculating the volume of oil film as:

Volume of the oil film, V  =

(b)  To find the thickness, t, of the oil film (i.e. size of a molecule)

The thickness, t, is found by using any of the following expressions for finding volume of a cylinder.

(i)  V = At from  Volume = Base area (A) x thickness (t)

(ii)  V = pr²t from  A = pr²

(iii)  V = ¼pd²t    from  2r = d, r = ,   \ r² = = ,

\V = ¼pd²t

Substituting for V in the above formulae, we have the following expressions.

V in (i) At  =

x y  
\
t  = x y  cm

V in (ii)  pr²t  =
x y  
\
t  = x y cm

V in (iii)  ¼pd²t  = x y  \
t  = x y cm

Assumptions made in the estimation of the size of a molecule

1. The oil film is assumed to be one molecule thick.
2. Each molecule is assumed to be a perfect sphere.
3. The spaces between the molecules in the oil film are assumed to be negligible.
4. The oil film is assumed to form a complete circle.

Example 1

1 cm³ of oleic acid was dissolved in 999 cm³ of alcohol to form 1000 cm³ of solution. A 1 cm³ drop of the solution was put on a water surface sprinkled with lycopodium powder. The alcohol dissolved in the water leaving the acid to spread forming a patch of diameter of 28 cm³.

(a)  Calculate the volume of oleic acid in the 1 cm³ drop of the solution.

(b)  Estimate the size of oleic acid molecule.

(c)  Why was lycopodium powder used?

Solution

Volume of oleic acid (solute) dissolved  = 1 cm³

Volume of alcohol (solvent) = 999 cm³

Volume of solution dropped on water  = 1 cm³

Total volume of solution, of solution = (1
+ 999) cm³

= 1000 cm³

If the total volume, 1000
cm³ of oleic-alcohol solution contains 1 cm³
of oleic acid, then the volume, 1 cm³
of oleic acid , of oil contained in 1 cm³

= x 1  cm³  = = 0.001 cm³

The volume of the oleic acid in 1 cm³ of the solution = 0.001 cm³.

(b)  Data:  d = 28 cm, volume, V, of oleic acid = 0.001 cm³, p = , t = ?

 Using the formula V  = ¼pd²t  

^0.001  =

x
x 28² x t

t  =

=

\
t  = 1.62 x 10^-6 cm or 1.62 x 10^-8 m

Alternatively you can first find the radius and then substitute the value in the formula

V = pr²t  From d = 28 cm, r = ½d = x 28  = 14 cm

 V  = pr²t

 0.001  = x 14 x 14 x t = = 1.62 x 10^-6 cm or 1.62 x 10^-8 m

1. To give clear circular patch of the oleic film.
   

Self-Check Question  4.1

1.  An oil drop of volume 10^-3 cm³ forms a patch of an area of 0.785 cm² on water surface during an experiment to estimate size of a molecule.
If the film is 1 molecule thick, what is the size of the molecule?

A. 4.06×10^-4 cm  B. 7.85X10^-4cm  C. 9.53×10^-4cm  D. 1.27X10^-3cm

2.  The diagram in figure 4 shows an arrangement for determining the size of an oil molecule.

(a)  State two assumptions made in the experiment

(b)  If 1.8 x 10^-4 cm³ of oil spreads to form a patch of area 150 cm².

 Calculate the thickness

3.  A solution is made by dissolving 1 cm³ of cooking oil in 199 cm³ of the methanol. When 0.004 cm³ of the solution is dropped on the surface of water, an oil film of diameter 12 cm is obtained.

(i)  Calculate the volume of the cooking oil in the film.

(ii)  Estimate the thickness of a molecule of the cooking oil

(iii)  State any assumption made in (b) (i).

4.  In an experiment to estimate the size of a molecule of olive oil, 0.12 mm³ of the solution was dropped on a clean water surface in a trough. The oil spreads to form a circular patch of an area of 1.0 x 10⁴ mm². Estimate the size of a molecule of olive oil.

5.  Suppose an oil drop on water surface sprinkled with lycopodium powder has a volume of 0.10 mm³ and forms a film of an approximate radius 10 cm, calculate the thickness of the oil film.