Elements, Compounds and Mixtures

Question 1

What is an element?

Answer 1

• A pure substance made up from the same type of atom (same number of protons)
• Examples: O and Mg₂

If there is only one atom in the substance, than it is just an atom, if it has more than one bonded together, it is a molecule. In this case, both are elements.

Question 2

What is a compound?

Answer 2

• A pure substance made up of two or more different elements bonded together, which cannot be seperated by physical means
• Examples: NH₄ and MgO

A compound must have different elements, if it is the same element bonded together than it is a molecule (although molecules can have different elements too).

Question 3

What is a mixture?

Answer 3

• A combination of two or more substance (elements or compounds) which are not bonded together and can be seperated by phsyical means
• Examples: Air

Question 4

What is a chemically pure substance?

Answer 4

• A substance which only contains one kind of compound or element

If anything else is added, it becomes a mixture

Question 5

Are the boiling and melting points in a chemically pure substance fixed or unfixed

Answer 5

As long as it is the same pure mixture, it will always be the same

Question 6

Are the boiling and melting points in a chemically unpure substance fixed or unfixed

Answer 6

Unfixed as each substance will have a different melting/boiling point inside of the mixture, broadening its range

Question 7

How do you test for the purity of a substance?

Answer 7

• Slowly heat a substance, and compare its melting/boiling point to data books
• The closer it is to the value, the purer the substance is

Question 8

What is simple distillation used to seperate?

Answer 8

A liquid from a soluble solid (that has dissolved to form a solution)

Important: This process is more favourable for obtaining the liquid, while crystallisation is more favourable for obtaining the solute

Question 9

What is the process of simple distillation?

Answer 9

• The solution is heated, and the liquid will evaporate, producing a vapour which rises and is filtered into a vapour condenser
• In the condenser, the vapour condenses and turns into the pure version of the liquid which is collected in a beaker
• The pure solute will be left behind

The condenser just cools the vapour to its condensation point, and it does this by having a constant stream of water running through it

Question 10

What is fractional distillation used to seperate?

Answer 10

Two or more liquids with different boiling points from a mixture

Question 11

What is the process of fractional distillation?

Answer 11

• The solution is heated to the temperature of the liquid with the lowest boiling point, which evaporates and produces a vapour which rises and is filtered into a vapour condenser
• It will then condense and the pure version of this liquid will be collected in a beaker
• If there are more than two liquids to seperate, you can repeat this with the liquid that has the 2nd lowest boiling point and so on, collecting each liquid in a different beaker
• Once done you will be left with the pure and unmixed versions of the liquids

Question 12

What is filtration used to seperate?

Answer 12

A liquid from an insoluble solid (which has not dissolved)

Question 13

What is the process of filtration?

Answer 13

• A piece of filter paper is placed in a funnel above a beaker
• The mixture of the insoluble solid and the liquid are poured onto the filter paper
• The small liquid particles will pass through the filter paper as a filtrate, leaving the larger insoluble solid molecules behind

Question 14

What is crystallisation used to seperate?

Answer 14

A soluble, dissolved solid from a liquid (in a solution)

Question 15

What is the process of crystallisation?

Make sure you understand

Answer 15

• The solution is heated, evaporating the solvent which increases the concentration of solute, leading to it becoming a saturated solution
• The saturated solution is left to cool, and crystals of the solute will begin to form due to decreasing solubility (caused by the temperature drop)
• Seperate the crystals and the remaining solvent/solution by decanting
• Wash them with water and leave them to dry in a warm place

• The new crystals only form because the solution is saturated - which is the why the solvent is evaporated at the start to form the saturated solution
• On top of that, the reason why the solute is dissolved at the starting temperature before the experiment is because there is enough solvent so the solution is not saturated

Question 16

What is chromatography used to seperate?

Answer 16

Different soluble substances that have a different solubility in a mixture

Question 17

What is the process of chromatography?

Answer 17

• Draw a pencil line on the chromatography paper, and place spots of the different samples across it
• Lower the paper into solvent, but make sure the pencil line sits above the solvent so the samples do not dissolve into the solvent (stationary phase)
• The solvent will travel up the paper by capillary action, taking some of the samples with it (mobile phase)
• The parts of the samples with a higher solubility will travel further up the paper than the parts with a lower solubility

Question 18

What will a chromatogram look like with a pure sample?

Answer 18

There will only be one spot

Question 19

What will a chromatogram look like with an impure sample?

Answer 19

There will be multiple spots up the chromatogram, the distance travelled of each depending on solubility

Question 20

What is the solvent front?

Answer 20

The line at the top of the solvent indicating how far the solvent has travelled

Question 21

How do you calculate the Rf value of a substance?

The Rf value of a substance will be constant, but will change depending on solvent

Answer 21

Distance travelled by substance/distance travelled by solvent

• The value will always be between 0-1, higher meaning higher solubility
• The distances travelled are always between the baseline (pencil line) and the substance/solvent front respectively

Question 22

What can Rf values be used for?

Answer 22

Comparing the Rf value of an unknown substance to that of a known substance to identify the substance