CBSE Class 9 Science Revision Notes

CBSE Class 9 Science Revision Notes. Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts made as per NCERT and CBSE pattern will help the student to understand the chapter and score better marks in the examinations.

MCQs-Particle Model of solids, Liquids and Gases

1.Which of the following best describes a SOLID?

• fills container, takes shape of container, flows easily, low density

• definite volume, definite shape, does not readily flow, high density

• definite volume, takes shape of container, flows easily, high density

• fills container, definite shape, does not readily flow, low density

2. The table shows the melting points and boiling points of some substances A to D. Which substance is a liquid at -l0°C and a gas at l0°C?

Melting Point °C and Boiling Point °C

• -66 and 42

• -142 and -78

• -95 and 4

• 90 and 189

3. BOILING is the state change from?

• liquidtogas

• gas to liquid

• liquid to solid

• solid to liquid

4. Air is in a bottle with a tight screw-top. When the bottle becomes cooler,the pressure inside decreases because the molecules of air?

• move faster

• move slower

• become bigger

• become smaller

5. The table below shows the melting points and boiling points of substances A to D. Which substance is suitable to use in a thermometer that measures in the range -4°C to 50°C? 

 Melting point 0 °C (mpt), Boiling Point °C  (bpt)

• mpt = 0, bpt = 100

• mpt = -110, bpt = 80

• mpt = 40, bpt = 350

• mpt = -200, bpt = 40

6. The diagram shows an experiment using a light colourless gas (air) and a heavier brown coloured gas (bromine or nitrogen dioxide). The gases fully mix together because?

• all the gas particles can move around freely

• of convection currents in the air

• the gases have different densities (one is 'lighter' or 'heavier' than the other)

• the air can sink into the coloured gas and the coloured gas can float on air

7. The diagram shows an experiment using a light colourless gas (air) and a heavier brown coloured gas (bromine or nitrogen dioxide). The gases spread around in the gas jars by a process called?

• evaporation

• condensation

• diffusion

• sedimentation

8. Which of the following is the opposite of evaporating?

• condensing

• melting

• freezing

• boiling

9. Which of the following best EXPLAINS why liquids flow much more easily than solids?

• liquid particles are lighter than solid particles

• liquid particle attractive forces are weaker than solid particle attractive forces

• liquid particles have more energy than solid particles

• liquid particles can change shape more easily than solid particles

10. CONDENSING is the state change from?

• liquid to solid

• gas to liquid

• liquid to gas

• solid to liquid

11. Which describes sand?

• a liquid

• a gas

• difficult to classify simply as gas, liquid or solid

• a solid

12. Starting with the solid,thegraph shows the results of measuringthe temperature of substance X as it was heated from a low to a high temperature.Which of the foilowingis TRUE7

• X is meltingbetween times tO and t1

• X is boiling between times t2 and t3

• temperature TEisl the melting point of X

• X is completely liquid between times t1and t2

13. Which of the followingbest describes what happens to the particles of water vapour when it condenses7

• They lose energy and and are closer and Iess free to move around

• They gain energy and increase their freedom to move without significant attractio

• They lose energy and Iose freedom to move about

• They gain energy and gain freedom to move about

14. Which of the followingbest EXPLAINS why the liquid in a glass thermometer expands with rise in temperature7

• the Iiquid particles get bigger expandingthe liquid

• the glass contracts forcing the liquid further up the tube

• the Iiquid particles gain more energy and collide more forcefully

• the glass particles gainmore energy and push the Iiquid particles up the tube

15. Which of these is a gas7

• sand

• water

• rock

• oxygen

16. Gases diffuse becausethey7

• have a low density

• easily move as convection currents

• are made of rapidly moving particles

• have low boiling points

17. Which describes carbon dioxide?

• a gas

• a liquid

• difficult to classify simply as gas, liquid or solid

• a solid

18. Water turning to a solid is called?

• freezing

• boiling

• condensing

• melting

19. The table shows the melting points and boiling points of some substances A to D. Which substance is a liquid at 20 °C?

Melting Point °C and Boiling Point °C

• -142 and -78

• -95 and 4

• -66 and 42

• 90 and 189

20. Which of the following best describes what happens to the particles of ice when it melts?

• They lose energy and lose freedom to move about

• They lose energy and escape into the atmosphere

• They gain energy and gain freedom to move about

• They gain energy and break up into atoms of hydrogen and oxy

THE THREE STATES OF MATTER

GAS-LIQUID-SOLID PARTICLE THEORY MODELS

• A gas has no fixed shape or volwne, but always spreads out to fill any container,
• There are almost no forces of attraction between the particles so they are completely free of each other.
• The particles are widely spaced and scattered at random throughout the container so there is no order in the system.
• The particles move rapidly in all directions, frequently colliding with each other and the side of the container.
• With increase in temperature, the particles move faster as they gain kinetic energy.

Using the particle model to explain the properties of a Gas

• Gases have a very low density ('light') because the particles are so spaced out in the container (density =mass I volume).

o Density order:solid> liquid>>> gases

• Gases flow freely because there are no effective forces of attraction between the gaseous particles­ molecules.

o Ease of flow order: gases> liquids>>> solids (no real flow in solid unless you powder it!)

o Because of this gases and liquids are described as fluids.

• Gases have no surface, and no fiXed shape or volwne, and because of lack of particle attraction, they always spread out and fill any container (so gas volume= container volume).
• Gases are readily compressed because of the 'empty' space between the particles.

o Ease of compression order: gases>>> liquids> solids (almost impossible to compress a solid)

• Gas pressure

o When a gas is confined in a container the particles will cause and exert a gas pressure which is measured in atmospheres (atm) or Pascals (Pa = N/m- pressure is force/area on which force is exerted.

• The gas pressure is caused by the force created by millions of impacts of the tiny individual gas particles on the sides of a container.

• For example- if the nwnber of gaseous particles in a container is doubled, the gas pressure is doubled because doubling the number of molecules doubles the number of impacts on the side of the container so the total impact force per unit area is also doubled.

This doubling of the particle impacts doubling the pressure is pictured in the two diagrams below.

• If the volume of a sealed container is kept constant and the gas inside is heated to a higher temperature, the gas pressure increases.

o The reason for this is that as the particles are heated they gain kinetic energy and on average move faster.

o Therefore they will collide with the sides of the container with a greater force of impact, so increasing the pressure.

DIFFUSION in Gases:

• The natural rapid and random movement of the particles in all directions means that gases readily 'spread' or diffuse.
• Diffusion is faster in gases than liquids where there is more space for them to move (experiment illustrated below) and diffusion is negligible in solids due to the close packing of the particles.
• Diffusion is responsible for the spread of odours even without any air disturbance e.g. use of perfume, opening ajar of coffee or the smell of petrol around a garage.
• The rate of diffusion increases with increase in temperature as the particles gain kinetic energy and move faster.
• Other evidence for random particle movement including diffusion:

When smoke particles are viewed under a microscope they appear to 'dance around' when illuminated with a light beam at 90° to the viewing direction.This is because the smoke particles show up by reflected light and 'dance' due to the millions of random hits from the fast moving air molecules.This is called 'Brownian motion' (see below in liquids). At any given instant of time, the hits will not be even, so the smoke particle get a greater bashing in a random direction.

• A liquid has a fixed volume at a given temperature but its shape is that of the container which holds the liquid.
• There are much greater forces of attraction between the particles in a liquid compared to gases, but not quite as much as in solids.
• Particles quite close together but still arranged at random throughout the container, there is a little close range order as you can get clumps of particles clinging together temporarily.
• Particles moving rapidly in all directions but more frequently collisions with each other than in gases due to shorter distances between particles.
• With increase in temperature, the particles move faster as they gain kinetic energy, so increased collision rates, increased collision energy and increased rate of diffusion.

Using the particle model to explain the properties of a Liquid

• Liquids have a much greater density than gases ('heavier') because the particles are much closer together because of the attractive forces.
• Most liquids are just a little less dense than when they are solid

o Water is a curious exception to this general rule, which is why ice floats on water.

• Liquids usually flow freely despite the forces of attraction between the particles but liquids are not as fluid' as gases.

o Note 'sticky' or viscous liquids have much stronger attractive forces between the molecules BUT not strong enough to form a solid.

• Liquids have a surface, and a fixed volume (at a particular temperature) because of the increased particle attraction, but the shape is not fixed and is merely that of the container itself.

o Liquids seem to have a very weak 'skin' surface effect which is caused by the bulk molecules attracting the surface molecules disproportionately.

• Liquids are not readily compressed because there is so little 'empty' space between the particles, so increase in pressure has only a tiny effect on the volume of a solid, and you need a huge increase in pressure to see any real contraction in the volume of a liquid.

• Liquids will expand on heating but nothing like as much as gases because of the greater particle attraction restricting the expansion (will contract on cooling).

o Note: When heated, the liquid particles gain kinetic energy and hit the sides of the container more frequently, and more significantly, they hit with a greater force, so in a sealed container the pressure produced can be considerable!

• The natural rapid and random movement of the particles means that liquids 'spread' or diffuse. Diffusion is much slower in liquids compared to gases because there is less space for the particles to move in and more 'blocking' collisions happen.

o Just dropping lumps/granules/powder of a soluble solid (preferably coloured!) will resulting in a dissolving followed by an observable diffusion effect.

o Again, the net flow of dissolved particles will be from a higher concentration to a lower concentration until the concentration is uniform throughout the container.

• Diffusion in liquids - evidence for random particle movement in liquids:

o If coloured crystals of e.g. the highly coloured salt crystals of potassium permanganate are dropped into a beaker of water and covered at room temperature.

o Despite the lack of mixing due to shaking or convection currents from a heat source etc. the bright purple colour of the dissolving salt slowly spreads throughout all of the liquid but it is much slower than the gas experiment described above because of the much greater density of particles slowing the spreading due to close proximity collisions.

o The same thing happens with dropping copper sulphate crystals (blue, so observable) or coffee granules into water and just leaving the mixture to stand.

• A solid has a fiXed volume and shape at a particular temperature unless physically subjected to some force.
• The greatest forces of attraction are between the particles in a solid and they pack together as tightly as possible in a neat and ordered arrangement.
• The particles are too strongly held together to allow movement from place to place but the particles vibrate about their position in the structure.
• With increasein temperature, the particles vibrate faster and more strongly as they gain kinetic energy.

Using theparticlemodel to explain the properties of a Solid

• Solids have the greatest density ('heaviest') because the particles are closest together.
• Solids cannot flow freely like gases or liquids because the particles are strongly held in fixed positions.
• Solids have a fixed surface and volume (at a particular temperature) because of the strong particle attraction.
• Solids are extremely difficultto compress because there is no real 'empty' space between the particles, so increase in pressure has virtually no effect on the volume of a solid.
• Solids will expand a little on heating but nothing like as much as liquids because of the greater particle attraction restricting the expansion and contraction occurs on cooling.

o The expansion is caused by the increased energy of particle vibration, forcing them further apart causing an increase in volume and corresponding decrease in density.

• Diffusion is almostimpossiblein solids because the particles are too closely packed and strongly held together with no 'empty space' for particles to move through.

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