2. Simple classification of substances Questions and Answers

Simple classification of substances Questions

1. The diagram below shows the heating curve of a pure substance. Study it and answer the

questions that follow:

 (a) What physical changes are taking place at points X and Z? (b)Explain what happens to the melting point of sodium chloride added to this substance

2. (a) State two differences between luminous flame and non-luminous flame

 (b) It is advisable to set a Bunsen burner to luminous flame prior to an experiment.

Explain

3. The paper chromatography of a plant extract gave the following results:

 (a) Which is the most suitable solvent for purifying the extract? Explain

(b) Ball pen cannot be used to mark solvent front in the above chromatography. Explain

4. Name the process which takes place when:

(a) Solid Carbon (Iv) Oxide (dry ice) changes directly into gas

 (b) A red litmus paper turns white when dropped into chlorine water

 (c) Propene gas molecules are converted into a giant molecule

5. A sample of copper turnings was found to be contaminated with copper (II) oxide. Describe

 how a sample of copper metal can be separated from the mixture

6. Copper (II) oxide and charcoal are black solids. How would you distinguish between the

 two solids?

7. a) What is chromatography?

 b) Give two applications of chromatography

8. The two elements P and R were separately burned in air, the products gave the results

recorded in the table below:

 (a) Suggest the identity of element R. ……………………………………………..……..

 (b) Describe how the nature of the solutions of the of the oxides were determined

9 The diagram below represents a paper chromatography for the three brands of soft drinks

containing banned artificial food additives.

 A and C found to contain the banned artificial food additives. Which numbers indicate the

banned artificial food additives?

10. Without using any laboratory chemical, describe a simple laboratory experiment to distinguish

between calcium hydrogen carbonate and sodium hydrogen carbonate

11. Substance Q has a melting point of 15^oC and boiling point of 70^oC.

 (a) On the same axes, draw the melting point and boiling point graph for Q and the room

temperature

(b) State the physical state of substance Q at room temperature

12. Cooking oils comprise of a mixture of compounds which have a boiling point range

of 23^oC to 27^oC.

 (i) What evidence is then to support the statement that cooking oil is a mixture?

 (ii)Name another experimental technique that could be used to confirm your answer

in part (i) above

13. A form 1 student carried out the separation as shown in the set-up below:-

(i) Identify the method above………………………………………………………………………

(ii) Give one of its disadvantages

(iii) Name a mixture which can be separated by the set-up above

14. What is meant by melting point and boiling point of a substance?

15. The apparatus below were used by a student to study the effect of heat on hydrated

copper II sulphate

(a) What is the role of the ice cold water ……………

 (b) Name liquid P …………………………………………………………

 (c) What observation is made in the boiling tube

17. The diagram below shows chromatograms of blood samples obtained from three athletes.

 One athlete used illegal drug to improve performance in competition.

(a) Name the line marked M ………………………………………………….

 (b)Identify the athlete who used illegal drug ……………… ……………………….

18. Classify the following processes as chemical changes or physical changes

Process
physical or chemical

Neutralization ………………………………………

Sublimation ………………………………………

Fractional distillation ………………………………………..

Displacement reaction …………………………………………

19. Give two reasons why a luminous flame is not used for heating purposes

20. Classify the following processes as chemical changes or physical changes

Process
physical or chemical

Neutralization ………………………………………

Sublimation ………………………………………

Fractional distillation ………………………………………..

Displacement reaction …………………………………………

21. Give two reasons why a luminous flame is not used for heating purposes

22. State two criteria for determining the purity of a substance

23. Study the information in the table below and answer the questions.

i) A mixture contains ethene, Hydrogen and ammonia gases. Explain how a sample of

hydrogen gas can be obtained from this mixture.

24. a)i) The diagram below show spots of a pure substance A, B, and C on a chromatography

paper. Spot D is that of a mixture

After development A, B, and C were found to have moved 8cm, 3cm and 6cm respectively.

 D had separated into two spots which had moved 6cm and 8cm

 On the diagram above;

I. Label the baseline (origin)

II. Show the positions of all the spots after development

ii) Identify the substances present in mixture D

 b) Describe how solid ammonium chloride can be separated from a solid mixture of

ammonium chloride and anhydrous calcium chloride

 c) The table below shows liquids that are miscible and those that are immiscible

 Use the information given in the table to answer that questions that follow;

i) Name the method that can be used to separate L₁ and L₂ from a mixture of the two

ii) Describe how a mixture of L₂ and L₄ can be separated

25. A student left some crushed fruit mixture with water for some days. He found the mixture

 had fermented. He concluded that the mixture was contaminated with water and ethanol with

 boiling point of 100^oC and 78^oC respectively. The set-up of apparatus below are used to separate

 the mixture.

(i) Name the piece of apparatus labelled W

 (ii) What is the purpose of the thermometer in the set-up?

 iii) At which end of the apparatus W should tap water be connected?……………………………

 (iv) Which liquid was collected as the first distillate? Explain

 (v) What is the name given to the above method of separating mixture?

 (vi) State two applications of the above method of separating mixtures

 (vi) What properties of the mixture makes it possible for the component to be separated

by the above methods?

26. The set-up below was used to separate a mixture:-

 (a) Name the apparatus missing in the set-up

 (b) Give one example of mixture T

 (c) What is the name of this method of separation

27. a) The diagram below shows a set – up used by a student to find out what happens

when Copper (II) sulphate crystals are heated.

(i) State the observations made when the blue copper (II) sulphate crystals are heated.

(ii) Identify liquid Y and write an equation for its formation.

 b) Pellets of sodium hydrogen and anhydrous Copper (II) sulphate were put in separate Petri-

dishes and left in the open for two hours. Explain the observation in each Petri-dish.

28. The chromatography below shows the constituents of a flower extract using an organic solvent:-

 (a) (i) Name a possible organic solvent you can use for this experiment

(ii) State one property that makes the red pigment to move the furthest distance from M

(iii) Describe how one could get a sample of yellow pigment

(iv) On the diagram indicate solvent front

 (b) Describe how Aluminium chloride can be separated from a mixture of aluminium chloride

and sodium chloride

29. Study the information below and answer the questions that follow:

Describe how the mixture of solid R, S, and V can be separated

30. Given a mixture of lead (II) oxide, ammonium chloride and sodium chloride, describe how this

mixture can be separated to obtain a sample of each.

31. The setup below was used to separate two miscible liquids Q and T

(Boling points; Q =98° C, T=78°C)

 (a) Identify the mistakes in the setup above

 (b)Identify Distillate X

32. Name the process which takes place when:

 a) Solid Carbon (IV) oxide (dry ice) changes directly into gas.

b) A red litmus paper turns white when dropped into chlorine water.

c) Propene gas molecules are converted into a giant molecule.

33. The following diagram shows a paper chromatogram of substances A, B, C, and D which

are coloured

(a) Indicate the solvent front on the chromatogram

(b) Which substance is pure? ………………………………………..

(c) Substance E is a mixture of C and D. Indicate its chromatogram in the diagram

34. Study the information below and answer the following questions. A mixture contains three

solids A, B, and C. The solubility of these solids in different liquids is as shown below:-

Explain how you will obtain sample C from the mixture

35. State and explain the observations made when iodine crystals is heated in a boiling tube?

2. Simple classification of substances Answers

1. a) X – melting point √ ½

Z – Boiling point √ ½ b) Its melting point is lowered and becomes less sharp due to the introduction of an impurity √1

2. Luminous flame produces soot while non- luminous flame does not√1

Luminous flame is yellow in colour while non- luminous flame is blue in colour

OR accept any correct answer

b) The luminous flame is moderately hot and is clearly visible hence no danger is posed

3. a) X

Gives the greatest number of spots hence the greatest number of pure substances √ 1

b) The ink is made of more than one pure substance hence will also undergo chromatography

4. (a) sublimation

(b) Bleaching action

(c) Polymerization

5. Adds excess dilute hydrochloric acid/ sulphuric (vi) acid

 Filter to obtain copper metal

 Wash with distilled water

6. To separate samples of CUO and charcoal in test tubes, dilute mineral acid is added with

shaking CUO black dissolves to form blue solution ½

 Charcoal does not dissolve in dilute mineral acids

7. a) Is the process for the separation of a mixture of solutes by their different rates of movement

over a porous medium caused by moving solvent

 b) – Separation of dyes

– To analyse and identify mixtures of substances which are difficult to separate by

other means

– Used to analyze dyes in food colouring (Any two each one mark)

8 a) Element R – Sulphur

b) Mix solid P oxide with water

put blue and litmus paper, Blue litmus paper remains blue, red litmus paper changes to blue.

Put blue and red litmus papers in water

Blue changes to red, red remain red.

9. 5 and 4 BOTH MUST BE CORRECT

10. EITHER

• In separate test tubes, boil about 5cm³ of each solution.
  

• Sodium hydrogencarbonate solution remains colourless forms no precipitate
  
• Calcium hydrogencarbonate solution changes from colourless to white precipitate
  

 OR

2NaHCO_3aq Na₂CO₃ + CO_2(g)n + H₂O_(e)

Ca (HCO₃)_2
(aq) CaCO_3(s) + CO_2
(g) + H₂O_(e)

HEAT must be mentioned or implied.

11. a)

b) Liquid

12. (i) Range of boiling points / no sharp boiling points

 (ii) Carry out fractional distillation

13. (i) Evaporation

 (ii) Uses a lot of fuel

 (iii) Any soluble salt and water

14. Melting points is the specific √ ½ constant temperature√ ½ for a particular substance when a solid √ ½

change to a liquid√ ½

16. (a) To cool/condense vapour. √1 (1 mk)

 (b) Water. √1 (1 mk) 3

 (c) Blue solid √1 changes to white solid. √1 (1 mk)

17. (a) Solvent front √

 (b) C √

18. a) Chemical √ ½

 b) Physical √ ½

 c) Physical √ ½

 d) Chemical √ ½

19. – Smoky/ sooty √

 – Not hot enough √

20. a) Chemical √ ½

 b) Physical √ ½

 c) Physical √ ½

 d) Chemical √ ½

21. – Smoky/ sooty √

 – Not hot enough √

22. – Boiling point

 – Melting point

 – Density

 – Refractive index

23. i) Pass the mixture of gases through concentrated sulphuric (vi) acid √ ½ . Ammonia and

ethane will dissolve √ ½

 – Hydrogen √ ½ being insoluble √ ½ is then obtained

24. a) i)

 ii) A and C

 b) Since NH4CL sublimes but CaCL2 does not, sublimation process would do. Heat the

mixture, NH4CL sublimates into vapour and condences on the upper cooler parts of the test

tube. CaCL2 remains at the bottom of the heating tube

 c) i) Fractional distilation

ii) Separating funnel method 8

Since the two liquids are immiscible pour the mixture into the separating funnel and

allow to settle. The denser liquid will settle down and the less dense one will form

the second layer on top. Open the tap and run out the liquid in the bottom layer leaving

the second layer in the funnel

25. (i) Condenser

 (ii) To indicate when a liquid is boiling, a thermometer reads a constant temperature

(iii) A

 (iv) Ethanol

Reason:- It has a lower boiling of 78^oC compared to water with a boiling point of 100^oC

or – The liquid with the lower boiling point boils first and its vapours are condensed

and the condenser to be collected as the first distillate

(v) Fractional distillation

(vi) – To separate components of crude oil

• To isolate O₂ and N₂ from air
  
• To manufacture spirits
  

 (vii)- They are immiscible liquids

• They have different but close boiling points
  

26. (a) Wire gauze

 (b) Sodium chloride solution (or any named slat solution)

 (c) Evaporation

27. a) i) – Colourless liquid is seen on the cooler parts of the test tube. 1 mk.

– Blue crystals change to a white powder. 1 mk

ii) Water 1 which was originally water crystallization.

CuSO₄ , 5H₂O(s) CuSO₄(s) + 5H₂O(l) 1

 b) NaOH(s) absorbs water from the air and forms a solution. It is a deliquescent substance. 1 Anhydrous CuSO₄ absorbs water from air to form hydrated Copper (II) sulphate which is blue

but no solution is formed 1 it is hygroscopic

28. a)i)Ethanol, acetone (any organic solvent)

ii) Its most soluble in the solvent and less sticky

iii) – Cut out the yellow pigment

• put in organic solvent to dissolve the pigment
  
• filter and evaporate the filtrate to get the pigment
  

 iv)Above the red pigment and below the edge.

b)-Heat the mixture aluminum chloride sublime and collect be cooler part of the tube

and sodium chloride left at bottom of the tube

– Scratch the condense alcl₃ place in a beaker

(c)Add cold water to the mixture, and stir to dissolve R. Filter to get solid S and V on residue . Evaporate the future to get R. put S and in no water and stir to dissolve and filter to get S as residue evaporate future to get V

29. Add cold water to the mixture, and stir to dissolve R. Filter to get solid S and V on residue . Evaporate the future to get R. put S and in no water and stir to dissolve and filter to get S as residue evaporate future to get V

30. Heat the mixture Ammonium chloride sublimes and is collected on the cooler parts. Add water

to the remaining mixture, stir and filter. Lead (ii) Oxide remains as residue. Evaporate the

filtrate to dryness to obtain sodium chloride

31. a) – Fractionating column must have beads

– Wrong cold water circulation in the condenser

 b) T

32. a) Sublimation. 1 (3 mks

 b) Bleaching 1

 c) Polymerisation 1

33.

(a) See Diagram above

– Solvent front should be slightly above the furthest pigment

(b) C

– It contains only one pigment

34 . – Add either to the mixture. Stir and filter

 – Add alcohol to the residue, stir and filter

– Evaporate to filtrate to obtain C

35. – Black crystals changes directly into purple vapour√1

– The iodine crystals (sublimes) changed directly into a purple vapour without passing

liquid state and changed back to black iodine crystals on the upper cooler parts of boiling

tube√ (Correct colour must be stated 2 mks