ChemistryU8- Chemical Analysis

Question 1

What is meant by pure substances?

Answer 1

A substance made up of only 1 element or compound

Question 2

What is the test for pure water?

Answer 2

We measure the boiling points and melting points- so for pure water the melting point is exactly 0 degrees Celsius and the boiling point is exactly 100 degrees Celsius.

Question 3

What does anhydrous mean?

Answer 3

A substance containing no water

Question 4

What is the test for water?

Answer 4

Turns white anhydrous copper sulfate a blue colour

Question 5

The melting/ boiling points of compounds are called fixed points- how do we identify and differentiate mixtures and pure substances?

Answer 5

Use their fixed points data

Question 6

1- Why do melting and boiling points differ for mixture? 2- What do impurities do to the MP and BP of a substance?

Answer 6

1- Fixed points vary due to mixture composition

2- Lower the melting point but raise boiling points

Question 7

What is the correlation between MP and BP for pure substances?

Answer 7

Range of melting points and boiling points are very narrow

Question 8

How do the graphs for melting points of impure and pure substances differ? Draw the graphs and label the axis’s

Answer 8

Pure substances have a sharp melting point but mixtures melt over a range of temperatures. This difference is most easily seen when the temperature of a liquid is measured as it cools and freezes. With pure substances the change of state to a liquid stays at a straight horizontal line at the same temperature but with impure substances its a curve.

Question 9

1- Define solvent

2- Define solute

Answer 9

1- the liquid in which a solute is dissolved to form a solution.
2- component in a solution, dissolved in the solvent.

Question 10

What is a formulation?

Answer 10

Useful mixtures that have been designed to make useful products and they carry out specific functions

Question 11

Why are formulations important?

Answer 11

They work to perform a specific function and allow us to use active drugs effectively and safely.

Question 12

Give 4 ingredients that can be added to a formulation and state its purpose

Answer 12

Pigment- Visual aesthetics
Solvent/ water- Makes the formulation more dilute and gives a desired consistency
Binder- Binds everything together so its useable
Flavouring- Makes the formulation better tasting and enjoyable for children or even adults.

Question 13

Give 3 examples of formulations

Answer 13

Any 3 from: pesticides, detergents, fertilisers, fuels, skincare, pharmaceutical drugs, cosmetics, paint etc

Question 14

What percentage of an active drug is allowed to be used?

Answer 14

5-10% of the active drug

Question 15

What is Chromatography?

Answer 15

A physical separation technique used to separate a mixture. Remember this means no chemicals are used and no new substances are produced

Question 16

Whats the pattern for melting and boiling of impure substances?

Answer 16

Over a range of temperatures

Question 17

Describe the melting and boiling point of a pure substance using a graph

Answer 17

Start with ice and increase temperature then at around 0 degrees Celsius, the temperature stays still and this is the melting point. Then the temperature increases again but comes to a standstill at 100 degrees Celsius and stops rising as this is the boiling point- as both BP AND MP are fixed temperatures we know the water is pure.

Question 18

Why must we carefully measure the quantities of a formulation?

Answer 18

So that the product we are designing has the properties we need

Question 19

How do our substances separate in Paper Chromatography?

Answer 19

Based on their different solubilities

More soluble= further distance and less soluble= shorter distance

Question 20

In Chromatography, what is the paper and the solvent called?

Answer 20

The paper is the stationary phase as it doesn’t move

The solvent is the mobile phase as it does move

Question 21

Describe a 6 mark Chromatography Experiment ( 10 steps)

Answer 21

1) Get Chromatography paper and draw a straight pencil line on the bottom of the paper (2 cm from base).
2) Get your ink/colouring and place dots of each substance on the line at equal distances apart (1cm) using a capillary tube and label them (A-Z)
3) Place the bottom of the chromatography paper in solvent making sure the solvent is 1cm below the pencil line.
5) Pour a solvent into a beaker
6) Attach the paper using a glass rod
7) Place chromatogram in beaker until the bottom of the paper touches the solvent BUT make sure the ink is not touching the water and put a lid on the beaker to reduce EVAPORATION
8) capillary action will draw up the solvent.
9) Remove chromatogram and draw a solvent line at the top of the paper to see how far the solvent reached
10) Dry the paper

Question 22

In a Chromatography experiment, why do we use pencil and not pen?

Answer 22

Pencil is insoluble in water whereas pen dissolves in water (soluble).

Question 23

When we see 1 spot after chromatography experiment, what does this mean? and what about 2 or more spots

Answer 23

The substance is pure and not a mixture- but if there was 2 spots+ the substance is a mixture

Question 24

Why does Chromatography work?

Answer 24

The chemical in the mixtures will be attracted to the stationary phase ( the paper) to different extents- so chemicals that strongly attach to the paper do not move far but chemicals that are weakly attracted will move further up.

Question 25

How do pure chemicals and chemicals in a mixture differentiate in solvents?

Answer 25

Pure chemicals produce a single spot in all solvents

Chemicals in mixtures separate into different spots depending on the solvent

Question 26

What is the equation for retention factor?

Answer 26

distance travelled by component/ distance travelled by solvent

Question 27

What does the Rf value tell us about the solubility of a separated component?

Answer 27

larger Rf value means the component is more soluble and smaller Rf value means the component is less soluble

Question 28

How would you measure Rf values?

Answer 28

measure distance of ink on pencil line to how far ink went and divide it by the distance between the pencil line and solvent line.

Question 29

What does it mean if a component stays on the pencil line and doesn’t move?

Answer 29

The component is insoluble

Question 30

What is a capillary tube?

Answer 30

A very thin glass tube which uses capillary action to draw liquids in

Question 31

In the Chromatography required practical- why mustn’t the sides of the paper touch the sides of the beaker?

Answer 31

It can interfere the way the water moves

Question 32

In chromatography, when spots line up it means…

Answer 32

they contain the same substance

Question 33

How do we identify the chemicals in colours?

Answer 33

Calculate the Rf values and look up Rf value in a data base to identify the chemical

Question 34

What the units for Rf value?

Answer 34

TRICK QUESTION! there are no units for retention factor

Question 35

What are the 2 problems with using Rf values to identify chemicals?

Answer 35

different chemicals could have the same Rf values so we need to repeat the experiment using different solvents to narrow it down. Also if chemicals have never been analysed before, they wont appear on the database

Question 36

Describe the test for hydrogen

Answer 36

We light a burning splint and as hydrogen burns rapidly it produces a pop sound

Question 37

Describe the test for oxygen

Answer 37

We place a glowing splint into the tube of oxygen and the splint relights

Question 38

Describe the test for carbon dioxide

Answer 38

Limewater is an aqueous (water) solution of calcium hydroxide. Draw some Co2 into a pipette and bubble the gas into the limewater. If c02 is present, the limewater will turn cloudy after repeating this

Question 39

Describe the test for chlorine

Answer 39

Insert damp litmus paper into the mouth of a test tube. Chlorine will bleach the litmus paper and turn it white.

Question 40

What colour is litmus paper originally?

Answer 40

blue

Question 41

Why does the hydrogen test produce a squeaky pop?

Answer 41

Hydrogen is highly flammable so the pop is like a mini explosion!

Question 42

Balance and displace the chloride: Mg + HCL

Answer 42

Mg +2HCL —– mgCL2 +h2

Question 43

What is limewater and state the formula?

Answer 43

Calcium hydroxide CA(OH)2

Question 44

Why does the limewater turn cloudy?

Answer 44

because a calcium carbonate precipate is formed

Question 45

What is the 2nd reaction for the test of carbon dioxide?

Answer 45

Light a lit splint near the gas in question and if the match is extinguished the gas is C02 as the C02 displaced oxygen and prevented combustion!

Question 46

What do we use to produce oxygen?

Answer 46

Hydrogen Peroxide —- O2 and H20

Question 47

What safety precautions should we take when working with Chlorine? ( level of usage)

Answer 47

Dont use too much chlorine as its toxic!

Question 48

Why is calculating Rf values important?

Answer 48

Rf values are specific to the compounds and allow us to identify unknown substances or compounds

Question 49

Why does chromatography work?

Answer 49

Chemicals will separate due to their attraction to the mobile and stationary phases. Molecules that are more soluble in the solvent have a higher attraction to the mobile phase so move up faster. And molecules less soluble in the solvent are more attracted to the stationary phase and do not move up as fast

Question 50

How can we tell via chromatography if there is the same element in 2 different mixtures?

Answer 50

They will have the same Rf value and will move up the solvent at the same rate/distance