656. Give the names, formulæ, and percentage composition of the three acids of phosphorus.

657. What happens when phosphorus is heated with (a) solution of potash, (b) solution of barium hydrate? Give equations.

658. How is hypophosphorous acid prepared? What occurs when it is (a) heated, (b) added to copper sulphate solution, (c) added to silver nitrate solution ? Give equations.

659. Give the constitutional formula of hypophosphorous acid, and the formulæ of its potassium and barium salts.

660. Give two different methods for obtaining phosphorous acid, and state what occurs when it is heated. Give equations.

661. Give the volume of the gaseous product at N.T.P. which should be obtained by heating 79 grammes of phosphorous acid.

662. Write the formulæ of the sodium salts of hypophosphorous, phosphorous, and phosphoric acids.

663. Give different methods which may be used for preparing ortho-phosphoric acid.

664. What changes occur when phosphoric acid is heated () to 215° C., (b) to a red heat? Give the formula of each product, and express the reactions by equations.

665. How may orthophosphoric acid be prepared from bone-ash?

666. What is meant by saying that phosphoric acid is tribasic? Give the formulæ of the sodium orthophosphates, and state how they are prepared.

667. How much phosphoric acid should be obtained by the oxidation of 1796 grammes of phosphorus with nitric acid?

668. What is the composition of the salt obtained by adding sodium carbonate to orthophosphoric acid till the latter ceases to redden litmus-paper? What salts are obtained by adding to this one, (a) phosphoric acid, (6) caustic soda?

669. Give the reaction of silver nitrate on each of the salts obtained by the processes mentioned in the previous question.

670. What is microcosmic salt, and how is it prepared? What occurs when it is heated? Give an equation.

671. When common sodium phosphate is heated till it ceases to lose water, what is the composition of the

H

residue? What weight of residue should be obtained by heating 193 grammes of the salt?

672. By what reactions could you distinguish normal from common sodium phosphate, and both these salts from sodium meta- and pyrophosphates?

673. Solution of magnesium sulphate is added to sodium phosphate in presence of ammonia and ammonium chloride, what substance is precipitated? What salt is obtained by igniting the precipitate?

674. To what substance is the name "glacial phosphoric acid" given, and how is it related to the orthophosphoric acid?

675. Give a short account of the phosphates obtained by Fleitmann and Henneberg, and show how they are related to the other phosphates.

676. Give a short account of the different sulphides of phosphorus.

COMPOUNDS OF ARSENIC.

677. Who discovered hydride of arsenic (arsine), and how did he obtain it? Give other methods.

678. What occurs when an electric current is passed through a solution of arsenious acid? To what purpose has this reaction been applied by Bloxam?

679. What occurs when arsine is (a) passed through a red-hot tube, (b) passed through solution of silver nitrate, (c) passed over sulphur exposed to sunlight, (d) passed over heated copper oxide?

680. Give equations to express the reactions which occur when phosphine and arsine are separately led into solution of silver nitrate.

681. Describe the tests for arsenic known as Marsh's test" and "Reinsch's test."

682. What occurs when arsine burns (a) in air, (b) in oxygen? How much oxygen is required for the complete combustion of 173 c.c. at N.T.P.?

683. Express by equations the reactions which occur

when arsine (a) is passed through solution of potassium permanganate, (b) is mixed with chlorine.

684. What is the ormula of the only known chloride of arsenic, and how is the substance obtained?

685. What is the action of arsenic chloride (a) on water, (b) on ammonia ?

686. In what different modifications does arsenic trioxide occur, and how may each be obtained?'

687. What weight of the oxide is formed by burning 3'567 grammes arsenic in a current of oxygen?

688. How may arsenic trioxide be obtained in octahedral and in rhombic crystals?

689. What molecular formula would you assign to arsenic trioxide knowing that its density is 13.85 (air = 1)?

690. How is arsenic pentoxide obtained, and what occurs when it is heated to a high temperature?

691. What occurs when arsenic trioxide is heated in contact (a) with charcoal, (b) with acetates?

692. Write the formula of the hydrides, chlorides, oxides, and acids of phosphorus and arsenic. In what respect do the pyro- and meta-acids of the two elements differ?