0-2 Particle Theory Of Matter

II. Particle theory of matter

Topic

II.

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Particle theory of matter

Reading Assignment Objectives

0.2 Scientific investigation Particle theory of matter

Notes

A. Scientific investigation When an experimental fact is observed, different explanations are proposed and criticised by other scientists. The explanation which defeats all crticisms is called a theory Law, hypothesis and theory about diffusion. law –

this is only a set of rules which helps to make prediction, this is not an explanation. e.g. Graham's law of diffusion of gas. The lower the density of a gas, the faster will be the rate of diffusion.

hypothesis – a tentative explanation that tries to explain the fact and still needs further testing. e.g. The diffusion is spreading of particles. theory –

a well tested hypothesis which can explain all the experimental facts and be used to make prediction accurately. e.g. Particle theory of matter – Matter is made up of tiny particles.

All scientific investigations start from the collection of experimental facts. This is why experiment is so important in science. Since the technology is getting better and better, an old theory may not be able to explain a newly discovered fact. If this happens, the whole cycle of investigation have to be started all over again. Many theory which had been thought to be correct 100 years ago were proved to be wrong..

DON'T ALWAYS INSIST ON WHAT YOU HAVE LEARNT BEFORE, KNOWLEDGE IS ALWAYS ADVANCING.

II. Particle theory of matter

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Can you answer the following questions : What happens when ice melts ? What happens when sugar dissolves in water ? What is the difference between melting and dissolving ? B. Particle theory of matter – A Linkage between the macroscopic (large) and microscopic (small) world Melting of ice at room temperature and dissolving of sugar in water is a very common phenomenon but how can this be explained. Scientists propose (postulate) that all substances are made up of tiny particles. The theory states that all matters are made of particles is called particle theory of matter.

Particle theory of matter is the most important theory in chemistry. It creates a link between the macro properties (e.g. reactivity, solubility) observed and the micro properties (e.g. bonding, structure) of a substance. 1.

Evidences of presence of particles

a) Diffusion When the glass plate is removed, the brown colour diffuses up and fills up the upper and lower gas jars evenly. Despite the fact that nitrogen dioxide is denser than air, the brown colour still moves up. Therefore, this must not be caused by gravity. This can be explained by the constant motion of air and nitrogen dioxide particles. Eventually, all particles mix up thoroughly.

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b) Brownian Movement When pollen grains are placed on water surface, it is observed that they move randomly even without being blown by the wind. This was firstly observed by Robert Brown in 1827 and is known as Brownian movement. Macroscopic Observation

Microscopic Interpretation

Pollen grains are hit by rapid moving water particles from all directions. This makes the pollen grains move randomly on the water surface.

c)

Physical states of matter

The existence of 3 physical states of matter (solid, liquid and gaseous state) can also be explained by particle theory of matter. In solid states, the particles are closely packed together and they can only vibrate about their equilibrium positions. Therefore, solid is incompressible and has a definite shape. In liquid states, the particles are fairly close packed but are free to move. Therefore, liquid is also incompressible but do not has a definite shape. In gaseous state, the particles are very far from each other and free to move at a great speed. Therefore, gas is freely compressible and do not has a definite shape.

When a cube of ice melts to water, there is almost no change in volume. But when a drop of water evaporates, the steam formed has a volume 1000 times larger than the drop of water. This proved that the particles in a gas are more widely separated than in a liquid. N.B.

The sizes of particles are all the same in different physical states, the difference is only the spacing among the particles.

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Summary of differences among solid, liquid and gas

Attractive force among particles Compressibility Volume Particle movement Shape

Solid strong

Liquid fairly strong

Gas very weak

incompressible

incompressible

fixed

fixed and similar to solid

can only vibrate

free to move slowly no definite shape (fill bottom of the container)

easy to compress no fixed volume (occupies the whole container) free to move quickly no definite shape (fill the whole container)

definite

Average kinetic energy (proportional to the temperature)

low

medium

high

The existence of the 3 physical states (macroscopic observation) can be explained by the balance between the attractions among the particles (microscopic interpretation) and the kinetic energy possessed by the particles. The attractions bring the particles together while the kinetic energy tends to separate the particles.

Solid

Liquid

Gas

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(1) Relationship between temperature and physical state of a substance

physical state

Solid

Liquid

melting point

Gas

boiling point

ambient temperature The physical state of a substance is depending on the melting point, boiling point of the substance and the ambient temperature. If the ambient temperature is lower than the melting point of the substance, the substance will be a solid. If the ambient temperature is higher than the boiling point, it will be a gas. If the ambient temperature is between the m.p. and b.p., the substance will be a liquid. e.g. water has m.p. 0 ºC and b.p. 100 ºC. What are the physical states of water at -20 ºC, 25 ºC and 700 ºC. The average temperature on Mars in - 33 ºC , what is the physical state of water on Mars? (2) Changing of physical state

Sublimation A few substances change directly from solid state to gaseous state when the temperature increases, e.g. carbon dioxide (dry ice), iodine, ammonium chloride and iron(III) chloride. The process in which solid changes to gas or gas changes to solid directly without going through the liquid state is called sublimation.

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Dissolution (Dissolving) Process

When a granule of sugar is placed in water, it is being hit constantly by water molecules. The water molecules pull individual sugar particles away from the granule of sugar. Eventually, the sugar particles are evenly distributed among the water particles and a sugar solution is formed. During the dissolving process, the sugar particles diffuse from the region of high concentration to the region of low concentration.

In a single granule of sugar, there is about 1,000,000,000,000,000,000 sugar particles. Individual sugar particle is extremely small. When they are separated from each other, the tiny particles can no longer be seen. There is why when sugar dissolves, no more sugar granule can be seen but the solution becomes sweet.

3.

Difference between Melting and Dissolution

Phenomenon of both melting and dissolving can be explained by particle theory of matter. When a solid melts or dissolves, the particles moves and separate. The only difference between melting and dissolving is that in melting process only one substance (ice) is involved. In dissolving process, two substances are involved, i.e. the solute (sugar) and the solvent (water). In melting of ice, water particles separate because they gain energy from the surrounding and moves faster and faster. In dissolving of sugar, sugar particles are constantly hit by water particles and loosen from the sugar granule to form a solution.

Glossary

Past Paper Questions

investigation facts law hypothesis theory macro properties micro properties macroscopic world equilibrium positions kinetic energy sublimation

particle theory of matter diffuse microscopic world state of matter