April 20, 2023

Unit 7: Kinetic Model of Matter

Discover Physics for GCE ‘O’ Level Science

April 20, 2023

7.1 The States of Matter

Learning OutcomesIn this section, you’ll be able to:• Compare the properties of solids, liquids and

gases• Describe the molecular structure of solids,

liquids and gases• Describe Brownian motion• Describe the effects of temperature on

Brownian motion

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7.1 The States of Matter

• Matter exists as a solid, liquid and gas.• These are referred to as the three states of

matter.• Similarly, substances can exist in three states.• The properties of a substance differ between

each states.

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7.1 The States of Matter

Table 7.1 Properties of solids, liquids and gases.

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7.1 The States of Matter

The Kinetic Model of Matter

• All matters are made up of tiny particles called atoms or molecules (group of atoms joined together)

• These particles are in constant motion.• The continuous motion of these particles is

known as kinetic model of matter.• This theory can explain the difference in

properties between each states.

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7.1 The States of MatterTable 7.2 Properties of solids, liquids and gases

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7.1 The States of MatterThe kinetic model of matter• The relative strengths of the intermolecular forces between the

particles differ between the different states of matter.

• The relative distances between the particles also differ between the different states of matter.

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7.1 The States of Matter

Evidence of molecular motion (Experiment 7.1)

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7.1 The States of Matter

Evidence of molecular motion

• From Experiment 7.1 in the textbook, we see that smoke particles in air move in a random and irregular way.

• This is referred to as Brownian motion.• Brownian motion occurs only in fluids where

the particles have the ability to flow or move freely.

• Fluids include liquids and gases.

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7.1 The States of Matter

Effects of temperature on molecular motion

• At higher temperatures, the motion of smoke particles will become more vigorous and agitated.

• This is because the air molecules are bombarding the smoke particles more vigorously and frequently.

• The air molecules have greater speeds at higher temperatures.

• The thermal energy is converted to kinetic energy of air molecules, causing it to move faster.

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7.1 The States of MatterKey Ideas1. All substances exist in three states of matter:

solid, liquid and gas.2. The kinetic model of matter describes matter as

being made up of tiny particles (atoms or molecules) in continuous motion.

3. Brownian motion is the random or irregular motion of smoke particles in air or pollen grains in water. When temperature increases, the smoke particles or pollen grains are observed to move faster and more vigorously.

4. Brownian motion is caused by the bombardment of air molecules of smoke particles in air or water molecules on pollen grains in water.

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7.1 The States of MatterTest Yourself1. Using the kinetic model of matter, explain

why the density of a gas is much lower than that of a solid.

Answer:• In solids, the particles (or atoms) are closely

packed.• In gases, the particles are far apart.• Since density is the mass per unit volume,

then solids must have a much higher density since there are many more particles packed closely together per unit volume compared to gases.

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7.1 The States of MatterTest Yourself2. What is Brownian motion in air? What causes

it?

Answer:Brownian motion refers to the random, irregular and jerky motion of smoke particles in air.The smoke particles are continually bombarded unevenly on different sides by air molecules. This results in the irregular movement of the smoke particles.

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7.1 The States of MatterTest Yourself3. (a) As temperature is increased, how would

the movement of smoke particles in Brownian motion change?

3. (b) What would you expect to see if the smoke particles are much larger?

Answer:3. (a) As temperature increased, the motion of

the smoke particles become more vigorous and agitated.

3. (b) Larger smoke particles tend to have less irregular and less jerky motion.

April 20, 2023