# BMAT Physics Notes: Matter

Advice & Insight From BMAT Specialists

## States of Matter

On a microscopic level we view matter to be made of atoms/molecules that have certain properties.
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SOLID
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In the solid state atoms areÂ vibratingÂ aboutÂ fixedÂ positions.
The atoms areÂ closeÂ together and can be arranged in aÂ regularÂ pattern for many hundreds of layers (e.g. in a metal).
There areÂ attractiveÂ andÂ repulsiveÂ forces acting on the atoms which cause them toÂ vibrate.
The atoms have bothÂ Â kinetic energyÂ ( because they areÂ moving) andÂ potential energyÂ (because they experienceÂ forces).
Most solids are difficult toÂ compressÂ because of the strong repulsive forces between them.
They also do notÂ expandÂ a lot when heated.
HeatÂ is transferred through a solid byÂ conduction.
Solids have a fixedÂ shape.

LIQUID
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In the liquid state the atoms doÂ notÂ vibrate and areÂ notÂ arranged in a regular pattern.
They are able to move around each other and they areÂ slightlyÂ further apart, on average, than atoms of a solid.
The atoms move inÂ straight lines,Â this is calledÂ translationalÂ motion.
The atoms are continuallyÂ makingÂ andÂ breakingÂ bondsÂ with neighbouring atoms.
Liquids areÂ difficultÂ to compress because of theÂ repulsiveÂ forces.
Most liquids areÂ poorÂ conductors.Â HeatÂ is transferred through a liquid mainly byÂ convection.
Liquids doÂ notÂ have a fixed shape but take the shape of theÂ containerÂ in which they are placed.
The atoms have bothÂ Â kinetic energyÂ andÂ potential energy.

GASES

In a gas the atoms areÂ muchÂ furtherÂ apart than those of a solid or liquid.The atoms/molecules move inÂ straight linesÂ and move withÂ different speeds. They doÂ notÂ experience anyÂ forcesÂ (except during collisions).
They haveÂ kinetic energy only.
Gases areÂ easyÂ to compress or expand because the atoms do not experience forces.
They expand aÂ lotÂ when heated.
Gases areÂ poorÂ conductors of heat.
Heat is transferred through a gas byÂ convection.

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## â€‹Change of State

â€‹Suppose a lump of metal is heated steadily. The temperature rises. Eventually, if it is heated long enough the metal reaches itsÂ melting point.Â If further heating occurs the metal will begin to melt andÂ change stateÂ i.e. change from aÂ solidÂ to aÂ liquid.
Heat is required to change the metal from a solid to a liquid but whilst it is melting theÂ temperatureÂ remainsÂ constant.
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The amount of heat required to changeÂ anyÂ mass of a solid into a liquid at its melting point is calledÂ latent heat of fusion.

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The amount of heat required to changeÂ 1 kgÂ Â of a solid into a liquid at its melting point is calledÂ specificÂ latent heat of fusion.

##### Examples

â€‹To change 2kg of ice into water would require 2 x 334 kJ,
To change 3kg of ice into water would require 3 x 334 kJ, and so on.
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In general, theÂ thermal energy (E)Â required to changeÂ mÂ kg of aÂ solidÂ into aÂ liquidÂ at itsÂ meltingÂ point is given by:

Example 1
â€‹

A lump of ice of mass 0.1 kg is at its melting point of 0Â°C. Heat is supplied to the ice by a small electrical heater of power 50 W.
â€‹
Calculate (i) theÂ heatÂ required to melt all the ice,Â  (ii) theÂ timeÂ taken.
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(iiâ€‹)Â â€‹Power = Energy/timeÂ Â Â  HenceÂ  time = energy/powerÂ  =Â  33400/50 =Â Â 668s

Learn time-efficient BMAT strategies and practice with reflective BMAT questions & worked solutions.

Suppose a liquid is heated steadily. The temperature rises. Eventually, if it is heated long enough the liquid reaches itsÂ boiling point.Â If further heating occurs the liquidÂ  willÂ change stateÂ i.e. change from aÂ liquidÂ to aÂ gas.
Heat is required to change the liquid into a gasÂ  whilst it is boiling but theÂ temperatureÂ remainsÂ constant.

The amount of heat required to changeÂ anyÂ mass of a liquid into a gas at its boiling point is calledÂ latent heat of vaporisation.
â€‹
The amount of heat required to changeÂ 1 kgÂ Â of a liquid into a gas at its boiling point is calledÂ specificÂ latent heat of vaporisation.

Example

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This means to change 1kg ofÂ water atÂ 100Â°Â CÂ Â to 1kg ofÂ vapour (gas) atÂ 100Â°Â CÂ requires 2.26 MJ
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To change 2kg of water (at 100Â°Â C) into gas ( 100Â°Â C) would require 2 x 2.26 MJ
To change 3kg of water (at 100Â°Â C) into gas ( 100Â°Â C) would require 3 x 2.26 MJ, and so on.

In general, theÂ thermal energy (E)Â required to changeÂ mÂ kg of aÂ liquidÂ into aÂ gasÂ at itsÂ boilingÂ point is given by:Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â
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Example 2
â€‹
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â€‹Calculate the specific latent heat of vaporisation of the liquid.
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Heat produced = power x time =Â  15 x 450 =Â  6750J
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## â€‹Microscopic explanation of PRESSURE of a gas

The molecules are moving inÂ randomÂ directions and collideÂ  with each other and with theÂ wallsÂ of the container. After each collision they are moving in different directions. Hence theirÂ momentumÂ has changed. From Newtonâ€™s 2nd law of motion the force acting on the molecules is equal to the rate of change of momentum. Hence there must be aÂ forceÂ acting on theÂ molecules. From Newton’s 3rd law there must be anÂ equalÂ andÂ oppositeÂ force acting on theÂ walls. Since many molecules collide with a given area of the container walls there is a force acting on a given area i.e.Â pressure.

## Microscopic explanation of TEMPERATURE of a gas

When a gas is heated the gas moleculesÂ gainÂ KE i.e. they moveÂ faster. The temperature of the gas rises.
Â Theory shows that the average KE of the gas molecules is proportional to the (Kelvin) Temperature.
[You do not need to know this for the exam].
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This means that when the temperature of a gasÂ risesÂ the molecules moveÂ faster, when the temperatureÂ decreasesÂ the molecules moveÂ slower.

## â€‹Ideal gas equation

Suppose a gas is enclosed in a piston chamber as shown.
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If the gas is compressed as shown then the molecules moveÂ closerÂ together. This means they collide more frequently and so theÂ pressure (P) increasesÂ and theÂ volume (V) decreases.
(TheÂ temperatureÂ is constant).

Experiment shows that:Â Â Â
Pressure x volumeÂ  = constantÂ  orÂ Â Â Â P V = constant
[You have to be able to use equation in calculations].
Any gas that behaves in this way is called anÂ ideal gas.

â€‹Example 3
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Gas leaks slowly out of a cylinder of constant volume. The temperature of the gas in the cylinder does not change. Which of the following is constant for the gas molecules in the cylinder?
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AÂ Â the number striking unit area of surface in unit time.
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BÂ Â the number of the collisions between molecules per unit time.
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CÂ Â the number per unit volume.
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DÂ Â the average speed.

## â€‹Density

Density is defined asÂ mass per unit volume.
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â€‹Example 4
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â€‹Example 5
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## Measurement of Density

1. MeasureÂ massÂ by placing object on a scale balance.
2. If the object isÂ regularÂ (e.g. a cube of a solid substance) itsÂ volumeÂ can be calculated.
3. If the object is small andÂ irregularÂ its volume can be found by placing it in a measuring cylinder and measuring the displacement of the liquid.
4. Using Density = mass/volume the density canÂ  be calculated.

Â  SOLIDS have the greatest densities and GASES the smallest.
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## PRESSURE

Pressure is defined asÂ force per unit areaÂ where the force isÂ normalÂ to the surface.
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â€‹Example 6
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(ii) theÂ massÂ andÂ weightÂ of the block,

(iii) theÂ maximumÂ andÂ minimumÂ pressure the block can exert.

## Variation of pressure with depth

Suppose a liquid of densityÂ rÂ is placed in a tank as shown.
The pressure p of the liquid acting on the base of the tank is given by:

This equation is valid for a container ofÂ anyÂ shape:

â€‹Example 3
D (The temperature is constant and so the average speed is constant).

â€‹Example 4
mass =Â  density x volume = 1000 x 0.2 = 200 kg
WeightÂ  = mgÂ  = 200 x 10 = 2000NÂ  (Taking g = 10 numerically)
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Example 6
volumeÂ  = 20x10x4 = 800 cm3 Â = 800 x 10-6 m3

Mass = density x vol = 5000 x 800 x 10-6 = 4 kg
â€‹
Weight = mg = 40N

MAXIMUM pressure will occur when the block rest on its SMALLEST face i.e. 4cm x 10cm
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Pressure = weight / Area = 40 /(40×10-4) = 10,000 N m-2

MINIMUM pressure will occur when the block rests on its LARGEST face i.e. 20cmx10cm
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Pressure = weight/area = 40/(200×10-4) = 2000 Nm-2

#### BMAT Physics Notes: Matter

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