11 Chemical Changes

Question 1

What is a physical change?

Answer 1

A change in which no new substance is formed.

Question 2

What is a chemical change?

Answer 2

A change in which new substances in the form of elements or compounds are formed.

Question 3

State one difference between a chemical change and a physical change.

Besides the fact that chemical changes produces new substances.

Answer 3

Chemical changes are irreversible while physical changes are reversible.

Reversible means the mixture can usually be seperated easily by separation techniques.

Question 4

Look at the word equation below:
methane + oxygen —–(heat)—–> carbon dioxide + water

Identify the reactants and products in this chemical reaction.

Answer 4

reactants: methane, oxygen
products: carbon dioxide, water

Question 5

Look at the number of atoms before and after a chemical reaction.
What can you conclude about the relationship between the number of atoms before and after the reaction?

chemical reaction between methane and oxygen

Answer 5

The number of each type of atoms remains the same.

Both before and after the reaction:
There are 4 hydrogen atoms, 4 oxygen atoms and 1 carbon atom.

Question 6

What is combustion?

aka burning in air

Answer 6

It is a chemical reaction in which a substance is heated in the presence of oxygen to form one or more new substances.

e.g.
carbon + oxygen –(heat)—> carbon dioxide

Question 7

What is thermal decomposition?

Answer 7

It is a process in which a substance is broken down into 2 or more simpler substances upon heating.

e.g.
calcuim carbonate —-(heat)—-> calcium oxide + carbon dioxide

Question 8

What is oxidation?

Answer 8

A chemical change is which a substance gains oxygen.

e.g.
iron + oxygen + water ——> rust

Question 9

What is neutralisation?

Answer 9

A reaction where acid mixes with alkali to form salt and water.

e.g.
hydrochloric acid + sodium hydroxide ———> sodium chloride + water

Question 10

State 2 properties of acid.

Think taste, litmus test

Answer 10

1) have a sour taste
2) turns blue litmus paper red

Question 11

State 2 properties of alkali.

Think taste, litmus test

Answer 11

1) have a bitter taste
2) turns red litmus paper blue

Question 12

Give three examples of acid found in the lab.

Answer 12

1) hydrochloric acid
2) nitric acid
3) sulphuric acid

Question 13

Give three examples of alkalis found in the lab.

Answer 13

1) sodium hydroxide
2) calcium hydroxide
3) potassium hydroxide

Note that the names of alkalis usually ends with hydroxide.

Question 14

The reaction below shows a neutralisation.
hydrochloric acid + calcium hydroxide –> salt + water

Identify the name of the salt.

The name of the salt takes one part from the acid and one part from the alkalis.

Answer 14

calcium chloride

calcium from the alkalis and chloride from the acid

Question 15

What are the products of an acid-metal reaction?

Think: metal + acid —–> ??

Answer 15

salt and hydrogen

Use this acronym to remember:
MASH
Metal Acid Salt Hydrogen

Question 16

Describe the test for hydrogen gas.

Answer 16

Introduce a lighted splint to the gas. Hydrogen burns with a ‘pop’ sound.

Question 17

What are the products of an acid-carbonate reaction?

Think: carbonate + acid —–> ??

Answer 17

salt, water and carbon dioxide

Use this acronym to remember
CACoWS
Carbonate Acid Carbon dioxide Water Salt

Question 18

Describe the test for carbon dioxide gas.

Answer 18

Bubble the gas through lime water. Carbon dioxide gas produces a white precipitate in lime water.

Do not use limewater turns milky or chalky.

Question 19

The pH scale ranges from 0 to 14.
What is the pH ranges of acidic solutions?
What is the pH ranges of alkaline solutions?
What is the pH value of neutral solutions?

Colours of a universal indicator

Answer 19

acidic soluions: 0 to 6.9
alkaline solutions: 7.1 to 14
neutral solutions: 7

Question 20

How do we test that a solution is neutral using only litmus paper?

Both the blue and red litmus paper must be used.

Answer 20

A blue litmus paper remains blue and a red litmus paper remains red in a neutral solution.