620. Why are nitrogen dioxide and tetroxide represented by the formulæ NO and NO, respectively, and not by N2O, and N2O?

621. What is the action on nitrogen tetroxide of small and large quantities of water?

622. How is nitrogen pentoxide obtained, and how is it related to nitric acid?

623. What is the action of phosphorus pentoxide on nitric acid, and how may nitric acid be obtained from the product?

624. What naturally occurring compounds may be used for the preparation of nitric acid? How do they generally occur, and how is their formation accounted for?

625. How is, nitric acid usually prepared (a) on the small scale, (b) on the large scale? What impurities are generally present in the commercial acid?

626. How may nitric acid be obtained starting with hydrogen, oxygen, and nitrogen?

627. How much sulphuric acid is needed to react on 129 tons of nitre so as to obtain nitric acid, and how much of the latter should be got from this weight of the salt?

628. What are the best tests for nitric acid, and how may these tests be applied to the detection of nitrates?

629. Starting with nitric acid, how may each of the oxides and acids of nitrogen be obtained? Give equations.

630. Name all the bodies which can be obtained by the action of nitric acid of different strengths on zinc. Explain the formation of each and give equations.

631. What is the action of strong nitric acid on (a) sulphur, (6) mercury, (c) sulphur dioxide, (e) tin, (ƒ) phosphorus ?

632. How much pure nitric acid is present in 159 tons of nitric acid, 100 grammes of which neutralize 44'3 grammes pure caustic potash?

633. Name four metals which are not acted on by nitric acid, and state how solutions of each may be obtained.

634. What is aqua regia? What substances are required to prepare it, and what gases does it contain?

635. What is the formula of hyponitrous acid, and how is it related to nitrous and nitric acids? How are its salts obtained?

COMPOUNDS OF PHOSPHORUS.

636. Give different ways for the preparation of phosphine, and describe its properties. Can it be prepared by direct union of its elements, and if so, how?

637. What occurs when spontaneously inflammable phosphine is left in contact with (a) charcoal, (3) sulphur, (c) potassium permanganate solution?

638. How is phosphine obtained quite pure? What change in volume occurs when electric sparks are passed through the pure gas?

639. Under what circumstances is phosphine spontaneously inflammable ? How may the gas be made to lose and regain the property of inflaming spontaneously?

640. Compare the properties of phosphine with those of ammonia. What is the action of each on silver nitrate solution? Give equations.

641. How may phosphine be combined with hydriodic

acid, what is the formula of the compound produced, and how may it be best obtained in quantity?

642. How are the liquid and solid hydrogen phosphides prepared, and what are their properties? Give the formula for each.

643. How much oxygen, by weight and volume, is required to burn completely 179 c.c. of phosphine?

644. Twenty-seven c.c. of phosphine are added to 39 c.c. of hydriodic acid. What occurs? Give the weight of the solid and the volume of the gaseous product.

645. How are the trichloride and pentachloride of phosphorus respectively obtained? How may the trichloride be obtained from the pentachloride and vice versâ?

646. What is the action of phosphorus trichloride on each of the following bodies? (a) water, (b) sulphur trioxide, (c) sulphuric acid, (d) alcohol, (e) sulphuretted hydrogen, (f) acetic acid. Give equations.

647. What is the action of phosphorus pentachloride on (a) water in small quantity, (b) excess of water? Give also its action on all the other substances mentioned in the previous question. Give equations.

648 What is the formula of the substance called phosphorus oxychloride? Give various reactions for obtaining it, and state its action on water and on alcohol.

649. How are the bromide and iodide of phosphorus obtained, and what is the action of each on water?

650. Give the formulæ of the two fluorides of phosphorus, and state how each is prepared.

651 What is the composition of the white fumes formed when a piece of dry phosphorus is exposed to air? How is the substance obtained in quantity?

652. Name the two oxides of phosphorus, give their formulæ, and state how they are prepared. Represent by equations the action of water on each.

653. Nine grammes of phosphorus pentoxide exposed to air gained 2.76 grammes. Find the percentage composition of the product so obtained.

654. How could you obtain phosphorus from phosphorous pentoxide?

655. How much water must be added to 187 grammes of phosphoric anhydride so as to convert it into phosphoric acid?