Purity (Purification).

Question 1

What is a mixture?

Answer 1

Two or more substances mixed together but not chemically combined.

NOTE - The chemical properties of each substance in a mixture are unchanged.

Question 2

How do we know if a substance is pure?

Answer 2

Pure substances have a sharp melting point and boiling point whereas if a substance is impure, the impurities will have an effect on the melting and boiling point.

FURTHER EXPLAINATION : Pure water has an exact boiling point of 100°C whereas impure water could have a boiling point of 103°C.

Question 3

Why is purity important (for consumption)?

Answer 3

Medical drugs and food additives must be pure to avoid harming people. To have a pure substance for food and drugs is very important as impurities could be dangerous even in small amounts.

Question 4

A solution contains a solid dissolved in a solvent. Which is the solute and which is the solvent?

Answer 4

The dissolved solid is called the solute and the liquid that dissolves the solid is the solvent.

Question 5

Define filtration.

Answer 5

Separating a solid from a liquid by using filter paper (the precipitate is insoluble).

Question 6

What is the filtrate and what is the residue?

Answer 6

The solution which passes through the filter paper is called the filtrate and the solid that stays on the filter paper is called the residue.

Question 7

Define decanting.

Answer 7

To gradually pour another liquid from one container into another in order to separate out sediment.

Question 8

What is centrifugation/ what is the use of a centrifuge machine?

Answer 8

The separation of a solid from a liquid by rapid spinning during the solid collects at the bottom of the test tube - the liquid can then be decanted off carefully.

Question 9

What is crystallization?

Answer 9

The formation of crystals when a saturated solution is left to cool.

Question 10

If you were to have produced a soluble salt, how would you separate this salt from the solution that it was dissolved in?

Answer 10

• Warm the solution in an open container allowing the solvent to evaporate, leaving a saturated solution..
• Allow this solution to cool.
• The solid will come out of the solution and crystals will start to grow, these can be collected and allowed to dry.

Question 11

What is simple distillation?

Answer 11

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

Question 12

When does simple distillation work?

Answer 12

Works when the liquids have different boiling points.

Question 13

What is the sequence of events in distillation?

Answer 13

Heating → evaporating → cooling → condensing.

Question 13

What is the sequence of events in distillation?

Answer 13

Heating → evaporating → cooling → condensing.

Question 14

How do you separate ethanol and water?

Answer 14

Ethanol has a lower boiling point than water (ethanol boiling point 78°C VS water boiling point 100°C) so it evaporates first. The ethanol vapor is then cooled and condensed inside the condenser to form a pure liquid.

Question 15

How does the fractional distillation of crude oil (petroleum) and products for fermentation work?

Answer 15

• The oil is heated in the fractionating column and the oil evaporates and condenses at a number of different temperatures.
• The many hydrocarbons in crude oil can be separated into fractions each of which contains molecules with a similar number of carbon atoms.
• The fractionating column works continuously, heated crude oil is piped in at the bottom. The vaporized oil rises up the column and the various fractions are constantly tapped off at the different levels where they condense.
• The fractions can be processed to produce feuls and feedstock for the petrochemical industry.

NOTE - The separation of the components in petroleum is achieved by fractional distillation on an industrial scale.

Question 16

What is fractional distillation used for?

Answer 16

Used to separate two or more liquids that are miscible with one another.

Question 17

What are the steps to separating liquids using fractional distillation?

Answer 17

• The solution is heated to the temperature of the substance with the lowest boiling point.
• This substance will rise and evaporate first, and vapors will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker.
• All of the substance is evaporated and collected, leaving behind the other components.

Question 18

What are the steps to separating liquids using simple distillation?

Answer 18

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

Question 19

What can simple distillation be used to do?

Answer 19

Simple distillation can be used to separate the products of fermentation, such as alcohol and water.

Question 20

How can immiscible liquids (oil and water) be separated?

Answer 20

Immiscible liquids can be separated using a separating funnel or by decanting.

Question 21

What are the steps of separating undissolved solid from a liquid using filter paper?

Answer 21

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

Question 22

Appropriate apparatus for measuring:

• Time.
• Temperature.
• Mass.
• Volume.

Answer 22

• Time = stopwatch/ clock.
• Temperature = thermometer.
• Mass = balance.
• Volume = measuring cylinder/ burette/ pipette.

Question 23

What is paper chromatography used for?

Answer 23

Used to separate mixtures and give information to help identify substances.

Question 24

What does paper chromatography involve?

Answer 24

Involves a stationary phase and a mobile phase.

Question 25

How do you carry out paper chromatography?

Answer 25

• Place substances on line near bottom of the paper.

- Place in solvent and observe how far the substances travel up the paper.

Question 26

What does paper chromatography separation depend on?

Answer 26

Separation depends on the distribution of substances between the phases.

Question 27

What is the formula for Rf value?

Answer 27

Rf value = distance moved by substance ÷ distance moved by solvent.

Question 28

Different compounds have different Rf values in different solvents, which can be used to help identify the compounds. How?

Answer 28

Compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.

Question 29

How will an impure substance show up on the paper and how will a pure substance show on the paper?

Answer 29

An impure substance will show up with more than one spot, a pure substance should only show up with one spot.

Question 30

How will a mixture show up on the paper?

Answer 30

If a substance is a mixture, it will separate on the paper to show all the different components as separate spots.

Question 31

How will two substances which are the same show up on the paper?

Answer 31

If two substances are the same, they will show up the same on the paper after carrying out the chromatogram.

Question 32

What happens on the paper when the substance is more soluble?

Answer 32

The more soluble a substance is, the further up the paper it travels.

Question 33

What are locating agents?

Answer 33

Locating agents are substances that react with the sample and produce a colored product which is then visible which allows for Rf values to be taken and molecules to be identified.

Question 34

When is a chromatogram treated with a locating agent?

Answer 34

The chromatogram is treated with the agent after the chromatography run has been carried out, making the sample runs visible to the naked eye.

Question 35

What are the examples of locating agents?

Answer 35

Ninhydrin used with proteins - breaks them down into amino acids and colors them purple.
UV light.