Acids, bases + salts

Question 1

What colours does litmus paper form when it is at a pH of acidic, alkaline and neutral

Answer 1

• acidic ~ red
• alkaline ~ blue
• neutral ~ purple

Question 2

How will phenolphthalein change in an acidic solution?

Answer 2

colourless to pink

Question 3

how will methyl orange change in acidic and alkaline solutions?

Answer 3

• acid ~ red
• alkaline ~ yellow

Question 4

acid definition

Answer 4

A source of hydrogen ions. They are proton donors and have a pH of less than 7

Question 5

base definiton

Answer 5

A substance that can neutralise an acid. They are proton acceptors.

Question 6

alkali definition

Answer 6

A source of hydroxide ions which have a pH greater than 7

Question 7

give the name of the reaction between an acid and a base (or an acid an alkali) and the chemical equation

Answer 7

• neutralisation
• H^+ + OH^- = H20

Question 8

give the general equation for a neutralisation reaction

Answer 8

acid + base = salt + water

Question 9

List how to do a titration reaction and what it is used for to find out (6 marks)

Answer 9

• Titrations allow you to find out exactly how much acid is needed to neutralise a quantity of alkali (or visa versa)
• Using a pipette and pipette filler, add some alkali (25cm^3) to a conical flask, along with two or three drops of indicator.
• fill a burette with acid ~ make sure this is done below eye level
• using the burette, add the acid to the alkali a bit at a time giving the conical flask a regular swirl ~ go slowly when you think the end-point is about to be reached
• the indicator changes colour when all the alkali has been neutralised

Question 10

How do you make soluble salts? (4-6 marks)

Answer 10

• you can make a soluble salt by reacting an acid that contains one of the ions you want in the salt with an insoluble base that contains the other ion you need (often a metal hydroxide or metal oxide)
• start by heating the acid in a water bath ~ this speeds up the reaction between the acid and the insoluble base. Do this in a fume cupboard to avoid releasing acid fumes into the room
• Then add the base to the acid ~ the base and the acid will react to produce a soluble salt and water. You will know when the base is in excess and all the acid has been neutralised because the excess solid will sink to thew bottom of the flask
• filter off the excess solid to get a solution containing only salt and water
• Heat the solution gently using a Bunsen burner to slowly evaporate off some of the water. Leave the solution to cool and allow the salt to crystallise.
• filter off the solid salt and leave it to dry

Question 11

Are common chlorides soluble or insoluble?

Answer 11

soluble (except silver chloride and lead chloride)

Question 12

are common sulfates soluble or insoluble?

Answer 12

soluble (except lead, barium, and calcium sulfate ones)

Question 13

are common carbonates soluble or insoluble?

Answer 13

insoluble (except for sodium, potassium and calcium ones)

Question 14

are common hydroxides soluble or insoluble?

Answer 14

insoluble (except for sodium, potassium and calcium ones)

Question 15

are nitrates soluble or insoluble?

Answer 15

soluble

Question 16

are common salts of sodium, potassium and ammonium soluble or insoluble?

Answer 16

soluble

Question 17

How do you make a pure, dry sample of insoluble salts?

Answer 17

• You can use a precipitate reaction
• You just need to pick the right two soluble salts and react them together to get your insoluble salt
• E.g. to make lead sulphate (insoluble), mix lead nitrate and magnesium sulfate (both soluble)

Question 18

How do you make lead sulfate (6 marks)

Answer 18

• add 1 spatula of lead nitrate to a test tube. Add water to dissolve it.
• You should use deionised water to make sure there are no other ions about
• Shake it thoroughly to ensure that lead nitrate has dissolved then in a separate test tube, do the same with 1 spatula of magnesium sulfate.
  Tip the two solutions into a small beaker, and give it a stir to make sure it’s all mixed together. The lead sulfate should precipitate out.
• Put a folded piece of filter paper into a filter funnel and stick the funnel into a conical flask.
• Pour the contents of the beaker into the middle of the filter paper. make sure that the solution doesn’t go above the filter paperas some of the solid can spill over
• swill out the beaker with deionised water and tip this into the filter paper to make sure you get all the precipitate from the beaker
• Rinse the contents of the filter with deionised water to make sure that all the soluble magnesium nitrate has been washed away
• then scrape the lead sulfate onto fresh filter paper and leave it to dry in an over or a desiccator

Question 19

Outline how you could prepare a pure, dry sample of copper sulfate in the lab from sulphuric acid and copper oxide (6 marks)

Answer 19

• Warm sulphuric acid in water bath
• Add excess copper oxide to there acid
• When the reaction is complete filter the mixture to remove excess copper oxide
• Heat the remaining solution gently (using Bunsen burner) to evaporate off some of the water
• leave the solution to cool and allow the salt to crystallise
• filter off the solid salt and leave the crystals to dry

Question 20

is silver chloride soluble or insoluble?

Answer 20

insoluble

Question 21

is magnesium hydroxide soluble or insoluble?

Answer 21

insoluble

Question 22

is sodium nitrate soluble or insoluble?

Answer 22

soluble

Question 23

A student is making a sample of a salt by reacting ammonium hydroxide with hydrochloric acid. Name the salt formed in this reaction

Answer 23

Ammonium chloride

Question 24

The student uses a titration method to add ammonium hydroxide solution to the acid until the reaction reaches its end point, which is shown by a change in colour of an indicator in the solution. The student then crystallises the solution to obtain a salt. Will this produce a pure sample pf the salt? Explain your answer.

Answer 24

No because the salt will be contaminated by the indicator