metal hydroxide precipitates, identifying anions (non-metal ions) + required practical

Question 1

describe how to use sodium hydroxide solution to identify calcium ions, magnesium ions and aluminium ions

how to distinguish between these three metal ions

Answer 1

If we add sodium hydroxide solution to these three solutions, then they all produce a white precipitate

That is a problem as we cannot distinguish between these three tests - and know which solutions contain which type of ions = They all produce a white precipitate

If we add excess sodium hydroxide solution, then the aluminium precipitate redissolves (this allows us to tell which of our three ions is aluminium - Al3+)

take original magnesium and calcium compounds
and perform a flame test .
We would need to do flame tests to work out which one is calcium.

In a flame test, calcium produces an orange-red flame.

This allows us to tell/know which compound is calcium and which one is magnesium
__________________________

take original magnesium and calcium compounds → flame
test,
calcium → (brick) red flame,
magnesium → no colour

Question 2

Write balanced chemical equations for these sodium hydroxide precipitate tests - sodium, calcium and magnesium

Answer 2

calcium nitrate + sodium hydroxide -> sodium nitrate + calcium hydroxide

Ca(NO3)2 (aq)+ 2NaOH (aq) –> 2NaNO3 (aq) + Ca(OH)2 (s)

magnesium nitrate + magnesium hydroxide -> sodium nitrate + magneium hydroxide

aluminium nitrate + sodium hydroxide -> sodium nitrate + aluminium hydroxide

Al(NO3)3 (aq)+ 3NaOH (aq) –> 3NaNO3 (aq) + Al(OH)3 (s)

We are mixing calcium nitrate with sodium hydroxide solution - would also work for other soluble calcium compounds

products - sodium nitrate solution (solution because it is aq solution) and also insoluble metal hydroxide

We can tell that calcium hydroxide is a precipitate as it has the state symbol for a solid

insoluble metal hydroxides which form the precipitates
calcium hydroxide, mg(OH)2 , Al (OH)3

Question 3

different ways to test for certain metal ions

Answer 3

flame tests
flame emission spectroscopy
we can also test for certain metal ions using their reaction with sodium hydroxide solution

Question 4

What other metal ions can we use to test for using sodium hydroxide solution

Answer 4

Copper II ions react with sodium hydroxide to form a blue precipitate of copper (II) hydroxide

copper (II) nitrate + sodium hydroxide –>sodium nitrate + copper (II) hydroxide
Cu(NO3)2 (aq) + 2NaOH (aq)-> 2NaNO3 (aq)+ Cu(OH)2 (aq)

Iron (II) ions react with sodium hydroxide to form a green precipitate of iron (II) hydroxide

iron (II) nitrate + sodium hydroxide –>sodium nitrate + iron (II) hydroxide
Fe(NO3)2 (aq) + 2NaOH (aq)-> 2NaNO3 (aq)+ Fe(OH)2 (aq)

Iron (III) ions react with sodium hydroxide to form a brown precipitate of iron (III) hydroxide

Fe(NO3)3 (aq) + 3NaOH (aq)-> 3NaNO3 (aq)+ Fe(OH)3 (aq)

solutions of iron (III) ions - > add sodium hydroxide solution - brown precipitate of iron (III) hydroxide

Question 5

describe how we can identify non-metal ions: CARBONATE

testing for the carbonate ion

Answer 5

1. Add dilute acid to our sample
2. If the carbonate ion is present then the acid will react with the carbonate to make carbon dioxide gas. We will see effervescence (fizzing). But That does not prove that we have carbon dioxide gas
3. Bubble the gas through lime-water. If the lime-water goes cloudy, then this prove that we have carbon dioxide gas.
   That means that we started with the carbonate ion

Question 6

describe how we can identify non-metal ions: HALIDE - CHLORIDE

testing for chloride ions

Answer 6

1. Add dilute nitric acid to the sample
2. Add dilute silver nitrate solution
3. Halide ions produce a precipitate of the silver halide. Each halide makes a different coloured precipitate

Chloride ions produce a white precipitate of silver chloride

Question 7

describe how we can identify non-metal ions: HALIDE - BROMIDE

testing for bromide ions

Answer 7

1. Add dilute nitric acid to the sample
2. Add dilute silver nitrate solution
3. Halide ions produce a precipitate of the silver halide. Each halide makes a different coloured precipitate

Bromide ions produce a cream precipitate of silver bromide

Question 8

describe how we can identify non-metal ions: HALIDE - IODIDE

testing for iodide ions

Answer 8

1. Add dilute nitric acid to the sample
2. Add dilute silver nitrate solution
3. Halide ions produce a precipitate of the silver halide. Each halide makes a different coloured precipitate

Iodide ions produce a yellow precipitate of silver iodide

Question 9

describe how we can identify non-metal ions: SULFATE

testing for sulfate ions

Answer 9

1. Add dilute hydrochloric acid to our sample
2. Add barium chloride solution
3. If sulfate ions are present, we will see a white precipitate of barium sulfate

Question 10

A student adds dilute nitric acid and silver nitrate solution to an aqueous solution of a calcium halide
A white precipitate forms
Give the balanced symbol equation for this reaction

cgp page 157

Answer 10

CaCl2 + 2AgNO3 —> 2AgCl + Ca(NO3)2

Question 11

CGP Page 161 Question 3.2

Answer 11

No the student is not correct.
The sample produced a white precipitate with hydrochloric acid and barium chloride solution, so he can be sure that it contains sulfate ions

Magnesium does produce a white precipitate with sodium hydroxide, but so do other metal ions - aluminium and calcium, so he cannot be sure that it contains magnesium ions

Question 12

ionic equation for test for sulfates

Answer 12

Ba2+ (aq) + SO42- (aq) —> BaSO4 (s)

HCL is added to get ride of any traces of carbonate ions before you do the test
carbonate ions would also produce a precipitate so they would confuse the results

Question 13

ionic equation for test for halides

Answer 13

Ag+ (aq) + Cl- —> AgCl(s)
Ag+ (aq) + Br- —> AgBr(s)
Ag+ (aq) + I- —> AgI (s)

ignore nitric acid

Question 14

describe how to use chemical tests to identify the ions in unknown compounds - required practicals

compound A produces a lilac flame test
A sample of compound A was reacted with silver nitrate solution and dilute nitric acid. A yellow precipitate was formed

Explain your answer

Answer 14

Compound A produced a lilac flame test. That tells us that compound A contains the potassium ion

when compound A was reacted with nitric acid and silver nitrate solution it produced a yellow precipitate.

The silver nitrate test produced a yellow precipitate. This tells us that compound A contains the iodide ion
So compound A is potassium iodide

Question 15

describe how to use chemical tests to identify the ions in unknown compounds - required practicals

compound B produced an orange-red flame test
A sample of compound B effervesced when tested with hydrochloric acid
The gas produced turned limewater cloudy
Identify compound B

Explain your answer

Answer 15

Compound B produced an orange-red flame test. That tells us that compound B contains calcium ion.

When compound B was reacted with an acid, it effervesced and the gas turned limewater cloudy
This tells us that compound B also contains the carbonate ion

So compound B is calcium carbonate

Question 16

describe how to use chemical tests to identify the ions in unknown compounds - required practicals

compound c was reacted with sodium hydroxide solution and produced a blue precipitate
compound c was then reacted with barium chloride solution and dilute hydrochloric acid. This reaction produced a white precipitate
Identify compound C

Explain your answer
write chemical formula

Answer 16

Compound C produced a blue precipitate with sodium hydroxide solution
This tells us that compound C must contain the copper (II) ion

Compound C also produced a white precipitate with barium chloride and hydrochloric acid. This tells us that compound C also contains the sulfate ion

So compound C is copper (II) sulfate

CuSO4

Question 17

Explain why for the test for sulfate ions, you use hydrochloric aid and not sulfuric acid

Answer 17

Sulfuric acid - contains sulfate ions
wont give an accurate result