Acids, Alkalis and Solubility

Question 1

Where do acids and alkalis lie on the pH scale?

Answer 1

Neutral - 7
Acids - Less than 7
Alkalis - More than 7

Question 2

What is universal indicator?

Answer 2

An indicator made from a mixture of different indicators and produces a range of colours based on the pH

Question 3

What colour does methyl orange indicator turn in acid/alkaline solutions?

Answer 3

Acid - red
Alkaline - yellow

Question 4

What colour does litmus indicator turn in acid/alkaline solutions?

Answer 4

Acid - red
Alkaline - blue

Question 5

What colour does phenolphthalein indicator turn in acid/alkaline solutions?

Answer 5

Acid - colourless
Alkaline - pink

Question 6

What’s an ion?

Answer 6

An atom that has become charged by losing/gaining electrons

Question 7

What would happen when an acid is dissolved in water?

Answer 7

When acids are dissolved in water, they produce an excess of hydrogen ions (H+)
e.g. hydrochloric acid is formed when hydrogen chloride dissolves in water and splits into H+ and Cl- ions

Question 8

What happens when alkalis are put in water?

Answer 8

Alkalis produce an excess of OH- ions when in water
e.g. sodium hydroxide splits into Na+ and OH- ions when it dissolves

Question 9

What happens to the acidity/alkalinity and pH of a solution,
as more hydrogen/hydroxide ions are added?

Answer 9

As more hydrogen ions (H+) are added to the same volume, the concentration increases. The higher the concentration of H+ ions, the more acidic the solution becomes, which makes the pH lower

The higher the concentration of hydroxide ions (OH-) in a solution, the more alkaline it becomes, which increases the pH

Question 10

How is the concentration of OH- ions and H+ ions in water?

Answer 10

Water is a pure substance with a pH of 7
So, it contains low and equal concentrations of OH- and H+ ions

Question 11

How do you work out the concentration of a solution?

Answer 11

Concentration (g dmcubed) = amount dissolved/volume of solution

Question 12

How do you convert cmcubed to dmcubed?

Answer 12

cmcubed —-> dmcubed = divide cmcubed by 1000

Question 13

What’s a concentrated solution?

Answer 13

A concentrated solution is a solution that contains a lot of dissolved solute per unit volume

Question 14

How does concentration of hydrogen ions in an acid relate to the pH scale?

Answer 14

If the concentration of hydrogen ions is increased by
a factor of 10, the pH decreases by 1.
If the concentration decreases by a factor of 10, the pH increases by 1
e.g. a solution with a pH of 5 would have a concentration of H+ ions 10 x 10 = 100 times greater than a solution of pH 7

Question 15

An acid with a pH of 3 is diluted by a factor of 10. What will the
new pH be?

Answer 15

pH 4

Question 16

What’s the difference between strong and weak acids?

Answer 16

Strong acids - Their molecules completely dissociate (break up) into ions when they dissolve in water, they produce a high concentration of H+ ions
weak acids - Their molecules do not dissociate completely into ions in solution,

Question 17

What are bases?

Answer 17

Bases are substances used to neutralise acids to form salt and water only - all metal oxides are bases

Question 18

What happens during neutralisation?

Answer 18

During neutralisations, the hydrogen ions in the acid combine with oxide ions to form water. This removes the hydrogen ions, therefore increasing the pH which makes the solution become more neutral and less acidic

Question 19

Complete the word equation: magnesium oxide + sulfuric acid —->

Answer 19

magnesium oxide + sulfuric acid —> magnesium sulfate + water

Question 20

How can you prepare and insoluble salt from an insoluble base? e.g. formation of tin chloride

Answer 20

1) add excess tin oxide to hydrochloric acid
2) Gently warm the mixture (e.g. water bath) to speed up the reaction
3) Filter to remove unreacted solid from solution, left with tin chloride solution
4) Heat to evaporate water and concentrate the salt solution
5) Leave water to evaporate, so that crystallisation can occur

Question 21

Why is an excess of the base always added to the reaction?

Answer 21

To ensure that all the acid is used up

Question 22

What’s a base that can dissolve in water called?

Answer 22

Bases that can be dissolved in water are called alkalis

Question 23

Why are all alkalis bases, but not all bases
are alkalis?

Answer 23

Not all bases are soluble, only soluble bases are alkalis

Question 24

Which bases are alkalis?

Answer 24

Group 1 hydroxides - e.g. sodium hydroxide
Group 2 hydroxides - e.g. calcium hydroxide

Question 25

What is the format for the chemical formula of group 2 hydroxides?

Answer 25

chemical symbol(OH)2

Question 26

What happens during a neutralisation reaction?

Answer 26

The hydrogen ions from the acid react with the hydroxide ions from the alkali. So, water is formed in the reaction
The other ions from the acid and alkali stay in the solution as ions of the dissolved salt. These ions combine to form a salt when the water evaporates

Question 27

What type of solution is it important to have before evaporating the water for crystallisation, when obtaining soluble salts from alkalis? - why?

Answer 27

It is important to have a neutral solution before evaporating the water, because otherwise you will contaminate the
salt with an excess of one reactant

Question 28

What are the products when acids react with metal carbonates?

Answer 28

Salt, water, carbon dioxide

Question 29

What would you see when an acid reacts with metal carbonates?

Answer 29

Bubbles of carbon dioxide
solid metal carbonate disappears if there is enough acid to react with it

Question 30

What happens in terms of ions during a reaction between and acid and metal carbonate?

Answer 30

The hydrogen ions from the acids react with the
carbonate ions to form water and carbon dioxide molecules

Question 31

What are the products when a metal reacts with an acid?

Answer 31

Salt + hydrogen

Question 32

What’s a precipitate reaction?

Answer 32

A reaction in which 2 soluble substances in solutions cause an insoluble precipitate to form

Question 33

Which substances are not soluble in water?

Answer 33

Silver and lead chlorides
Lead, barium and calcium sulphates
Most carbonates
Most hydroxides

Question 34

What’s a precipitate?

Answer 34

The insoluble salt formed when two soluble salts react