162 PEACTICAL PHYSIOLOGY 



Thus, if ethylic alcohol be heated with potassium bichromate and 

 sulphuric acid, it is oxidised and acetic aldehyde is formed : 



CH 3 - CH 2 OH + O = CH 3 - CHO + H 2 0. 



Ethyl alcohol. Acetic aldehyde. 



This group - CHO is, however, not a stable one, but very readily 

 undergoes further oxidation to produce the acid radicle COOH, 



CH 3 - CHO + = CH 3 - COOH. 



Acetic aldehyde. Acetic acid. 



Aldehydes are consequently strong reducing agents, and it is this 

 property which constitutes one of their most important group reactions, 

 for the reaction is frequently accompanied by a visible change in 

 the colour of the solution. Thus, in the above experiment, reduction 

 causes the yellow chromate to be changed into the green chromate. 



Their power of reducing cupric hydrate, which is blue in colour, 

 to cuprous hydrate, which is red, and of reducing argentic nitrate 

 to metallic silver, is of especial value as a test. Similar reactions 

 are obtained with certain bismuth and mercury salts. In order to 

 produce these reactions it is necessary that the solution be alkaline in 

 reaction. 



EXPERIMENT I. Demonstrate the reducing power of a simple alde- 

 hyde, such as formalin, using -cupric hydrate as the metallic salt. Place 

 one drop of a weak solution of cupric sulphate in the test tube. Add 

 about ten drops of formalin (or aldehyde), and then, drop by drop, 

 a strong solution of caustic potash. The first drop or so of the latter 

 produces a precipitate of cupric hydrate, but it afterwards becomes 

 redissolved, as aldehydes have the power of dissolving cupric hydrate 

 in alkaline solution. Now boil and note that a reddish-yellow pre- 

 cipitate of cuprous oxide is produced. The chemistry of the reaction 

 is illustrated by the following equations : 



1st Stage. CuS0 4 + 2KOH Cu(OH) 2 + K 2 S0 4 . 



Cupric sulphate + caustic potash. Cupric hydrate + pot. sulphate. 

 The cupric hydrate is kept in solution by the aldehyde to form clear 

 blue solution. 



2nd Stage. 2Cu(OH) 2 + RCHO = Cu 2 (OH) 2 + ECOOH + H 2 0. 



Cupric hydrate + aldehyde. Cuprous hydrate + acid. 



By heating the cuprous hydrate loses a molecule of water and 

 changes into the oxide : Cu 2 (OH) 2 - H 2 = Cu 2 O. 



EXPERIMENT II. Demonstrate the reduction of silver nitrate. 



Place about 5 c.c. of an ammoniacal solution of silver nitrate (pre- 

 pared by adding ammonia to a solution of silver nitrate till the preci- 



