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IFE Unit 1: Fire Engineering Science > Physical Properties of Matter > Flashcards

Flashcards in Physical Properties of Matter Deck (29)
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1

What is matter?

Matter is the name given to all material things (solids, liquids, gases.
The amount of matter is known as the mass (measured in kg) but for liquids gases and vapours we are more used to using volume (the amount of space occupied by a given substance).

2

Density

Density of a burning liquid determines whether you will be able to cover it with water to extinguish it or if you have to use foam.
Important factor is how well the burning liquid mixes with water, this is known as miscibility.

3

Miscibility

How well a liquid mixes with water.

4

Density definition

Mass of one cubic meter of a material.
One cubic meter is the standard unit volume.

5

Density Formula

Density=Mass / Volume.

If mass is kg and volume is m^3, unit of density will be kg/m^3 (kilograms per cubic meter).

If mass is grams and volume is cm^3 then the density will be g/cm^3 (grams per cubic centimetre).

6

Mercury v Water

Water is 1000 kg/m^3 and Mercury is 13600 kg/m^3.

7

Floating on water

If something has a density lower than water, and is not miscible in water then it will float on the surface.

e.g. wood will float on water but iron of the same size will not as iron has a higher density.

8

Relative Density

AKA

Specific Gravity

AKA Specific gravity is sometimes used to measure density.

It is the ratio of the mass of any volume of it to the mass to an equal amount of water.

=

mass of any volume of the substance/
mass of an equal volume of water


=


density of that substance/
density of water

9

Vapour Density

Gases and vapours have a low density compared to liquids and solids.
At normal temp and pressure (20 degrees and one atmosphere) 1m^3 of water is 1000kg and 1m^3 of air is 1.2kg.
For this reason we do not use specific gravity.
Therefore density of gas or vapour is given in relation to an equal volume of oxygen, air or hydrogen under same temp and pressure. Air more commonly used for FRS purposes.

10

Liquids of different density

Problem with ff'ing comes with flammable liquids of lower specific gravity than water, as the water will not smother the combusting liquid.

2 tanks on flat ground with horizontal pipe connecting the two, water will assume the same level in both as for the system to balance, there must be an equal head (height) in each tank above the lowest point of the pipe.

The head determines water pressure at any depth, and with tanks example, levels adjust to ensure pressures at the level of the pipe are equal.

Imagine petrol is poured into first tank on top of water, petrol has lower specific gravity than water (floats on top), so larger head of petrol is required in tank 1 in order to balance the system. Therefore, tank 1 will be more full as larger combo of petrol and water needed in order to create the same head of pressure of the (just) water in tank 2 above the lowest point of the pipe.

11

Gases of different density

All gases and vapours are completely miscible.

All gases heavier than air will accumulate in low places (wells or cellars) e.g. CO2 and petrol vapour,
and all gases lighter than air will accumulate in high places.

Differences in density can be created by changes in temp. Increase in temp causes expansion and therefore a lowering of density.

Density of gases e.g. o2 compared to unburned products of combustion is responsible for fire gases being forced out through gaps and air being drawn into a compartment or building. Heated, therefore lighter gas draws to top, if no where to escape, an over pressure will form.

12

Matter and Energy

Most substances, at normal temps are only ever found in 1 of the 3 states (solid, liquid, gas).

All matter is made up of very small particles called ATOMS.

13

Atoms

Atoms have a smaller core (nucleus) containing smaller particles called protons and neutrons.

Protons have a positive electrical charge.

Neutrons have no charge.

Nucleus is surrounded by a system of electrons, which each carry a negative charge.

14

Atoms Continued

Atoms are neutral if number of protons and electrons are matched, and each proton possesses and equal and opposite charge to each electron.

Number and arrangement of electrons around nucleus determines the chemical behaviour and which other atoms with which it will combine with.

15

Chemical Reactions

Take place when electrons move between atoms.

An atom that has lost or gained one or more electrons in a chemical interaction will possess an electrical charge and is called either a positive or negative ion.

16

Max number of protons in a nucleus

92-Becomes unstable with anymore and falls apart.

17

Element

An element is a substance containing atoms all of the same type. Meaning, same number of protons.
As there can be up to 92 protons in an atom, it means that there are 92 stable elements.
Atoms of different elements form to become molecules.

18

Molecules and Compounds.

Some consist of 2 or more atoms of the same kind e.g. oxygen molecules consist of 2 oxygen atoms.

Others consist of 2 or more atoms of different kinds e.g. Carbon Dioxide, 2 oxygen and 1 carbon atom, and water, 2 atoms pf hydrogen and 1 atom of oxygen.

CO2 and H2O are chemical compounds and can be split into their component elements.

19

Splitting Atoms

Atoms can be split or combined with other particles to make new atoms of other elements.

20

Energy

Energy is the availability to do work.
Cannot be created or destroyed and is found in a number of forms e.g. mechanical, heat, light, electrical, chemical, nuclear, gravitational, magnetic, sound, etc...

21

Heat Energy

FF's regularly experience energy in form of heat energy, produced by a chemical change e.g. combustion, in which we say chemical energy is released as heat energy.

22

Temperature

The measure of how hot something is, and is related to how fast the constituent molecules are moving.

Temperature also determines which way the heat will flow. Heat can only move from something of a high temp to a low temp.

23

Attraction between molecules

Molecules that make up any substance are always continually moving, even in a solid.
They continually vibrate around a fixed spot.
They also exert a force of attraction to each other which is greater the closer they are to one another.
Movement of molecules tends to spread them out whilst the attraction force, or force of cohesion binds them together.

24

Applying heat energy to a solid

As you apply heat energy to a solid it is stored in the substance as vibrational energy. As more energy is stored the molecules vibrate faster and take up more space.
At same time, temp of the solid rises and thermal expansion occurs.

A temp is reached where the molecules are vibrating so much, they break free, from rigid framework where they were held together by cohesive forces, and begin to be able to slide past each other, although they do not have complete freedom.

At this point, the solid melts and becomes a liquid.

25

Further Heating applied to a solid

Further heating causes temp to increase and the energy is stored as kinetic energy of the molecules.
They move with increasing rapidity until they overcome the adhesive forces completely.
A liquid then boils and becomes a gas (or a vapour).
If heat is reduced the process reverses; kinetic energy of the molecules reduces.

26

Melting, Boiling and Evaporation

Melting point is temp at which a solid becomes a liquid.
Freezing point is liquid turning to a solid.
Boiling point is temp at which a liquid boils and becomes a vapour.

Since energy is required to overcome the cohesion when a substance melts or boils, the heat which is supplied during these processes does not cause a rise in temp of the substance itself.
Conversely, when a vapour condenses or a liquid solidifies, it gives up heat without any fall in temperature as long as the change is taking place.
e.g. water freezes at 0 degrees and boils at 100 degrees.

27

Evaporation

Even at temps below boiling point, some molecules at the surface of a liquid may gain enough energy with colliding with other molecules to escape as vapour. This is known as evaporation.

28

Vapour pressure

For a liquid in an enclosed space e.g. a saucepan of water, where there is air.

Even if not heated some evaporation will still take place. The evaporating molecules will build up a pressure known as the vapour pressure.
At same time, some molecules will re enter the liquid.

For any temp below boiling temp there is a definitive vapour pressure at which the number of escaping molecules is balanced by the number re entering the liquid.

29

Boiling and Vapour pressure

Boiling occurs when the vapour pressure has become equal to the surrounding atmospheric pressure, the pressure of the air.
Vapour then forms not only on surface of liquid, but also in the body of the liquid and bubbles can be seen.

If external pressure is increased, the vapour pressure at which boiling will take place is increased so the temperature must increase.
If external pressure falls, the reverse is true, so the temp where boiling occurs is reduced.