1elements, compounds and mixtures

Question 1

element

Answer 1

a substance made up of atoms that all contain the same number of protons and cannot be split into anything simpler

Question 2

compound

Answer 2

a pure substance made up two or more elements chemically combined together. there are unlimited types of compounds. cannot be separated by physical methods of separation

Question 3

mixture

Answer 3

a combination of two or more substances that are not chemically joined together. can be separated by physical methods of separation

Question 4

pure substances

Answer 4

melt and boil and specific and sharp temperatures

Question 5

mixtures have

Answer 5

a range of melting and boiling points as they consist of different substances that tend to lower the melting point and broaden the melting point range

Question 6

melting and boiling points data can be used to

Answer 6

distinguish pure substances from mixtures

Question 7

melting point analysis is routinely used to accesss the

Answer 7

purity of drugs

Question 8

the influence of impurities can be seen more clearly on

Answer 8

heating / cooling curves

Question 9

simple distillation is used to

Answer 9

separate a liquid and soluble solid from a solution or a pure liquid from a mixture of liquids

Question 10

how does simple distillation work

Answer 10

the solution is heated and pure water evaporates producing a vapour which rises through the neck of the round bottomed flask. the vapour passes through the condenser where it cools and condenses, turning into the pure liquid that is collected in a beaker. after all the water is evaporated from the solution, only the solid solute will be left behind

Question 11

fractional distillation is used to

Answer 11

separate two or more liquids that are miscible with one another

Question 12

how does fractional distillation work

Answer 12

the solution is heated to the temperature of the substance with the lowest boiling point. this substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid the will be collected in a beaker. all of the substance is evaporated and collected, leaving behind the other components of the mixture.

Question 13

filtration is used to

Answer 13

separate an undissolved solid from a mixture of the solid and a liquid/ solution

Question 14

how does filtration work

Answer 14

a piece of filter paper is placed in a filter funnel above a beaker. a mixture of insoluble solid and liquid is poured into the filter funnel. the filter paper will only allow small liquid particles to pass through as filtrate. solid particles are too large to pas through the filter paper so will stay behind as a residue

Question 15

crystallisation is used to

Answer 15

separate a dissolved solid from a solution when the solid is much more soluble in hot solvent than in cold

Question 16

how does crystallisation work

Answer 16

the solution is heated, allowing the solvent to evaporate, leaving a saturated solution behind. test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution- if solution is saturated crystals will form on the rod. the saturated solution is allowed to cool slowly, crystals begin to grow as solids will come out of solution due to decreasing solubility. the crystals are collected by filtering the solution, they are washed with cold distilled water to remove impurities and are allowed to dry.

Question 17

paper chromatography is used to

Answer 17

separate substances that have different solubilities in a given solvent

Question 18

how does paper chromatography done

Answer 18

a pencil line is drawn on chromatography paper and spots of the sample are placed on it. pencil is used for this as ink would run into the chromatogram along with the samples. the paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent so the samples don’t wash into the solvent container. the paper is called the stationary phase. the solvent travels up the paper by capillary action, taking some of the coloured substances with it; it is called the mobile phase. different substances have different solubilities so will travel at different rates causing the substances to spread apart. this will show the different components of the ink/dye. if two or more substances are the same they will produce identical chromatograms. if the substance is a mixture it will separate on the paper to show all the different components as separate spots. an impure substance will show up with more than one spot, a pure substance should only show up with one spot

Question 19

pure substances will produce

Answer 19

only one spot on the chromatogram

Question 20

if two or more substances are the same

Answer 20

they will produce identical chromatograms

Question 21

if the substance is a mixture it will

Answer 21

separate on the paper to show all the different components as separate spots.

Question 22

an impure substance will produce

Answer 22

a chromatogram with more than one spot

Question 23

Rf values are used to

Answer 23

identify the components of mixtures

Question 24

the Rf value of a particular compound is always

Answer 24

the same but it is dependant on the solvent used- if the solvent changes then the value changes

Question 25

calculating the Rf value allows chemists to identify

Answer 25

unknown substances because it can be compared with Rf values of known substances under the same conditions- they are known as reference values

Question 26

Rf=

Answer 26

distance travelled by substance/ distance travelled by solvent

Question 27

hypothesis of investigating paper chromatography using inks and food colouring

Answer 27

Rf values can be used to identify the components of an unknown mixture by comparison with Rf values of known substances

Question 28

materials needed to investigate paper chromatography using inks and food colouring

Answer 28

a 250cm3 beaker, a wooden spill, a rectangle of chromatography paper, four known food colourings labelled A-D, an unknown mixture of food colourings labelled U, five glass capillary tubes, paper clip, pencil and ruler

Question 29

method to investigate paper chromatography using inks and food colouring

Answer 29

Use a ruler to draw a horizontal pencil line 2 cm from the end of the chromatography paper
Use a different capillary tube to put a tiny spot of each colouring A, B, C and D on the line
Use the fifth tube to put a small spot of the unknown mixture U on the line
Make sure each spot is no more than 2-3 mm in diameter and label each spot in pencil
Pour water into the beaker to a depth of no more than 1 cm and clip the top of the chromatography paper to the wooden spill. The top end is the furthest from the spots
Carefully rest the wooden spill on the top edge of the beaker. The bottom edge of the paper
should dip into the solvent
Allow the solvent to travel undisturbed at least three quarters of the way up the paper
Remove the paper and draw another pencil line on the dry part of the paper as close to the wet edge as possible. This is called the solvent front line
Measure the distance in mm between the two pencil lines. This is the distance travelled by the water solvent
For each of food colour A, B, C and D measure the distance in mm from the start line to the middle of the spot

Question 30

evaluation of investigating paper chromatography using inks and food colouring

Answer 30

The Rf values of food colours A, B, C and D should be compared to that for the unknown sample as well as a visual comparison being made

Question 31

conclusion of investigating paper chromatography using inks and food colouring

Answer 31

The use of chromatography and Rf values is a viable method of identifying unknown mixtures given reference material