1. D. Crowley, 2007

2.  To understand the terms melting and boiling point

3.  Recap - how can we separate the air into its constituents (nitrogen; oxygen; argon; carbon dioxide and water vapour)?  The air can be split into its components by cooling it until it becomes liquid  In liquid form, we can increase the temperature steadily, collecting each component of the air because they boil off at different temperatures…

4.  What are the three states?  Solid  Liquid  Gas

5.  Substances change states at given temperatures  This is very useful information to know  E.g. a mercury thermometer will not work below -39  C - can you guess why?  Below this temperature the mercury will freeze (become solid), so cannot move up or down the thermometer! SolidLiquidGas

6.  This is the temperature a substance turns from a solid to a liquid  Different substances have different melting points  E.g. water melts at 0  C; ethanol melts at -114  C

7.  So there is a fixed temperature where a substance turns from a solid to a liquid  Don’t forget, this point is also the temperature where a substance turns from a liquid to solid (its freezing point)  I.e - water melts at 0  C and above (solid to liquid); and it freezes at 0  C and below (liquid to solid)!

8.  This is the temperature at which a substance evaporates as quickly as it can (liquid to gas)  Different substances have different boiling points  E.g. water boils at 100  C; ethanol boils at 78  C

9.  So there is a fixed temperature where a substance boils from a liquid to a gas  Don’t forget, this point is also the temperature where a substance turns from a gas to liquid (its condensing point)  I.e - water boils at 100  C and above (liquid to gas); and it condenses at 100  C and below (gas to liquid)!

10.  You can identify the state of a substance if you know its temperature, melting and boiling point:  If the temp. is below the melting point, the substance will be a  If the temp. is between the melting and boiling point, the substance will be a  If the temp. is above the boiling point, the substance will be a Complete the sentences, using the either solid; liquid; or gas

11.  You can identify the state of a substance if you know its temperature, melting and boiling point:  If the temp. is below the melting point, the substance will be a solid  If the temp. is between the melting and boiling point, the substance will be a  If the temp. is above the boiling point, the substance will be a

12.  You can identify the state of a substance if you know its temperature, melting and boiling point:  If the temp. is below the melting point, the substance will be a solid  If the temp. is between the melting and boiling point, the substance will be a liquid  If the temp. is above the boiling point, the substance will be a

13.  You can identify the state of a substance if you know its temperature, melting and boiling point:  If the temp. is below the melting point, the substance will be a solid  If the temp. is between the melting and boiling point, the substance will be a liquid  If the temp. is above the boiling point, the substance will be a gas

14.  If you add other substances then the temperature your original substance freezes, melts and boils can change  E.g. salt is added to water to lower the melting point (and freezing point)  This is why salt is spread on roads during winter  Normally the water would freeze at 0  C  Adding the salt means the temperature must be even colder than 0  C for the ice to freeze - useful for cold nights!  E.g. solder is a mixture of tin and lead - this mixture results in the melting point being even lower than pure tin or lead, making it useful to join metals together

15.  Using Exploring Science 8 read through pages 78 and 79 and answer questions 1 to 7…

16. 1. Tungsten melts at 3410  C 2. Liquid sodium boils at 882  C 3. Liquid oxygen boils at -182  C 4. Boiling points are always higher than melting points, because the boiling point is the temperature where the liquid can get no hotter (although the gas can) - so must be higher than the melting point! 5. Iron freezes at 1535  C; oxygen freezes at -218  C; and sodium freeze at 97  C (these are also the temperatures they melt at)! 6. Wax A will be pure - a constant melting point. Wax B had different melting points, suggesting it contained other substances which altered the melting point, depending upon how much of them were present in the sample 7. Air is a mixture of substances (nitrogen; oxygen; argon etc…) so there is no fixed melting or boiling points.