chem 13.3

Question 1

what is an element

Answer 1

a pure substance that contains only one kind of atom, which cannot be broken down into anything simpler by chemical methods

Question 2

what is a compound

Answer 2

a pure substance that contains two or more elements chemically bonded together
its formation is a chemical change, as it involved the formation of new substances. as a result, it can be broken downinto its constituent elements through chemical methods

Question 3

what is a chemical change

Answer 3

a chemical change is a change in which one or more new substances are formed

Question 4

what is a mixture

Answer 4

made up of two or more substances (elements/compounds) that are not chemically bonded together
its formation is a physical change, in which no formation of new substances are involved. as a result, its components can be separated by physical methods

Question 5

what is a physical change

Answer 5

a change in which no new substances are formed

Question 6

5 features of a compound

Answer 6

• constituent elements must be in a fixed ratio
• energy is usually taken in or released during formation
• its properties are very different from its constituent elements
• compounds have sharp melting / boiling points
• compounds can only be separated into its constituent elements through chemical methods

compounds also have a chemical formula

Question 7

what requires energy to be taken in

Answer 7

breaking things down requires energy to be taken (endothermic) as the products > reactants
reactants > products (exothermic)

Question 8

5 features of a mixture

Answer 8

• substances in a mixture can be mixed together in any proportion
• no or little energy change when a mixture is formed
• properties are similar to those of the substances making up the mixture
• purity: mixtures do not have a sharp mp / bp
• constituents can be separated by simple physical method

Question 9

what does having sharp melting / boiling points mean

Answer 9

absolute value / whole number

Question 10

what is an electrolyte

Answer 10

a substance that can be decomposed by electricity
a substance that can conduct electricity in its molten state / in aqueous solution

Question 11

what is a molten oxide

Answer 11

the melted version of an ionic compound

Question 12

how can copper be separated from copper (ii) oxide

Answer 12

by heating it with carbon (carbon reduction), which helps to extract metals from metal oxides

Question 13

how can aluminum be separated from aluminum oxide

Answer 13

by electrolysis of its molten oxide (electrolysis can be only used for substances that can conduct electricity)

Question 14

what are alloys

Answer 14

mixtures involving metals (the solvent of the mixture must be a metal)

Question 15

electrolysis of water experiment procedure

Answer 15

1. label 2 test tubes A / B, fill w/ pure water
2. fill up the electrolytic cell w/ pure water, add 5cm3 of dilute sulphuric acid to the pure water (helps increase electrical conductivity)
3. turn on power supply for 10 min
4. turn off after 10 minutes, stopper test tubes quickly
5. use a glowing splint to test the gas in test tube A (oxygen, relight)
6. use a burning splint to test the gas in test tube B (hydrogen, burn with pop sound)

Question 16

using the reaction of iron and sulphur as an example to explain the difference between a mixture and a compound in terms of composition

Answer 16

(variable composition/fixed composition)
mixture has no fixed ratio, for compound each iron particle combines with another sulphur particle

Question 17

using the reaction of iron and sulphur as an example to explain the difference between a mixture and a compound in terms of energy change during formation

Answer 17

(none or little / usually released or taken)
the constituent elements need to be heated to form the compound, while heat is not required in making a mixture

Question 18

using the reaction of iron and sulphur as an example to explain the difference between a mixture and a compound in terms of general properties:

Answer 18

(retains its own properties / different from constituent elements)
in the mixture iron is attracted to the magnet while sulphur isn’t + hydrogen gas is produced when reacting w hydrochloric acid
in the compound a bad egg smell is produced when reacting w hydrochloric acid + not attracted to a magnet

Question 19

using the reaction of iron and sulphur as an example to explain the difference between a mixture and a compound in terms of separation of constituents

Answer 19

(physical / chemical)
mixture can be separated using magnet (iron is attracted sulphur isn’t), while there is no simple method to separate iron (ii) sulphide back into its original elements