Thursday, October 20, 2011

GCE 'O' 2011 Science (Chemistry) Paper 5

  8:00 AM    

You are provided with a sample of solid salt, Q. Solid salt Q contains three diferent ions.
Carry out the following experiments and test any gases evolved. Carefully record your observations.
The volumes given below are approximate and should be estimated rather than measured.
test
observations
(a)
Use a spatula to transfer approximately one third of solid Q into a dry hard glass test-tube.

Heat the sample gently for 30 seconds and then very strongly until no further changes are seen.

Test any gases evolved using red and blue litmus paper, together with any other tests you think are appropriate.

*Potassium dichromate(VI) can’t be use!
Condemned by MOE already.

On heating gently, a colourless, odourless gas that turned anhydrous blue cobalt chloride paper pink was evolved. Water vapour was evolved.
Upon strong heating, a colourless, choking gas that turned blue litmus red was evolved.
The gas turned acidified orange potassium dichromate(VI) paper green.
Sulfur dioxide gas was evolved.

A brown residue remains.
[4]
 Transfer the remainder of solid Q into the large test-tube labelled solution Q. Half fill this test-tube with deionised water. Insert the stopper into the test-tube and shake or invert the tube until all of the solid has dissolved. This is solution Q.
You must retain solution Q for use in parts (b), (c) and (d).
(b)
Place 3cm3 of solution Q in a clean test-tube.

To this solution, add 3 or 4 pieces of magnesium ribbon.


Solution Q is green in colour.

The magnesium ribbon becomes smaller as grey solids are formed.

Gently warm the mixture.
Leave this test-tube to stand in the test-tube rack for a few minutes.

While you are waiting, continue with part (c).


The green solution turns colourless.



[4]

(c)
Place 2 cm3 of solution Q in a clean test-tube.

To this solution, add 2cm3 of dilute sodium hydroxide.

A dirty green precipitate was obtained.

Gently warm the mixture.


On warming the mixture, a colourless, pungent gas that turned red litmus blue was evolved.
Ammonia gas was evolved.

The green precipitate turns red brown.
[3]
(d)
Place 2 cm3 of solution Q in a clean test-tube.

To this solution, add 1 cm3 of dilute hydrochloric acid, followed by 1 cm3 of barium chloride solution.


A white precipitate is formed.




[1]

(e) Consider the results of your experiments.

Name the three ions present in salt Q. Give evidence to support each of your choices.

ion present
evidence
Iron (II)

Dirty green precipitate of Fe(OH)2 formed in part (c).
Ammonium

Ammonia gas evolved in part (c) when solution Q with dilute sodium hydroxide is warmed.
Sulfate

White precipitate of BaSO4 formed in part (d).

[3]

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Thursday, October 21, 2010

GCE 'O' 2010 Science (Chemistry) Paper 5

  8:00 AM    

You are provided with a sample of solid salt, X, in a large stoppered test-tube. Solid X is a corrosive and so should not be touched.
Carry out the following experiments and test any gases evolved. Carefully record your observations.
The volumes given below, unless referring to drops of solution, are approximate and should be estimated rather than measured.
test
observations
(a)
Dissolve the solid salt X, in the large test-tube, in distilled water and then add more distilled water until the test-tube is about two-thirds full. Stopper and shake the test-tube thoroughly.
You will use this solution in parts (b) and (c).

Orange/Yellow solution formed
When orange solution dissolved in distilled water.
[1]
(b)
Place 4cm3 of solution from part (a) in a clean test-tube.

To this test-tube, add 4cm3 of dilute sulfuric acid and 3-4 pieces of granulated zinc. Gently warm, but do not boil, this mixture.

Leave this test-tube to stand in the test-tube rack.

Keep this mixture for use in parts (d) and (e).

Only proceed to part (d) when no further changes are observed in this test-tube.

While you are waiting, continue with part (c).

Bubbles of gas occur, orange solution turns yellow

After 20min, the yellow solution turns colourless.


Use lighted wooden splint to test for ‘pop’ sound. Hydrogen gas evolved.
[3]
(c)
(i) Place 2cm3 of the solution from part (a) in a clean test-tube.

To this test-tube, add dilute sodium hydroxide solution dropwise, with shaking, until no further change is observed.

Reddish/ orange brown ppt formed which is insoluble in excess formed

(ii) Place 2cm3 of the solution from part (a) in a clean test-tube.

To this test-tube, add 1-2 drops of solution Y.

Reddish brown solution formed.
The yellow solution turns orange/ darker.

(iii) Place 2cm3 of the solution from part (a) in a clean test-tube.

To this test-tube, add 5-10 drops of aqueous silver nitrate.

Now add dilute aqueous ammonia dropwise, with shaking, until no further change is observed.

White / Light yellow ppt formed.

White ppt dissolve on adding aq ammonia.

Reddish brown/ white  ppt formed insoluble in excess ammonia
[4]
(d)
Using a teat pipette, remove 1-2cm3 of the solution from part (b) in a clean test-tube.

To this test-tube, immediately add dilute sodium hydroxide solution dropwise, with shaking, until no further change is observed.

Carry on with part (e), but contimue to observe this test-tube for several minutes.

Dark/ dirty green ppt formed.

Affter sometimes, reddish brown ppt is formed at the sides of test-tube
[2]
(e)
Using a teat pipette, remove 1-2cm3 of the solution from part (b) in a clean test-tube.

To this test-tube, immediately add 1-2 drops of solution Y.

Reddish brown solution formed.
The orange solution turns pale yellow
[1]

Conclusions
Consider the results of the experiments in parts (b) to (e).

Give four different conclusions about the observations you have made in parts (b) to (e).

You are not expected to make any conclusions about solution Y.

Give evidence (a reason) to support each of your conclusions.

Conclusion 1 …Zinc displaces the less reactive metal from the solution of X…...……………
Evidence 1 …..Grey solid of the metal from X seen in (b). White ppt of zinc hydroxide seen in (d).……………………………………..…………………………………………………………...
Conclusion 2 …Redox reaction takes place in (b).………………………………………………
Evidence 2 ……Zinc is oxidized while salt X is reduced as seen in the vigorous reaction in (b) and the evolution of gas………………………………………………………………………...
Conclusion 3 …Salt X contains a chloride……………………………….. ………………………
Evidence 3 ……The white ppt formed with silver nitrate solution in c(iii)….……………...…...
Conclusion 4 …Salt X contains iron (III) ions..……………………………………………………

Evidence 4 …..The reddish brown ppt formed with sodium hydroxide solution in c(i)……..[4]

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Thursday, October 22, 2009

GCE 'O' 2009 Science (Chemistry) Paper 5

  8:00 AM    

2          You are provided with a sample of solid salt, X.

Carry out the following experiments and carefully record your observations.

The volume given below, unless referring to drops of solution, are approximate and should be estimated rather than measured.

(a)  Using the smaller tip of the spatula, transfer two spatula tips (about 0.5 g) of solid X into a large dry hard-glass boiling tube. Place about 3cm3 of limewater in a test-tube.
Heat the solid carefully but strongly for several minutes. During the heating process, withdraw several samples of gas from inside the hard-glass boiling-tube using a teat pipette. The teat pipette should not touch the boiling-tube. Each time, bubble the gas from the teat pipette through the limewater.


(i)            Record your observations.

………………………………………………………………………………………….

(i)            A solid residue, Y, remains in the hard-glass boiling-tube. Suggest identities for the other products formed in this decomposition reaction.

Carbon dioxide gas and water………………………………………………………

………………………………………………………………………………………….
                                                                                                                        [4]


Allow the hard-glass boiling-tube to cool completely. While you are waiting, continue with the tests (b) and (c) below.

(b)       Adjust the Bunsen burner to give a blue flame. Moisten the end of a wooden splint
            With water and dip the moist end of the splint into the sample of solid X. Place this
            end of the splint in the Bunsen burner flame. 

            Record your observations.
.  
(c)        Using the smaller tip of a spatula, transfer a small amount of solid X into a test-tube and add to this about 2cm3 of dilute hydrochloric acid.

            Record your observations.

To the cold hard-glass boiling-tube from part (a), add about 2cm3 of distilled water. Using a glass stirring rod, stir the mixture until all the residue has dissolved. Now add more distilled water until the test-tube is about three-quarters full. Stir the mixture with the stirring rod. This is solution Y.  

 

(d)       Using the solution Y, perform the following tests and record your observations.

(i)            To about 2cm3 of the solution Y, add about 2cm3 of barium chloride solution. Then add dilute hydrochloric acid drop-by-drop until about 2cm3 has been added.

                    Observations with barium chloride solution 

………………………………………………………………………………………….

Observations with hydrochloric acid 

  …………….........................................................................................................
ii) To about 2cm3 of the solution Y, add about 2cm3 of copper (ll) sulphate solution; shake the test-tube thoroughly. Then transfer the mixture into a boiling-tube. Heat this boiling-tube carefully, until the mixture begins to boil. Continue gently heating the boiling mixture until no further change is seen.

Observations with copper (ll) sulphate solution 

  

………………………………………………………………………………………….

Observations on heating 

  

………………………………………………………………………………………….

(iii)         To about 2cm3 of the solution Y, add about 2cm3 of dilute hydrochloric acid.

          Observation        

             

 ………………………………………………………………………………………….
 (e)

 Suggest the name of the anion present in salt X and the name of the anion present in solution Y.

            Anion present in salt X  …Hydrogencarbonate, carbonate…………………………….
            Anion present in solution Y….carbonate…………………………………………………

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Thursday, October 16, 2008

GCE 'O' 2008 Science (Chemistry) Paper 5

  8:00 AM    

You are provided with a sample of solid salt, T, which contains the element K, Br and O only. Carry out the following experiments and carefully record your observations.
The volumes given below, unless referring to drops of solution, are approximate and should be estimated rather than measured.

(a) Adjust the Bunsen burner to give a blue flame. Moisten the end of a wooden splint with water and dip the moist end of the splint into the sample of solid T.
Record your observations.
Observations ...................................................................................................................................... [1]


  
Carry out the following tests on salt T. Record your observations in the spaces provided.
(b)
Test
observations
Use the smaller tip of a spatula to transfer a small sample of solid T into a clean and dry hard-glass test-tube.

Heat the sample carefully until it has melted completely; then heat the sample strongly until no further changes are seen.

Test any gas evolved using a
glowing splint.

                                                                                                   [4]




















Leave the test-tube used in part (b) cool. Keep the test-tube and its contents for use in part (c); only proceed to part (c) when this test-tube is cool.
(c) 
(i)
Dissolve the residue in the cool test-tube from part (b) in distilled water and then continue adding water until the test-tube is half full.
Stopper and shake the test-tube thoroughly.

  
(ii)
Place 2cm3 of the solution from part (c)(i) in a clean test-tube.
Add 2cm3 of dilute HNO3 and shake the test-tube.

Then add, with shaking, 10 drops of KI solution.

Then add 2 to 4 drops of starch solution and
shake the test-tube.

 
(iii)
Place 2cm3 of the solution from part (c)(i) in a
clean, dry test-tube.
Add 5 drops of aqueous AgNO3.

Then add, with shaking, about 2cm3 of dilute
aqueous ammonia solution.

                                                 [2]










































(d)
Use a spatula to transfer a small sample of solid T into a clean, dry test-tube and dissolve it in about 2cm3 of distilled water.

To this test-tube, add 5 drops of aqueous AgNO3, followed by 1cm3 of dilute aqueous ammonia, with shaking.

                                             [1]





















Conclusions

Consider the results of the experiments in parts (b), (c) and (d). State at least four conclusions you can draw about the chemical reactions involved.

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