WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.895 Unit S P 14: Particle model. 00:00:09.567 --> 00:00:16.115 In a solid, particles are closely packed and vibrate about fixed positions; a solid has a fixed shape and volume. 00:00:16.265 --> 00:00:21.736 In a liquid, close particles move past each other, giving fixed volume but variable shape. 00:00:24.400 --> 00:00:32.685 In a gas, particles are widely separated and move rapidly in random directions; a gas fills its container and is readily compressed. 00:00:32.835 --> 00:00:36.045 Particles themselves do not swell when the gas expands. 00:00:36.195 --> 00:00:40.931 Gas particles are further apart; their individual sizes need not change. 00:00:45.600 --> 00:00:50.389 Density is mass per unit volume: rho equals mass divided by volume. 00:00:50.539 --> 00:01:01.215 Use mass in kilograms and volume in cubic metres for density in kilograms per cubic metre; grams and cubic centimetres instead give grams per cubic centimetre. 00:01:03.867 --> 00:01:09.925 Rearrange to mass equals density times volume, and volume equals mass divided by density. 00:01:10.075 --> 00:01:16.187 Convert carefully: one gram per cubic centimetre equals one thousand kilograms per cubic metre. 00:01:16.337 --> 00:01:20.790 One cubic centimetre equals ten to the power minus six cubic metres. 00:01:23.467 --> 00:01:29.108 The same material generally has lower density as a gas because particles are much further apart. 00:01:29.258 --> 00:01:35.861 Solids are often denser than liquids, but ice is less dense than liquid water because of its more open structure. 00:01:38.533 --> 00:01:42.792 If mass stays constant while volume increases, density decreases. 00:01:42.942 --> 00:01:47.774 Changes of state conserve mass in a closed system even when volume changes. 00:01:50.433 --> 00:01:53.440 Measure an object's mass using a zeroed balance. 00:01:53.590 --> 00:01:59.907 For a regular block, measure length, width and height and calculate volume equals length times width times height. 00:02:02.567 --> 00:02:11.920 For an irregular solid that does not absorb water (a non-porous solid), measure its volume by water displacement using a measuring cylinder or overflow can. 00:02:12.070 --> 00:02:14.651 Fully submerge it and avoid trapped bubbles. 00:02:14.801 --> 00:02:18.285 The volume of water displaced equals the solid’s volume. 00:02:18.435 --> 00:02:21.786 Subtract the starting volume; avoid trapped bubbles. 00:02:26.467 --> 00:02:34.550 Choose a method suitable for the material: an object that dissolves, absorbs water or floats needs a different liquid or an adapted procedure. 00:02:34.700 --> 00:02:38.502 Do not simply call a partly submerged volume its whole volume. 00:02:41.167 --> 00:02:46.637 For a liquid, weigh the empty dry container and then container plus a measured liquid volume. 00:02:46.787 --> 00:02:50.726 Subtract the empty-container mass before calculating density. 00:02:53.400 --> 00:02:55.725 Read liquid volume at eye level. 00:02:55.875 --> 00:02:59.441 For water, read the bottom of the curved surface (meniscus). 00:02:59.591 --> 00:03:04.529 Use an instrument with suitable scale divisions (resolution) and repeat readings. 00:03:07.200 --> 00:03:09.504 Use the same unit system throughout. 00:03:09.654 --> 00:03:20.937 A 54 grams block with volume 20 cubic centimetres has density 2.7 grams per cubic centimetre, equal to 2700 kilograms per cubic metre. 00:03:23.600 --> 00:03:28.456 Temperature is linked to the average kinetic energy of a substance’s particles. 00:03:28.606 --> 00:03:33.917 Internal energy is the total kinetic and potential energy of all its particles. 00:03:34.067 --> 00:03:39.150 It depends on how much substance there is, the material, its temperature and its state. 00:03:41.800 --> 00:03:51.675 Heating transfers energy into a system, usually increasing particle kinetic energy and temperature, or changing particle potential energy during a state change. 00:03:51.825 --> 00:03:54.555 A temperature is not an amount of energy. 00:03:57.233 --> 00:04:06.129 Specific heat capacity C is the energy needed to raise the temperature of 1 kilogram of a substance by 1 degree Celsius (or 1 kelvin). 00:04:06.279 --> 00:04:11.909 Units are joules per kilogram per degree Celsius, equivalently joules per kilogram per kelvin. 00:04:14.567 --> 00:04:23.425 Change in thermal energy, delta Q, equals mass times specific heat capacity times temperature change: M times C times delta theta. 00:04:23.575 --> 00:04:31.777 Use mass in kilograms, specific heat capacity in joules per kilogram per degree Celsius, and temperature change in degrees Celsius. 00:04:31.927 --> 00:04:36.769 A temperature increase of one degree Celsius equals an increase of one kelvin. 00:04:39.433 --> 00:04:54.772 For 0.5 kilograms water, C equals 4200 joules per kilogram per degree Celsius and a 10 degrees Celsius rise, delta Q equals 0.5 times 4200 times 10 equals 21000 joules. 00:04:54.922 --> 00:04:58.643 Specific heat capacity includes “per kilogram” in its units. 00:05:03.300 --> 00:05:08.536 A material with greater C needs more energy for the same mass and temperature rise. 00:05:08.686 --> 00:05:14.259 A larger mass also needs more energy; C is a material property, not the mass itself. 00:05:16.933 --> 00:05:23.738 Insulation, a lid and reducing exposed area limit unwanted energy transfers during heating or cooling. 00:05:23.888 --> 00:05:27.750 Low thermal conductivity and trapped air can reduce cooling. 00:05:30.400 --> 00:05:32.448 Melting is solid to liquid; 00:05:32.598 --> 00:05:34.603 freezing is liquid to solid; 00:05:34.753 --> 00:05:37.612 boiling or evaporation is liquid to gas; 00:05:37.762 --> 00:05:40.130 condensation is gas to liquid; 00:05:40.280 --> 00:05:42.477 sublimation is solid to gas; 00:05:42.627 --> 00:05:44.825 deposition is gas to solid. 00:05:47.500 --> 00:05:53.002 These are physical changes: the material can recover its original properties when reversed. 00:05:53.152 --> 00:05:56.991 In a closed system, mass is conserved; the volume may change. 00:05:59.667 --> 00:06:03.357 During melting or boiling of a pure substance at fixed pressure, 00:06:03.507 --> 00:06:12.050 temperature stays constant while supplied energy changes the particle arrangements and potential energy rather than average kinetic energy. 00:06:14.700 --> 00:06:23.051 Specific latent heat L is energy needed to change the state of 1 kilogram without changing temperature, in joules per kilogram. 00:06:23.201 --> 00:06:30.677 Latent heat of fusion refers to melting or freezing; latent heat of vaporisation refers to liquid or gas change. 00:06:33.333 --> 00:06:36.942 Use Q equals M times L, with mass in kilograms. 00:06:37.092 --> 00:06:47.481 Melting 0.2 kilograms ice with L equals 334000 joules per kilogram requires 66800 joules for the state change alone. 00:06:50.133 --> 00:06:59.551 If a question includes warming and a state change, calculate separate stages using M times C times delta theta and M times L, then add energies. 00:06:59.701 --> 00:07:03.228 Do not use M times L for an ordinary temperature rise. 00:07:05.900 --> 00:07:10.237 During cooling, freezing and condensation release energy to surroundings. 00:07:10.387 --> 00:07:16.593 A flat region on a heating or cooling curve identifies a state-change stage under the stated conditions. 00:07:16.743 --> 00:07:21.391 Flat stages show energy used for a state change, not a temperature rise. 00:07:26.067 --> 00:07:29.841 Evaporation can occur at the liquid surface below the boiling point. 00:07:29.991 --> 00:07:33.963 Boiling occurs throughout the liquid at the boiling temperature for the pressure. 00:07:36.633 --> 00:07:46.264 To estimate water's specific heat capacity, measure its mass, initial temperature and the energy transferred by a low-voltage immersion heater over a timed interval. 00:07:48.933 --> 00:07:55.556 Measure heater voltage and current and use E equals V times I times T, or use a suitable joulemeter. 00:07:55.706 --> 00:08:04.483 Stir gently for uniform temperature and record the final temperature; calculate C equals E divided by the product M times delta theta. 00:08:04.633 --> 00:08:08.622 Keep the heater immersed and use insulation to reduce heat loss. 00:08:13.300 --> 00:08:16.728 Insulate the container and use a lid to reduce losses. 00:08:16.878 --> 00:08:23.266 Keep the heater immersed and follow electrical safety procedures; avoid contact with hot water or heaters. 00:08:25.933 --> 00:08:35.211 Energy heating the container or escaping to surroundings can make calculated C too high if all supplied energy is incorrectly attributed to the water. 00:08:35.361 --> 00:08:38.823 Use appropriate corrections or discuss this limitation. 00:08:41.500 --> 00:08:50.114 For a melting-ice temperature, time graph, place a temperature probe in ice or water and record at regular intervals while energy is supplied. 00:08:50.264 --> 00:08:53.855 Mix carefully and keep the probe away from the heater and vessel wall. 00:08:56.533 --> 00:09:02.729 At ordinary pressure, a pure ice, water mixture stays near 0 degrees Celsius while the ice melts. 00:09:02.879 --> 00:09:05.997 Once all the ice has melted, the water warms up. 00:09:06.147 --> 00:09:13.484 A real graph may not have a perfectly flat section (plateau) because heat flow and the thermometer’s response affect the readings. 00:09:16.167 --> 00:09:20.403 Gas particles collide with container walls and exert forces on them. 00:09:20.553 --> 00:09:24.191 Pressure is force per area and is measured in pascals (Pa). 00:09:26.867 --> 00:09:32.828 For a fixed mass of gas at constant volume, raising temperature increases average particle speed. 00:09:32.978 --> 00:09:36.345 More frequent and stronger wall impacts increase pressure. 00:09:36.495 --> 00:09:40.815 Fixed volume and fixed gas mass are essential to the temperature comparison. 00:09:45.467 --> 00:09:54.170 In the ideal particle model, absolute zero corresponds to no random thermal motion and zero extrapolated gas pressure at fixed volume. 00:09:54.320 --> 00:10:01.944 It is approximately minus 273 degrees Celsius or 0 kelvin; real gases condense before reaching it. 00:10:04.600 --> 00:10:11.981 Convert approximately using temperature in kelvin equals temperature in degrees Celsius plus two hundred and seventy-three. 00:10:12.131 --> 00:10:17.783 Temperature in degrees Celsius equals temperature in kelvin minus two hundred and seventy-three. 00:10:17.933 --> 00:10:21.057 Kelvin temperatures are written without a degree symbol. 00:10:23.733 --> 00:10:28.110 20 degrees Celsius is approximately 293 kelvin. 00:10:28.260 --> 00:10:38.370 A 10 degrees Celsius temperature increase is a 10 kelvin increase; adding 273 is for converting absolute readings, not temperature differences. 00:10:41.033 --> 00:10:47.625 The simple gas model has limits: particles have finite size, attractions matter and phase changes occur. 00:10:47.775 --> 00:10:54.259 Do not extrapolate an ordinary gas-pressure experiment as though the gas stays gaseous down to absolute zero. 00:10:56.933 --> 00:11:01.489 A gas is compressible because there is substantial space between its particles. 00:11:01.639 --> 00:11:08.485 Pressure produces a net force perpendicular to a surface through particle collisions; it does not act only downwards. 00:11:11.167 --> 00:11:19.655 For a fixed mass of gas at constant temperature, reducing volume increases collision frequency with the container walls and increases pressure. 00:11:19.805 --> 00:11:24.744 Pressure is inversely proportional to volume: pressure times volume is constant. 00:11:24.894 --> 00:11:31.305 The curve follows relative pressure equals one divided by relative volume, rather than a straight-line fall. 00:11:35.967 --> 00:11:42.962 Use pressure one times volume one equals pressure two times volume two with consistent pressure and volume units. 00:11:43.112 --> 00:11:46.741 Use absolute pressure, which includes atmospheric pressure. 00:11:46.891 --> 00:11:53.225 If a pressure gauge shows pressure above atmospheric pressure (gauge pressure), add atmospheric pressure first. 00:11:55.900 --> 00:11:59.259 The fixed-mass and constant-temperature conditions matter. 00:11:59.409 --> 00:12:07.174 A leaking sample changes its particle number; rapid compression can heat a gas and depart from the constant-temperature relationship. 00:12:09.833 --> 00:12:15.317 (Higher tier) Doing work on a gas can increase its internal energy and temperature (Higher tier). 00:12:15.467 --> 00:12:23.123 A bicycle pump can warm during compression, so allow gas to return to the controlled temperature when investigating Boyle’s law. 00:12:25.800 --> 00:12:34.438 In a pressure-volume investigation, use suitable sealed apparatus, vary volume, allow temperature to settle and record corresponding pressure. 00:12:34.588 --> 00:12:41.445 Plot pressure against one divided by volume to test the inverse relationship; follow the apparatus pressure limits. 00:12:44.100 --> 00:12:46.127 That completes Particle model. 00:12:46.277 --> 00:12:49.622 Revisit the notes and test yourself on the revision website.