WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.829 Unit S C 1: States of matter. 00:00:09.500 --> 00:00:15.025 Matter can be modelled as particles: atoms, molecules or ions depending on the substance. 00:00:15.175 --> 00:00:19.295 Drawn circles are a model, not a literal picture of every particle. 00:00:21.967 --> 00:00:26.533 In a solid, particles are close together and vibrate around fixed positions. 00:00:26.683 --> 00:00:30.105 In a crystalline solid they form a regular arrangement. 00:00:30.255 --> 00:00:32.559 A solid has a fixed shape and volume. 00:00:35.233 --> 00:00:39.469 In a liquid, particles are close together but can move past one another. 00:00:39.619 --> 00:00:45.080 A liquid flows and takes the shape of its container, while keeping approximately the same volume. 00:00:47.733 --> 00:00:52.639 In a gas, particles are much farther apart and move rapidly in random directions. 00:00:52.789 --> 00:00:57.099 A gas expands to fill its container and has no fixed shape or volume. 00:00:57.249 --> 00:01:00.340 Same particles; different arrangement and spacing. 00:01:00.490 --> 00:01:02.709 A simplified model, not to scale. 00:01:07.367 --> 00:01:12.391 Solids and liquids are difficult to compress because particles are already close together. 00:01:12.541 --> 00:01:17.183 Gases are readily compressed because there is much empty space between particles. 00:01:19.833 --> 00:01:26.105 The particles themselves do not expand, melt or change identity during a physical change of state. 00:01:26.255 --> 00:01:29.887 Their arrangement, separation, movement and energy change. 00:01:32.567 --> 00:01:34.551 Particles attract one another. 00:01:34.701 --> 00:01:38.100 The type and strength of attraction depend on the substance. 00:01:38.250 --> 00:01:47.013 For example, an ionic solid has strong attractions between ions, while a molecular solid has attractions between molecules (intermolecular forces). 00:01:49.667 --> 00:01:58.487 Heating a substance within one state generally increases particle kinetic energy: particles move faster, or vibrate more strongly in a solid. 00:02:01.167 --> 00:02:07.596 During melting or boiling, transferred energy helps overcome attractions and rearrange or separate particles. 00:02:07.746 --> 00:02:12.875 For a pure substance at constant pressure, temperature remains steady during the change. 00:02:15.533 --> 00:02:21.473 Melting and boiling take in energy; freezing and condensation transfer energy to the surroundings. 00:02:21.623 --> 00:02:25.237 Cooling within one state reduces average kinetic energy. 00:02:27.900 --> 00:02:37.007 Comparing states for one substance, gas particles usually have higher energy than liquid particles, which usually have higher energy than solid particles. 00:02:37.157 --> 00:02:41.692 Do not claim this comparison applies regardless of temperature or substance. 00:02:44.367 --> 00:02:48.077 Gas pressure results from particles colliding with container walls. 00:02:48.227 --> 00:02:53.623 A hotter sealed gas can exert greater pressure when volume and amount of gas are unchanged. 00:02:56.300 --> 00:03:00.833 Melting changes solid to liquid; freezing changes liquid to solid. 00:03:00.983 --> 00:03:03.437 These are reverse physical processes. 00:03:06.100 --> 00:03:11.720 Boiling or evaporation changes liquid to gas; condensation changes gas to liquid. 00:03:14.400 --> 00:03:19.679 Sublimation changes a solid directly into a gas, without becoming liquid first. 00:03:19.829 --> 00:03:23.735 The reverse change, from gas directly to solid, is deposition. 00:03:23.885 --> 00:03:27.051 Each pair has a forward and reverse physical change. 00:03:31.733 --> 00:03:35.770 Melting gives particles greater freedom to move while they remain close. 00:03:35.920 --> 00:03:38.821 Boiling separates particles much more widely. 00:03:41.500 --> 00:03:48.218 Freezing restricts particles to fixed positions, while condensation brings gas particles close enough to form a liquid. 00:03:50.900 --> 00:03:53.779 A physical change does not make a new substance. 00:03:53.929 --> 00:04:01.469 Water vapour, liquid water and ice all contain H 2 O molecules; suitable cooling or heating can reverse the change. 00:04:04.133 --> 00:04:09.115 Evaporation occurs at the surface of a liquid and can occur below its boiling point. 00:04:09.265 --> 00:04:12.802 Particles with sufficient energy escape into the gas phase. 00:04:15.467 --> 00:04:20.205 Boiling happens throughout a liquid at its boiling point and forms bubbles of vapour. 00:04:20.355 --> 00:04:22.637 The boiling point depends on the pressure. 00:04:22.787 --> 00:04:25.601 Both are physical changes from liquid to gas. 00:04:30.267 --> 00:04:35.345 A puddle drying on a cool day illustrates evaporation, not necessarily boiling. 00:04:35.495 --> 00:04:42.129 Water on the outside of a cold glass usually comes from condensation of water vapour in surrounding air. 00:04:44.800 --> 00:04:50.197 Dry ice is solid carbon dioxide and sublimes at normal atmospheric pressure. 00:04:50.347 --> 00:04:55.083 Frost can form by deposition of water vapour on a sufficiently cold surface. 00:04:57.733 --> 00:05:04.026 The visible white cloud above hot water consists mainly of tiny liquid droplets formed by condensation. 00:05:04.176 --> 00:05:06.587 Water vapour itself is invisible. 00:05:09.267 --> 00:05:12.551 Different substances have different melting and boiling points. 00:05:12.701 --> 00:05:17.874 Pressure also affects boiling point; always use the conditions supplied by the question. 00:05:20.533 --> 00:05:23.200 At a temperature below its melting point, 00:05:23.350 --> 00:05:25.057 a substance is solid; 00:05:25.207 --> 00:05:28.236 between its melting and boiling points it is liquid; 00:05:28.386 --> 00:05:30.605 above its boiling point it is gas, 00:05:30.755 --> 00:05:34.393 assuming the stated pressure and no chemical decomposition. 00:05:37.067 --> 00:05:42.409 For a substance melting at minus 10 degrees Celsius and boiling at 80 degrees Celsius, 00:05:42.559 --> 00:05:45.353 it is solid at minus 20 degrees Celsius, 00:05:45.503 --> 00:05:50.165 liquid at 25 degrees Celsius and gas at 100 degrees Celsius. 00:05:50.315 --> 00:05:54.733 At a transition temperature, two states can coexist during the change. 00:05:59.400 --> 00:06:05.105 At exactly a melting or boiling point, two states can coexist while the change is taking place. 00:06:05.255 --> 00:06:09.848 The temperature alone does not tell you that the whole sample has completed the change. 00:06:12.500 --> 00:06:20.104 Water at ordinary atmospheric pressure melts at about 0 degrees Celsius and boils at about 100 degrees Celsius. 00:06:20.254 --> 00:06:23.686 These familiar values are not the values for every substance. 00:06:26.367 --> 00:06:36.232 On a heating curve, sloping sections show temperature rising within a state; flat sections show melting and boiling of a pure substance under suitable constant conditions. 00:06:36.382 --> 00:06:41.224 Flat sections correspond to changes of state, rather than no energy input. 00:06:45.900 --> 00:06:54.411 Take care with negative temperatures: minus 20 degrees Celsius is colder than minus 10 degrees Celsius because it is further below zero. 00:06:57.067 --> 00:07:01.876 A useful state diagram should show spacing and arrangement as well as motion. 00:07:02.026 --> 00:07:09.128 Keep the same number and identity of particles when comparing one closed sample before and after a physical change. 00:07:11.800 --> 00:07:20.054 A simple sphere model cannot show all internal structure, real particle sizes, the full range of forces or three-dimensional motion. 00:07:20.204 --> 00:07:24.330 Arrows and labels communicate information the circles alone cannot. 00:07:27.000 --> 00:07:32.311 Gas particles move randomly between collisions; they do not all drift permanently upwards. 00:07:32.461 --> 00:07:39.307 Diffusion produces net movement from high to low concentration even though individual movements remain random. 00:07:41.967 --> 00:07:46.382 When gases mix in a closed container, they spread through available space. 00:07:46.532 --> 00:07:51.321 Mixing does not by itself mean a chemical reaction has formed a new substance. 00:07:54.000 --> 00:07:57.401 Use a substance’s properties to identify its state. 00:07:57.551 --> 00:08:06.679 A fixed shape suggests a solid; flowing while keeping a fixed volume suggests a liquid; easily compressing and expanding to fill a container suggests a gas. 00:08:09.333 --> 00:08:13.067 Practical heating and cooling observations need suitable equipment, 00:08:13.217 --> 00:08:15.819 eye protection and care with hot surfaces; 00:08:15.969 --> 00:08:19.827 temperature measurements and controlled pressure support interpretation. 00:08:22.500 --> 00:08:24.579 That completes States of matter. 00:08:24.729 --> 00:08:28.075 Revisit the notes and test yourself on the revision website.