Matter as Particles PDF - Junior Secondary Science

6 Matter as particles 6.1 Matter and particle theory (Book 1B, p. 164–177) 1 Matter is anything which has mass and takes up space. 2 The particle theory can be summarized as follows: All matter is made up of particles. The particles are tiny....

6 Matter as particles 6.1 Matter and particle theory (Book 1B, p. 164–177) 1 Matter is anything which has mass and takes up space. 2 The particle theory can be summarized as follows: All matter is made up of particles. The particles are tiny. The particles are moving randomly all the time. There are empty spaces between the particles. Different substances are made up of different particles. Different particles have different sizes and masses. 3 The particles that make up matter can be atoms or molecules. 4 Atom is the basic unit of matter. 5 Most molecules are formed from two or more atoms. The atoms in molecules can be of the same kind or of different kinds. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 1  6 Matter as particles 6.2 Particle model (Book 1B, p. 178–185) 1 Matter commonly exists in three states: solid state, liquid state and gas state. 2 The particle model helps describe the arrangement and movement of particles in solids, liquids and gases. 3 Key features of particles in the three states of matter: State of matter Arrangement of particles Movement of particles Solid Particles are closely packed and Each particle vibrates arranged in a regular pattern. about a fixed position. The spaces between particles are small. Liquid Particles are closely packed but Particles move around not arranged regularly. easily. The spaces between particles are small. Gas Particles are far apart. Particles move freely The spaces between particles in all directions. are large. 4 Properties of solids, liquids and gases: Solids have a fixed shape because their particles are in fixed position and cannot move around. Liquids and gases do not have a fixed shape because their particles can move around. Solids and liquids have a fixed volume because their particles are closely packed. Gases do not have a fixed volume because their particles are far apart. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 2  6 Matter as particles 6.3 Dissolving (Book 1B, p. 186–193) 1 During dissolving, the solute particles move apart and become evenly distributed among the solvent particles. crystal particle water particle The crystal particles are The crystal particles The crystal particles are surrounded and collided move apart and mix evenly distributed by the water particles. with the water particles. among the water particles. The colour of the solution becomes uniform. 2 The total mass of the solute and solvent does not change in dissolving. In other words, mass is conserved. Mass of solute + mass of solvent = mass of solution 3 The volume of the final solution may be different from the sum of the volume of the solute and the solvent. 6.4 Thermal expansion and contraction (Book 1B, p. 194–204) 1 Most substances expand when they are heated. They contract when they are cooled. This is called thermal expansion and contraction. 2 The movement of particles is affected by temperature. At higher temperature, the particles move faster; at lower temperature, the particles move slower. low high temperature Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 3  6 Matter as particles 3 The particles of a substance move faster during heating. This causes the spaces between the particles to increase. As a result, the substance expands. The particles of a substance move slower during cooling. This causes the spaces between the particles to decrease. As a result, the substance contracts. heating cooling The particles lose energy The particles gain energy and move slower. The and move faster. The spaces between particles spaces between particles become smaller. become larger. 4 Thermal expansion and contraction only changes the volume of a substance, not its mass. 5 The size of the particles of a substance does not change during thermal expansion and contraction. The substance expands because the spaces between its particles increase; the substance contracts because the spaces between its particles decrease. 6 Different substances expand to different extents when heated. In general, gases expand more than liquids and liquids expand more than solids. 7 Daily applications of thermal expansion and contraction: Liquid-in-glass thermometers When temperature increases, the liquid (e.g. alcohol) inside the narrow tube of the thermometer expands and rises. The temperature can then be read from the scale. Dental filling materials They should expand and contract at a similar rate as the teeth. Otherwise, cracks might occur in the fillings when they come in contact with cold or hot food or drinks. Opening jar lids The jar is inverted and just the lid is dipped in hot water. After some time, the lid can be opened easily as the lid gets slightly expanded. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 4  6 Matter as particles Fire sprinklers The glass bulb blocks and holds the water inside the water pipe hiding in the ceiling. The increase in temperature makes the alcohol in the bulb expand and breaks the glass bulb. This allows water to flow out of the sprinkler for putting out the fire. Thermostats Thermostats can turn the appliances on or off when they reach a certain temperature. Some thermostats contain a bimetallic strip. When the strip is heated, it bends because the two metals in the strip expand in different amounts. This separates the contacts and cuts the power. For example, in an electric iron: The bimetallic strip straightens when cooled. The circuit is completed. The iron is turned on. power cable heater bimetallic strip The bimetallic strip bends when heated. The circuit breaks. The iron is turned off. 8 Problems caused by thermal expansion and contraction: Roads may crack when they expand in hot weather or contract in cold weather. Expansion gaps are left on road surface to allow for expansion and contraction of the road. Overhead power cables are kept slack to allow for contraction. This makes sure that the cables will not become too tight or break in cold winters. Oil pipelines are made bent to prevent cracking when hot and cold oil flows through them. Same bending can be found in water and gas pipelines. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 5  6 Matter as particles E 6.5 Gas pressure (Book 1B, p. 205–216) 1 Bombardment of gas particles on a surface exerts a gas pressure on the surface. 2 When the volume of a closed container decreases, the gas becomes compressed. The gas particles bombard the container walls more often and thus the gas pressure increases. gas particle particles bombard the walls particles bombard of the bottle more often when the walls of the the gas is compressed bottle 3 When the gas temperature increases, the gas particles move faster and bombard the container walls harder and more often. As a result, the gas pressure increases. 4 When the gas temperature decreases, the gas particles move slower and bombard the container walls less hard and less often. As a result, the gas pressure decreases. 5 Atmospheric pressure is the gas pressure exerted by gas particles in the atmosphere. 6 The atmospheric pressure acts on our body. The matter in our body also exerts a pressure on our body. This balances the atmospheric pressure. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 6  6 Matter as particles 7 We can observe the existence of atmospheric pressure when the gas pressure inside and outside a container is imbalanced: When the can is heated, the water boils and changes to steam. The steam pushes the air out of the can. The can (filled with water) number of air particles in the can decreases. When the can is cooled, the steam condensers. There are fewer air particles bombarding the inside of the can. The gas pressure inside the can becomes much lower than the atmospheric pressure. The higher cold water pressure outside crushes the can. When air is drawn out of the Magdeburg hemispheres hemispheres, the number of gas particles inside the hemispheres decreases. valve rope rubber tubing (to vacuum pump) Much fewer particles then bombard the inside of the hemispheres. The gas pressure inside the hemispheres becomes much lower than the atmospheric pressure. The higher pressure outside presses the two hemispheres firmly together. gas particles atmospheric pressure gas pressure inside the to vacuum Magdeburg hemispheres pump Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 7  6 Matter as particles 8 Applications of atmospheric pressure in daily life: Drinking with a straw Sucker on a wall Vacuum cleaner 6.6 Density (Book 1B, p. 217–232) 1 The density of a substance is its mass per unit volume. 2 Objects of the same volume can have different masses. These objects have different densities. mass 3 Density = volume 4 The unit of density is g/cm3 or kg/m3. 5 If the density of an object is higher than that of its surrounding medium, it sinks in the medium. 6 If the density of an object is lower than that of its surrounding medium, it floats in the medium. 7 Why a steel ship floats in the sea: a A steel ship holds a large volume of air. Air is much less dense than water. b The overall density of the ship is lower than that of sea water, so the ship floats in the sea. E 8 In general, the density of a substance decreases as its temperature increases. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 8