WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.799 Unit C P 12: Particle model. 00:00:09.467 --> 00:00:16.015 In a solid, particles are closely packed and vibrate about fixed positions; a solid has a fixed shape and volume. 00:00:16.165 --> 00:00:21.636 In a liquid, close particles move past each other, giving fixed volume but variable shape. 00:00:24.300 --> 00:00:32.585 In a gas, particles are widely separated and move rapidly in random directions; a gas fills its container and is readily compressed. 00:00:32.735 --> 00:00:35.945 Particles themselves do not swell when the gas expands. 00:00:36.095 --> 00:00:40.831 Gas particles are further apart; their individual sizes need not change. 00:00:45.500 --> 00:00:50.289 Density is mass per unit volume: rho equals mass divided by volume. 00:00:50.439 --> 00:01:01.115 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. 00:01:03.767 --> 00:01:09.825 Rearrange to mass equals density times volume, and volume equals mass divided by density. 00:01:09.975 --> 00:01:16.087 Convert carefully: one gram per cubic centimetre equals one thousand kilograms per cubic metre. 00:01:16.237 --> 00:01:20.690 One cubic centimetre equals ten to the power minus six cubic metres. 00:01:23.367 --> 00:01:29.008 The same material generally has lower density as a gas because particles are much further apart. 00:01:29.158 --> 00:01:35.761 Solids are often denser than liquids, but ice is less dense than liquid water because of its more open structure. 00:01:38.433 --> 00:01:42.692 If mass stays constant while volume increases, density decreases. 00:01:42.842 --> 00:01:47.674 Changes of state conserve mass in a closed system even when volume changes. 00:01:50.333 --> 00:01:53.340 Measure an object's mass using a zeroed balance. 00:01:53.490 --> 00:01:59.807 For a regular block, measure length, width and height and calculate volume equals length times width times height. 00:02:02.467 --> 00:02:11.820 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. 00:02:11.970 --> 00:02:14.551 Fully submerge it and avoid trapped bubbles. 00:02:14.701 --> 00:02:18.185 The volume of water displaced equals the solid’s volume. 00:02:18.335 --> 00:02:21.686 Subtract the starting volume; avoid trapped bubbles. 00:02:26.367 --> 00:02:34.450 Choose a method suitable for the material: an object that dissolves, absorbs water or floats needs a different liquid or an adapted procedure. 00:02:34.600 --> 00:02:38.402 Do not simply call a partly submerged volume its whole volume. 00:02:41.067 --> 00:02:46.537 For a liquid, weigh the empty dry container and then container plus a measured liquid volume. 00:02:46.687 --> 00:02:50.626 Subtract the empty-container mass before calculating density. 00:02:53.300 --> 00:02:55.625 Read liquid volume at eye level. 00:02:55.775 --> 00:02:59.341 For water, read the bottom of the curved surface (meniscus). 00:02:59.491 --> 00:03:04.429 Use an instrument with suitable scale divisions (resolution) and repeat readings. 00:03:07.100 --> 00:03:09.404 Use the same unit system throughout. 00:03:09.554 --> 00:03:20.837 A 54 grams block with volume 20 cubic centimetres has density 2.7 grams per cubic centimetre, equal to 2700 kilograms per cubic metre. 00:03:23.500 --> 00:03:28.356 Temperature is linked to the average kinetic energy of a substance’s particles. 00:03:28.506 --> 00:03:33.817 Internal energy is the total kinetic and potential energy of all its particles. 00:03:33.967 --> 00:03:39.050 It depends on how much substance there is, the material, its temperature and its state. 00:03:41.700 --> 00:03:51.575 Heating transfers energy into a system, usually increasing particle kinetic energy and temperature, or changing particle potential energy during a state change. 00:03:51.725 --> 00:03:54.455 A temperature is not an amount of energy. 00:03:57.133 --> 00:04:06.029 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). 00:04:06.179 --> 00:04:11.809 Units are joules per kilogram per degree Celsius, equivalently joules per kilogram per kelvin. 00:04:14.467 --> 00:04:23.325 Change in thermal energy, delta Q, equals mass times specific heat capacity times temperature change: M times C times delta theta. 00:04:23.475 --> 00:04:31.677 Use mass in kilograms, specific heat capacity in joules per kilogram per degree Celsius, and temperature change in degrees Celsius. 00:04:31.827 --> 00:04:36.669 A temperature increase of one degree Celsius equals an increase of one kelvin. 00:04:39.333 --> 00:04:54.672 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. 00:04:54.822 --> 00:04:58.543 Specific heat capacity includes “per kilogram” in its units. 00:05:03.200 --> 00:05:08.436 A material with greater C needs more energy for the same mass and temperature rise. 00:05:08.586 --> 00:05:14.159 A larger mass also needs more energy; C is a material property, not the mass itself. 00:05:16.833 --> 00:05:23.638 Insulation, a lid and reducing exposed area limit unwanted energy transfers during heating or cooling. 00:05:23.788 --> 00:05:27.650 Low thermal conductivity and trapped air can reduce cooling. 00:05:30.300 --> 00:05:32.348 Melting is solid to liquid; 00:05:32.498 --> 00:05:34.503 freezing is liquid to solid; 00:05:34.653 --> 00:05:37.512 boiling or evaporation is liquid to gas; 00:05:37.662 --> 00:05:40.030 condensation is gas to liquid; 00:05:40.180 --> 00:05:42.377 sublimation is solid to gas; 00:05:42.527 --> 00:05:44.725 deposition is gas to solid. 00:05:47.400 --> 00:05:52.902 These are physical changes: the material can recover its original properties when reversed. 00:05:53.052 --> 00:05:56.891 In a closed system, mass is conserved; the volume may change. 00:05:59.567 --> 00:06:03.257 During melting or boiling of a pure substance at fixed pressure, 00:06:03.407 --> 00:06:11.950 temperature stays constant while supplied energy changes the particle arrangements and potential energy rather than average kinetic energy. 00:06:14.600 --> 00:06:22.951 Specific latent heat L is energy needed to change the state of 1 kilogram without changing temperature, in joules per kilogram. 00:06:23.101 --> 00:06:30.577 Latent heat of fusion refers to melting or freezing; latent heat of vaporisation refers to liquid or gas change. 00:06:33.233 --> 00:06:36.842 Use Q equals M times L, with mass in kilograms. 00:06:36.992 --> 00:06:47.381 Melting 0.2 kilograms ice with L equals 334000 joules per kilogram requires 66800 joules for the state change alone. 00:06:50.033 --> 00:06:59.451 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. 00:06:59.601 --> 00:07:03.128 Do not use M times L for an ordinary temperature rise. 00:07:05.800 --> 00:07:10.137 During cooling, freezing and condensation release energy to surroundings. 00:07:10.287 --> 00:07:16.493 A flat region on a heating or cooling curve identifies a state-change stage under the stated conditions. 00:07:16.643 --> 00:07:21.291 Flat stages show energy used for a state change, not a temperature rise. 00:07:25.967 --> 00:07:29.741 Evaporation can occur at the liquid surface below the boiling point. 00:07:29.891 --> 00:07:33.863 Boiling occurs throughout the liquid at the boiling temperature for the pressure. 00:07:36.533 --> 00:07:46.164 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. 00:07:48.833 --> 00:07:55.456 Measure heater voltage and current and use E equals V times I times T, or use a suitable joulemeter. 00:07:55.606 --> 00:08:04.383 Stir gently for uniform temperature and record the final temperature; calculate C equals E divided by the product M times delta theta. 00:08:04.533 --> 00:08:08.522 Keep the heater immersed and use insulation to reduce heat loss. 00:08:13.200 --> 00:08:16.628 Insulate the container and use a lid to reduce losses. 00:08:16.778 --> 00:08:23.166 Keep the heater immersed and follow electrical safety procedures; avoid contact with hot water or heaters. 00:08:25.833 --> 00:08:35.111 Energy heating the container or escaping to surroundings can make calculated C too high if all supplied energy is incorrectly attributed to the water. 00:08:35.261 --> 00:08:38.723 Use appropriate corrections or discuss this limitation. 00:08:41.400 --> 00:08:50.014 For a melting-ice temperature, time graph, place a temperature probe in ice or water and record at regular intervals while energy is supplied. 00:08:50.164 --> 00:08:53.755 Mix carefully and keep the probe away from the heater and vessel wall. 00:08:56.433 --> 00:09:02.629 At ordinary pressure, a pure ice, water mixture stays near 0 degrees Celsius while the ice melts. 00:09:02.779 --> 00:09:05.897 Once all the ice has melted, the water warms up. 00:09:06.047 --> 00:09:13.384 A real graph may not have a perfectly flat section (plateau) because heat flow and the thermometer’s response affect the readings. 00:09:16.067 --> 00:09:20.303 Gas particles collide with container walls and exert forces on them. 00:09:20.453 --> 00:09:24.091 Pressure is force per area and is measured in pascals (Pa). 00:09:26.767 --> 00:09:32.728 For a fixed mass of gas at constant volume, raising temperature increases average particle speed. 00:09:32.878 --> 00:09:36.245 More frequent and stronger wall impacts increase pressure. 00:09:36.395 --> 00:09:40.715 Fixed volume and fixed gas mass are essential to the temperature comparison. 00:09:45.367 --> 00:09:54.070 In the ideal particle model, absolute zero corresponds to no random thermal motion and zero extrapolated gas pressure at fixed volume. 00:09:54.220 --> 00:10:01.844 It is approximately minus 273 degrees Celsius or 0 kelvin; real gases condense before reaching it. 00:10:04.500 --> 00:10:11.881 Convert approximately using temperature in kelvin equals temperature in degrees Celsius plus two hundred and seventy-three. 00:10:12.031 --> 00:10:17.683 Temperature in degrees Celsius equals temperature in kelvin minus two hundred and seventy-three. 00:10:17.833 --> 00:10:20.957 Kelvin temperatures are written without a degree symbol. 00:10:23.633 --> 00:10:28.010 20 degrees Celsius is approximately 293 kelvin. 00:10:28.160 --> 00:10:38.270 A 10 degrees Celsius temperature increase is a 10 kelvin increase; adding 273 is for converting absolute readings, not temperature differences. 00:10:40.933 --> 00:10:47.525 The simple gas model has limits: particles have finite size, attractions matter and phase changes occur. 00:10:47.675 --> 00:10:54.159 Do not extrapolate an ordinary gas-pressure experiment as though the gas stays gaseous down to absolute zero. 00:10:56.833 --> 00:10:58.860 That completes Particle model. 00:10:59.010 --> 00:11:02.355 Revisit the notes and test yourself on the revision website.