Acids, Alkalis and Solubility (PAPER ONE)

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 (CHARGED PARTICLE)

Question 7

What would happen when an acid is dissolved in water? - how is hydrochloric acid formed?

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. They tend to have low pHs (0-2)
weak acids - Their molecules do not dissociate completely into ions in solution produce a low concentration of H+ ions, ionisation of weak acids are reversible reactions, pHs 2-6

Question 17

What’s a base

Answer 17

Any substance that reacts with an acid to form a salt and water only - all metal oxides are bases

Question 18

What happens during acid-alkali neutralisation reactions? (detail)

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

Core practical: Preparing copper sulfate

Answer 20

1) Measure 20cmcubed of dilute sulfuric acid using measuring cylinder and pour it into a conical flask
2) Warm the acid in a 50’c water bath
3) Add copper oxide powder to acid and stir
4) If all the copper oxide reacts, so disappears, add some more and stop when the copper oxide is in excess and no longer reacts
5) Filter the mixture and transfer the filtrate into evaporating basin
6) Heat evaporating basin by placing it over a beaker of water heated by a Bunsen burner and stop heating when crystals start to form
7) Pour solution into a watch glass and leave for a few days to allow all the water to evaporate

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? (for the formation of salt)

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 an 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

What's a precipitate?

Answer 33

The insoluble salt formed when two soluble salts react

Question 34

Test for hydrogen

Answer 34

Use a burning splint held at the open end of a test tube of the gas ▪ Creates a ‘squeaky pop’ sound

Question 35

Test for carbon dioxide

Answer 35

Bubble the gas through the limewater (calcium hydroxide solution) and it will turn cloudy

Question 36

What's a neutralisation reaction?

Answer 36

Reaction between an acid and a base

Question 37

What's the ionic equation for an acid and alkali neutralisation reaction?

Answer 37

OH- + H+ -----> H2O

Question 38

If the concentration of H+ ions increases by a factor of 100, what happens to the pH?

Answer 38

Decreases by 2

Question 39

In terms of ions, why is pH 7 neutral?

Answer 39

Has an equal concentration of OH- and H+ ions

Question 40

What are the formulas for... Hydrochloric acid Nitric acid Sulfuric acid ----- Sodium hydroxide Potassium hydroxide Calcium hydroxide

Answer 40

HCl HNO3 H2SO4 ---- NaOH KOH Ca(OH)2

Question 41

Which ionic compounds are soluble/insoluble in water?

Answer 41

SOLUBLE: - All common sodium, potassium and ammonium salts - All nitrates - Most chlorides (Silver and Lead chloride are INSOLUBLE) - Most sulfates (Lead, barium and calcium sulfate are INSOLUBLE) INSOLUBLE: - Most carbonates (Sodium, potassium and ammonium carbonate are SOLUBLE) - Most hydroxides (sodium, potassium, ammonium hydroxide are SOLUBLE)

Question 42

What do titrations do?

Answer 42

They allow us to accurately measure the volumes of an acid and an alkali required for neutralisation

Question 43

Explain how to carry out a titration.

Answer 43

1) Use a volumetric pipette to transfer 25cmcubed alkali into a conical flask (use a pipette filler) 2) Add 5 drops of e.g. phenolphthalein indicator to the alkali in the conical flask 3) Place on white tile to see the colour change more clearly 4) Fill the burette with an acid. 5) Add acid to the alkali until the solution is neutral. Once the solution begins to go colourless/a colour change is seen, add the acid drop by drop, swirling the conical flask 6) Once a full colour change is completed, record the volume of acid added from the burette - bottom of the meniscus at eye level to prevent parallax error

Question 44

What does the solubility table tell us?

Answer 44

If the product of a precipitate reaction is insoluble, it will form as a precipitate

Question 45

Lead Nitrate + Sodium Chloride -->

Answer 45

Lead Chloride (precipitate as it's insoluble) + Sodium Nitrate

Question 46

What's an example of a base?

Answer 46

Metal oxides (insoluble in water)

Question 47

In terms of ions, why does the pH increase when acid is added to a base

Answer 47

H+ ions are NEUTRALISED So, the conc of OH- ions increases and H+ ions decreases