Unit 2 Flashcards
Element definition
An element is a pure substance that cannot be broken down into anything simpler by chemical methods.
Compound DEFINITION
A compound is a pure substance that consists of two or more elements chemically joined together.
Mixture
A mixture consists of two or more substances (elements or compounds) that are not chemically joined together.
Iron, sulphur, their mixture and compound
Iron: dark grey powder, attracted to magnet, hydrogen gas is produced when added HCl
Sulphur: yellow powder, not attracted to magnet, no effect when HCl is added
Mixture of iron and sulphur: yellowish black powder, only iron powder attracted to it , hydrogen gas is produced when HCl is added
Iron (II) sulphide: dark brown solid, not attracted to magnet, a gas with a bad egg smell hydrogen sulphide is produced
Main difference between mixture and compound (1)
Composition:
Mixture: it has a variable composition — you can vary the amount of each constituent in the mixture.
Compound: definite composition — you cannot vary the amount of each element in a compound.
Main difference between mixture and compound (2)
Energy change during formation:
Mixture: There is no or little energy change when a mixture is formed.
Compound: energy is usually released or taken in when a compound is formed.
Main difference between mixture and compound (3)
General properties:
Mixture: each constituent in a mixture retain its own properties.
Compound: the properties are different from the elements in it.
Main difference between mixture and compound (4)
Separating the constituents:
Mixture:
Compound:
Fractional distillation of liquid air (liquefied air)
- Liquid nitrogen boils at -196 degree Celsius
- Liquid oxygen boils at -183 degree Celsius.
- Liquid argon boils at -186 degree Celsius.
Different stages of fractional distillation
A. Purification
- Dust, water vapour and carbon dioxide are removed from air.
- If cooled, it turns into ice (solidified), which block the pipes
B. Liquefaction of air
- the purified air is compressed to a high pressure.
- air becomes hot during the process. The hot air is cooled down by water.
- the cold, compressed air is passed into the expansion chamber. It expands rapidly and becomes colder during the process.
- the process of compression followed by expansion is repeated until the air reaches -200 degree Celsius. At this temperature, air becomes a liquid.
C: fractional distillation of liquified air
- liquid air is bumped into the fractionating column. The column is warmer at the bottom than it is at the top where it is piped off and stored. Liquid oxygen is collected at the bottom of the column.
- since the boiling points of oxygen and argon are close, a second fractionating column is often used to separate them.
Test for oxygen
Oxygen supports burning. Test the unknown gas by placing a glowing splint in the sample of gas. If the splint relights, the gas is oxygen
