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351. Sketch Hofmann's apparatus for showing the volume of steam obtained by the explosion of a mixture of hydrogen (2 volumes) with oxygen (1 volume), and explain how it is used.

352. What happens when hydrogen is passed over heated copper oxide ? Express the reaction by an equation.

353. Explain how the action of hydrogen on copper oxide may be used as a means of determining the composition of water.

354. Berzelius and Dulong heated 53.821 grammes copper oxide in contact with hydrogen. The residual copper weighed 42-989 grammes, and 12.197 grammes water were obtained. Calculate from these data the percentage composition of water.

355. Sketch Dumas' apparatus for the determination of the composition of water, and state the precautions he took to ensure the purity of the hydrogen employed.

356. As the result of numerous experiments Dumas found that 840°161 grammes of oxygen were required in the production of 945 439 grammes of water. Calculate from these data the percentage composition of water and the combining weight of oxygen.

357. One gramme of hydrogen is passed over 172 grammes of red-hot copper oxide. Find the weight of water produced, and the loss of weight of the

copper

oxide. 358. 53.5 c.c. of water were obtained by the action of hydrogen on heated copper oxide. Find the volume of hydrogen used, and of the oxygen with which it combined (both gases measured at N.T.P.).

359. What weight of iron sesquioxide, if heated in hydrogen, would be required to yield 12-7 grammes water, and what volume of hydrogen at N.T.P. would be used in the reaction ?

360. Twenty litres of hydrogen measured at 15o C. and 754 m.m. are burned in air, and all the water produced is collected. Find the weight of the water and the volume of air required.

361. Give six equations to show that water is a frequent product of chemical reactions.

362. Explain each of the following terms-(a) water of crystallization, (6) hydrates, (c) dęliquescent, (a) hygroscopic, (e) efflorescence, and (f) cryohydrates. Illustrate the meaning in each case by examples.

363. At what temperature is the density of water greatest ? How may this be shown experimentally ?

364. Starting with water at 15° C., explain the physical changes which occur (a) when it is heated to ico° C., (6) when it is cooled to o° C.

365. How does the pressure of the atmosphere affect the temperature at which water boils ?

366. What is the crystalline form of ice and snow ? How may it be observed in each case ?

367. What use is made of water in graduating a thermometer ? Why is it chosen for this purpose ?

318. At what temperatures does water boil and freeze respectively on the three thermometric scales in common use?

369. How may degrees on one scale be calculated into degrees on the others? What temperatures on the centigrade and Reaumur scales correspond to 15o Fahrenheit?

370. Sulphur melts at 115o C. and boils at 440° C. and sulphur dioxide boils at 8° C. Calculate these temperatures on the two other scales.

371. What changes in bulk occur (a) when ice melts, (6) when water freezes, and (c) when water is heated ?

372. Two thin glass tubes are filled with water and sealed up ; one is placed in boiling water, the other in a freezing mixture. What occurs in each case?

373. Equal quantities of water at 79° C. and of ice are mixed together. What is the temperature of the mixture ? Instead of ice, water at o° C. is used, what is the temperature in this case ?

374. Explain the term thermal unit.

375. Describe an experiment to show how the latent heat of steam may be ascertained.

376. What is meant by the term "tension of aqueous vapour," and how may it be measured ?

377. In measuring a moist gas in a eudiometer, how would you ascertain the correction to be made for the tension of aqueous vapour?

378. Describe any experiment which proves that during the evaporation of water heat is absorbed.

379. What methods have been suggested for obtaining large quantities of ice artificially?

380. On what physical conditions does the amount of gas capable of being dissolved by water, depend?

381. When water is decomposed by the galvanic current the oxygen measures rather less than half the hydrogen. How do you account for this?

382. Aqueous solutions of carbon dioxide, ammonia,

a

and oxygen are frozen. Is the gas expelled or retained at the moment of solidification ?

383. On what physical conditions does the amount of a solid capable of being dissolved in water usually depend?

384. Describe the construction and use of an oxyhydrogen blowpipe. What is the temperature of the flame of an oxyhydrogen blowpipe, and how may it be used to fuse platinum ?

385. If the water produced by burning hydrogen in oxygen be collected, is it pure or impure ?

386. Classify the various kinds of impurities found in natural waters. Which impurities may be removed by filtration ?

387. What do you understand by the terms “hard”: and “soft," as applied to water?

388. How may the degree of hardness of a water be a certained ?

389. What produces the incrustation on the sides of a vessel in which large quantities of water have been boiled ?

390. How may water be freed from all impurities and obtained chemically pure ?

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