D. Crowley, 2007. To understand the terms melting and boiling point

D. Crowley, 2007

To understand the terms melting and boiling point

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…

What are the three states?

Solid Liquid Gas

Substances change states at given temperatures This is very useful information to know

E.g. a mercury thermometer will not work below -39C - can you guess why?

Below this temperature the mercury will freeze (become solid), so cannot move up or down the thermometer!

Solid Liquid Gas

This is the temperature a substance turns from a solid to a liquid

Different substances have different melting points E.g. water melts at 0C; ethanol melts at -114C

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 0C and above (solid to liquid); and it freezes at 0C and below (liquid to solid)!

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 100C; ethanol boils at 78C

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 100C and above (liquid to gas); and it condenses at 100C and below (gas to liquid)!

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


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 between the melting and boiling point, the substance will be a liquid




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

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 0C Adding the salt means the temperature must be even colder

than 0C 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

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

1. Tungsten melts at 3410C2. Liquid sodium boils at 882C3. Liquid oxygen boils at -182C4. 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 1535C; oxygen freezes at -218C; and sodium freeze at 97C (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.

Documents

D. Crowley, 2007. To understand the terms melting and boiling point