F2 1st term SE 02 answers

1st Term SE 02 Submission date：_____________ Name：______________________ Class & no.：2 _________(     )

7.1 AB Composition of air II

i      Concept Checking

         Decide whether the following statements are true or false, and try to rewrite the true statements.                                                                                                                                           (10 marks)

1       A glowing splint relights when it is put into a tube of carbon dioxide.                                True / False

A glowing splint relights when it is put into a tube of oxygen.

/ A glowing splint goes out when it is put into a tube of carbon dioxide.

2        We can use hydrogencarbonate indicator to absorb carbon dioxide.                                  True / False

We can use hydrogencarbonate indicator to test for carbon dioxide.

3        Cobalt chloride paper is used to test for the presence of water.                                       True / False

4        Breathed air contains more oxygen than nitrogen.                                                           True / False

Breathed air contains more nitrogen than oxygen.

/ Breathed air contains less oxygen than nitrogen.

5        The carbon dioxide content of breathed air is higher than that of unbreathed air.              True / False

i      Short Questions

1       To test for the presence of water, we should handle dry cobalt chloride papers with a pair of forceps instead of hands. Why?                                                                                                        (2 marks)

This prevents the moisture on hands from turning the cobalt chloride papers from blue to pink (1), interfering the experimental results (1).

2       If dry cobalt chloride paper is exposed in air for a long time, its colour changes. Name the substance that causes this colour change.        The water vapour in air.                            (1 mark)

3       What indicator can we use to detect the content of carbon dioxide?                                 (1 mark)

Hydrogencarbonate indicator / Lime water

2      A student has mistakenly mixed up 4 test tubes of gases, which were unlabeled.         In order to find out what these gases were, he carried out several tests and the results are shown below.                                            

	Glowing splint	Lime water	Hydrogencarbonate indicator solution
Test tube 1	the splint relights	remains clear	remains red
Test tube 2	the splint goes out	turns milky	turns yellow
Test tube 3	the splint goes out	remains clear	remains red
Test tube 4	the splint goes out	not carried out	turns yellow

(a)      Identify the gases in the four test tubes.                                                                        (4 marks)

Test tube 1	Oxygen
Test tube 2	Carbon dioxide
Test tube 3	Nitrogen
Test tube 4	Carbon dioxide

(b)      Suggest TWO uses of the gas in test tube 2.                                                                 (2 marks)

To make dry ice (1); To make fire extinguisher (1).

(c)       (i)      With reference to the above test results, which gas is the most unreactive? Explain.  (2 marks)

The gas in test tube 3 (nitrogen) is the most unreactive among the three gases (1), as it shows no reaction to any of the above tests (1).

(ii)     Suggest ONE use of the gas in (i).                                                                       (1 mark)

To fill food package to extend the shelf life. (1)

(d)      What will be the result if the gas in test tube 4 is tested with lime water?                      (1 mark)

The lime water turns milky (1).