REFERENCE LIBRARY
- 8 MAY 1970
育教係華 頁三笋張六第日三初月四年戌庚懸 W
報日僑筆
10英文中學會考試題預習專欄
堅道英文書院主編
化學科
CHEMISTRY (271
SOLUTIONS FOR LAST WEK
(廿七)
raen powered sulphur 18 heated gently in s test-tube, it melts at 1150 as an amber- coloured mobile liquid. As heating continued, the colour of the liquid becomes brown and its viscosity increases. At about 160 0. the liquid is so viscous that it cannot be poured. On further heating, the liquid becomes almost black and becomes some that nobile again, At 444.6°C, the molten sulphur boils and a pale yellow vapour is given off..
Originally the sulphur is X-sulphur When heated to the liquid state, the changes in colour and viscosity are due to the presence
of two forms, A-sulphur and sulphur. At tanpanature balow 160 C mare 7-sulphur is present, whereas at higher temperaturas nore
-sulphur is present. If sulphur is coolan slowly, it is turned to monoolinio sulphur, and when-ulphur is cooled suddenly, plastio sulphur, is obtained.
An elemant whion dan exist an more than one fora, with different physical properties, but the same chemical properties is said to sxhibit allotropy, and the different forms are called allotropes or allotropic forms of the alement. For example, diamond, graphite, and coke are allotropes of carbon.
Preparation of monoclinic sulphur z powdered sulphur is dissolved in hot toluene in a boiling tube. Some felt is wrapped round the tube, so that cooling is slog. The solution crystallizes above 96.5 0 to give monoclinic sulphur,
Preparation of rhombie sulphurs powdered sulphur de dissolved in carbon disulphide, and the solution is filtered into a watch glass. As the solution evaporates, pale yellow cotahedral crystals of rhombie sulphur will asparate.
Preparation of plastic sulphurs sulphur is heated to the boiling point and then poured into a beaker of cold water Transparent non-crystalline mass of rubber-like material is formed.
Preparation of colloidal sulphuri when Bodium thiosulphate solution is treated with dilute hydrochloric acid, colloidal sulphur,
amorphous form of sulphur in formed.
+ 2HC1 2NaCl + 50,
Nitric acid is prepared in the laboratory by heating a mixture of roughly equal weights of potassium nitrate and conc. sulphuric acid in a retort. The acid fumes that are given off are Condensed and collected in a flask cooled by stream of water,
KNO + SO
+ HNO
To show that nitric acid contains nitrogen, nitrio oxide is first obtained by adding dilute nitric acid to oopper.
3Cu + 8HWỌ - 3Cu(NO3); + 2N0 ↑ + 4H,0
3
The nitric oxide is then passed over heated nopper turninge, which reduces it to nitrógen
2N0 + 20u - 2000 + N2 /
The gas may be concluded to be nitrogen for gives a negative result to all the standard laboratory tests for gases.
A
As the metal is unattacked by dilute sulphurfo aoid but by dilute nitric acid i must be one between lead and silver in the electrochemical series. The fact that the solution obtained gives a black precipitate with hydrogen sulphide further confirms that A is either lead, copper, mercury or silver. However, among the four, only aalts of lead will gives a white precipitate with sodium hydroxide that is soluble in excese of alkali.
Thus is lead and v Cons involved
3Pb
2007
3Pb(NO3)2
2HNO3 Fu(OH)2V + 2NaNO3 Kabo 2820
Pb(NO3)2 + 2Nạoa Pb(OH), + 2NaOH
As the powder yields carbon axoxlas on meating it must be a bioarbonate or an insoluble carbonate. Since there is a characteristic colour change ̈ for the residue during temperature changes, it 18 probably zinc oxide, This is confirmed by the fact that it dissolves in dilute acid, and furter to give a white precipitate on addition of ammonium sulphide.
Thus B is zino Carbonata and the reactionB involved are t
+ Ca(OH);
CaCO V + x20
+28HC1
The subsvands as a Brumine, Depause aqueous solutions of bronides will fora insoluble silvei bromide with silver nitrate solution. Silvar bromide is pale yellow, it is insoluble in dilute nitric acid and somewhat soluble ti ammonia. The flame test is characteristic of potassium- compounds.
Thus C is potassium bromide and the reactiona
nvolved are
+ AgH0,- ARTER
HNO:
( 接第六張第三頁)
升中試英文科試題及答案
Now read these sentences and in the same way put the letters in the right order.
ندان هت
He then asked us some questions.
When they had left we were told that he was an important govemment officer.
We all stood up as soon as we saw them
One morning a gentleman came into our destroom with our headmaster, The gentleman told us to sit down, and said a few words to our teacher
B
D
4 A
There was a swimming pool in the.rari
It followed us all the way
Last Saturday, my brother and I went to Victoria Park
When we were walking near the swimming pool, we saw a little dog.
At last a young girl came and carried it away
A
"My uncle is working there, Jack replied.
Japan for me,*Jack cried.
I'd like to go to England, John spoke first
"Where would you like to go for your holidays?" the teacher asked the class
"Why do you want to go to Japan, Jack ? the teacher asked.
D
3B
A
At last David's train arrived at the railway station at Shath
After about ten minutes they came to the fann-yard.
His uncle was there to meet him.........
David was very pleased to greet his two cousins in front of the rann-housa:
They walked to his uncle's farm at the foot of a hill.
A
E
4. B
vire
gauze
Heat
vater
Ulasa, retort
from tap
OS H
ixture of consi
and
nitric acid
The apparatus used must be entirely of glass Decause nitrio acid will attack rubber and oork.
The acid collected is yellow, This is due dissolved nitrogen dioxide formed by thermal.
composition of nitric acid,
+
4HNO - 28,0 + 4N021 027
Fure nitric acid itself is colourless, the yellow colour can be removed by blowing a current of air: through the acid.
To show that nitric acid contains nydrogen, magnesium ribbon is added to a very dilute solution of nitric acid. Effervescence will docu and a colourless gas may be collected over
•Ag(NO3)2 + #21
The gas is shown to be hydrogen because when mixed with air and ignited with a lighted splant it explodes,
To show that nitric acid containa hydrogen, the acid is heated in the following arrangement
added dropw18's
clay pipe
11110
strong heating
water
The mold decomposes to nitrogen dioxide and oxygen 68. mentioned above, Nitrogen dioxide readily dissolves in water, and oxygen is collected over water. The gas is shown to be oxygen, because it can re-ignite a glowing splint.
The green substance is a sulphate, as the solution gives with acidified barium chloride solution a white precipitate. It is also probably a ferrous salt, as ferrous salts are green. The fact that with sodium hydroxide yields a green precipitate strongly suggests the above conclusion. Upon exposure to air, the groom precipitate is tur dark green and then brown. This is explained by progressive oxidation of ferrous hydroxide to the brown ferric hydroxide.
Thus D Le ferrous sulphate and the reactions involved are
FeSO
+ 2NaOH = Fa(OH)2 ▼
478(OH),
280 - 4F (OE)
Ba30
(a) Sulphur alczide, and hydrogen chlorida
The gases are each passed inte a tent-taba containing acidified potassium permanganate. Sulphur dioxide is a strong enough reducing agent to decolorias, the permanganate, by not So for hydrogen chloride.
2KX=0 + 580 + 2H2 0 = X,50 + 230 + 2H,80
If both gases are each passed into a solution of noidified silver nitrate, only hydrogen chloride will give a white precipitate.
ONG
HCL
- Ago + HNO
The two gases may be mistaken as botn nave al pungent amell, colourless. acidio. ani hath fumes in moist air.
Carson monoxide and nyarogan
Take a test-tube of each gas a BLOW BONG air to mix. When a lighted splinter is introduced, the tube containing hydrogen will burn with a pop sound, whereas the carbon monoride will bura quietly.
+
- 2H
20
200 + 02 - 2002
then the
gases are each passed into a tibe containing ammoniacal silver nitrate solution, only carbon monoxide will reduce the silver altrate to silver which appeara an precipitataj
blanke
he two gases may be mistaken for one another, as both are colourless and odourless both burn with a blue flame, but does not. support combustion. Both are reducing gases, so that a number of reactions are similar 9.6. both reduce heated cupric oxide to netallio copper-
Nitrogen and carbon dioxide
Fach gas is passed into test tube containing line-water. Only carbon dioxide will turn time-water milky
· ro2 + Ba(OH)2 = CaCO3 v + E2®
trip of burning magnesium ribbon is puty into sach jar of gas. Magnesium will continum to burn in both ca
218
3MB
To the residue in each jar, a little water is added, the jar that originally contain nitrogen will give a pungent gas that turn – damp red litmus blue, due to hydrolysis of the nitride.
Ms N2 + 65,0 = 3Mg(OH)2
2 f
The two gases might be mistaken for one another, as both are colourless and odourless, and are quite inert, being unable to burn nor support combustion.
(d) Potassium nitrate and potassium chlorate
A solution of each is prepared, and ferrou sulphate and cono, H2504
added, Only the nitrate solution will zive a positive. brow ring test. A
Both substanong are heated strongly until no further evolution of gas. The residue is dissolved in water and teat for chloride by silver nitrate solution. Only the specimen. originally containing the chlorate will give a positive test.
2К6103
2XH03
2KC1 → 302 1
2KNO
The two way ve MISTaken, Because coth are colourless crystalline substances both evolve oxygen on heating and both behava na ozidi agents.
QUESTIONS FOR NEXT WEEK
1. How would you prepare and c011eat several jars of
hydrogen sulphide if powdered sulphur is provided. Describe experiments to study the properties of the gas (a) as a combustible substance, (b) as reducing agent, (c) as an acid. What is the teat for hydrogen sulphide?
What
the gases evolveu when dono, sulphuric acid is warmed with (a) sodium chloride, (b) sodium) nitrate, (c) sodium formate, (d) sodium oxalate? Describe and explain what happens when conc. sulphuric acid is added to (a) sugar, (b) paper, (o) blue supric sulphate crystals.
Describe one method for the industrial preparation of chlorina. What are the reactions of chlorine with (a) potassium iodide solution, (b) ammonia, (c) litmus solution, (d) water. (●) hydrogen. (f) sulphur,
Describe how oval gas 13 munuraotured,
Mention the by-products of the process and state their use
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