Elements, compounds and mixtures

Question 1

What are elements

Answer 1

Elements are pure substances made up of only one type of atom

Question 2

Name the diatomic elements

Answer 2

Iodine
Bromine
Chlorine
Fluorine
Oxygen
Nitrogen
Hydrogen

Question 3

What are compounds

Answer 3

Compounds are pure substances made up of two or more different elements that are chemically joined together

Question 4

What are mixtures

Answer 4

Mixtures are made up of two or more substances which are not chemically joined together

Question 5

What are mixtures of metals called

Answer 5

Alloys

Question 6

What are solutions

Answer 6

Solutions are a type of mixture which form when a solute dissolves in a solvent

Question 7

What is a homogeneous mixture

Answer 7

A homogeneous mixture is a mixture in which the composition is the same throughout

Question 8

What is a heterogeneous mixture

Answer 8

A heterogeneous mixture is a mixture where the composition is not uniform throughout

Question 9

What are miscible liquids

Answer 9

Miscible liquids are liquids that form a clear solution when mixed together

Question 10

What are immiscible liquids

Answer 10

Immiscible liquids are liquids that cannot mix together and form separate layers instead

Question 11

Give an example of miscible liquids and immiscible liquids

Answer 11

Miscible: Ethanol and Water
Immiscible: Oil and Water

Question 12

How can you identify pure substances

Answer 12

Pure substances have a sharp melting/boiling point

Question 13

How do impure substances melt/boil

Answer 13

Impure solids tend to melt over a range of temperatures always lower than the pure substances. Impure liquids boil over a range of temperatures, however if a solid is dissolved in a liquid, it raises the boiling temperature of the liquid

Question 14

Give differences between mixtures and compounds in terms of composition, properties, melting and boiling temperatures, separation and preparation

Answer 14

Composition: Compounds have a fixed composition where the percentage of each element is always the same, while mixtures don’t have a fixed composition and the percentage of each substance is not always the same

Properties: A compound has its own set of physical and chemical properties, while a mixture keeps the same properties as the substances it is made of

Melting and boiling temperatures: A compound has a fixed melting and boiling temperature, while a mixture melts and boils over a range of temperatures

Separation: A compound needs to be broken down by a chemical reaction, while a mixture can easily be broken down by physical means

Preparation: A chemical reaction needs to take place when a compound is made, while mixtures do not require a chemical reaction

Question 15

What is filtration used to separate

Answer 15

An insoluble solid from a liquid
Two solids, where one is soluble and the other is insoluble in a solvent

Question 16

Describe the process of filtration

Answer 16

The mixture is poured into a filter funnel lined with a piece of filter paper. The solid particles are too big to pass through the filter paper, and the solid thus remains on the filter funnel and is collected as the residue. The liquid on the other hand manages to pass through and is collected in a conical flask as the filtrate. The solid is then dried between two pieces of filter paper or in an oven

Question 17

Where is filtration used in real life

Answer 17

In the making of drinking water, as all solid impurities are removed by filtration

Question 18

What is distillation used to separate

Answer 18

A solvent from a solution
Two miscible liquids with a large difference in their boiling temperature

Question 19

Describe the process of simple distillation

Answer 19

The mixture is heated together with anti-bumping granules in a distillation flask. The substance with the lowest boiling temperature turns into vapor, rises up the the flask and enters the condenser. In the cold condenser, the vapor condenses back into a liquid and is collected in a conical flask as the distillate. The substance with the highest boiling point is left behind in the distillation flask

Question 20

Why is water coming in from the bottom of the condenser instead from the top

Answer 20

When water enters the condenser from the top, both the gravitational force and the force of input are acting downwards, so the water falls quickly and does not reach full volume. When water enters from the bottom, the gravitational force is acting downwards while the input force is acting upwards. Thus, the condenser is filled more slowly and reaches full volume. This leads to cooling being more uniform and no air bubbles forming

Question 21

Write down 6 precautions for simple distillation

Answer 21

Type of heating: If flammable liquids are to be separated a naked flame should be avoided. Use a water bath or an electric heated instead

The bulb of the thermometer: The bulb of the thermometer should be placed at the mouth of the condenser so that the exact boiling temperature of the liquid can be recorder

Anti-bumping granules: Anti-bumping granules such as glass beads must be used to allow the mixture to boil smoothly by preventing violent boiling

Connections to the condenser: Water in the condenser should be coming in from the bottom and out from the top. This prevents air from being trapped in the condenser, something that would reduce efficiency and increase the risk of the condenser cracking

Collection vessel: The collection vessel should have a narrow opening to reduce the evaporation of volatile distillates

Open apparatus: The apparatus should be open to prevent pressure form building up when it is heated

Question 22

What are some uses of distillation in real life

Answer 22

Used to make whisky, gin and brandy
Drinking water

Question 23

What is fractional distillation used to separate

Answer 23

Miscible liquids with a small difference in their boiling temperature (less than 30 degrees)

Question 24

What is a fractionating column and what does it do

Answer 24

The fractionating is a glass tube packed with glass beads that provide a large surface area on which condensation can take place

Question 25

Describe the process of fractional distillation

Answer 25

The mixture of water and ethanol is heated in a distillation flask together with some anti-bumping granules. Both liquids evaporate together and a mixture of vapors enters the fractionating column. The large surface area provided by the glass beads inside the column allows separating of vapors to take place. The vapor from the liquid with the lower boiling temperature enters the condenser first, where it condenses back into a liquid and is collected in a conical flask. Once all of the first liquid has distilled, on further heating the higher boiling point liquid will distill over and be collected

Question 26

Where is fractional distillation used in real life

Answer 26

To obtain ethanol from a mixture of ethanol and water in the fermentation of sugar Separation of compounds in crude oil

Question 27

What is crystallization used to separate

Answer 27

A soluble solid from its aqueous solution

Question 28

Describe the process of crystallization

Answer 28

Heat the solution in an evaporating dish to the point of crystallization until a saturated solution is formed. This can be checked by removing a small quantity of the solution with a glass rod and holding it in the air. If crystals form quickly, then the solution is saturated. The saturated solution is then slowly cooled and crystals start to form. These crystals are filtered from the rest of the solution, washed with cold solvent and dried between two pieces of filter paper

Question 29

How does the speed of the cooling process of crystallization affect the crystals formed

Answer 29

The slower the cooling process, the bigger and purer the crystals will be The faster the cooling process, the smaller and less pure the crystals will be

Question 30

Where is crystallization used in real life

Answer 30

To obtain salt from sea water To purify sugar

Question 31

How can two solids be separated with crystallization

Answer 31

When cooling, the most soluble solid will remain in the solution, while the least soluble will crystallize out

Question 32

What is chromatography used to separate

Answer 32

Dyes from ink Pigment from plants Amino acids

Question 33

Describe the process of chromatography

Answer 33

A spot of the mixture is placed on a pencil line above the bottom edge of a strip of chromatography paper. The chromatography paper is then placed in a boiling tube containing a suitable solvent. The boiling tube is closed with a rubber bung to prevent the evaporation of the solvent that can make the results inaccurate. The solvent begins to rise up the chromatography paper and the different components of the mixture begin to separate. A solute that is very soluble in the solvent travels along the paper faster than a solute that is slightly soluble. The final result is a chromatogram

Question 34

What does the retardation factor calculate

Answer 34

The Rf factor calculates the movement of a substance during chromatography relative to the movement of the solvent

Question 35

What is the equation for the Rf factor

Answer 35

distance traveled by solute/ distance traveled by solvent

Question 36

What precautions do you need to take for a chromatography experiment

Answer 36

A pencil line and not ink line is used to mark the position of the spot of the mixture. This is because ink will also separate and interfere with the results The beaker should be covered so that the filter paper is surrounded by the vapor of the solvent. This prevents the solvent from escapin which would result in an inefficient separation The level of the solvent should be below the spot of the mixture. Otherwise, the mixture will dissolve in the solvent and separation won't occur