II] THE COLLOIDAL STATE 13 



Expt. 1. Formatio7i of a suspension. Precipitate a solution of barium chloride 

 with some sulphuric acid and shake up well the fine precipitate of barium sulphate. 

 Note the gradual settling of the precipitate. 



Expt. 2. Formation of an eynulsi'on. Take a drop of olive oil in a test-tube and 

 half fill the tube with alcohol. Shake well and pour into a beaker of water. A fine 

 white emulsion of oil in water will be formed from which the oil will not separate. 

 By this method the oil is obtained in such small drops that stability is ensured. 



Take about equal quantities of olive oil in two test-tubes and add an equal 

 quantity of water to each. To one tube add a drop or two of 10 % caustic alkali 

 solution. Shake both test-tubes well. An emulsion is formed in both, but in the 

 tube without alkali the oil will separate out on standing. In the other tube the 

 emulsion is permanent. This is due to the fact that the olive oil (unless specially 

 purified) contains some free fatty acid. The latter forms soap with the alkali (see 

 p. 93) and renders the emulsion permanent. 



Expt. 3. Preparation of suspensoid sols, (a) Gold. Take two 100 c.c. measuring 

 cylinders and thoroughly clean them with nitric acid, and afterwards wash well with 

 freshly distilled water. In one make a 0*5 % solution of tannic acid (using the purest 

 sample obtainable) in water. In the other, 2 c.c. of commercial 1 % gold chloride are 

 made up to 100 c.c. with water. Use freshly distilled water in both cases. Mix equal 

 portions of the two solutions in a clean beaker. A purple colloidal solution of gold 

 will be formed. If three parts of the chloride solution are mixed with one part of the 

 tannin solution, and both solutions heated before mixing, a red colloidal solution is 

 obtained. (6) Silver. Take 5 c.c. of a 1 % solution of silver nitrate and add dilute am- 

 monia solution until the precipitate first formed just disappears, and then dilute with 

 100 c.c. of water. Mix equal volumes of this solution and the tannic acid prepared 

 for (a). A colloidal solution of silver will be formed which is clear brown by trans- 

 mitted light, but has a green fluorescence by reflected light, (c) Ferric hydroxide. 

 Take 5 c.c. of a filtered 33% solution of ferric chloride and pour into 500 c.c. of 

 boiling distilled water in a beaker. A colloidal ferric hydroxide sol is formed and the 

 colour changes to a deep brown-red. The yellow solution of ferric chloride is de- 

 composed by excess of water with the production of a soluble colloidal form of ferric 

 hydroxide, and hydrochloric acid is set free, (d) Arsenic tristdphide. Take 2 gms. 

 of arsenious acid and boil with 150 c.c. of distilled water, filter and cool. Then pass 

 sulphuretted hydrogen through the solution. A colloidal solution of the sulphide is 

 formed which is orange, with a greenish surface. 



The above sols should be kept for further experiment [see Expt. 8]. 



Expt. 4. Preparation ofemulsoid sols, (a) Starch. Weigh out 2 gms. of dry starch, 

 and mix well with a little cold distilled water. Boil 100 c.c. of distilled water in a 

 flask, and, when boiling, pour in the starch paste and boil for a few minutes longer, 

 stirring well all the time. A colloidal solution of starch is obtained which is faintly 

 opalescent. It is not afifected by heating and does not change its state on cooling, 

 (6) Gum arabic. Make a 5 7o solution of gum arabic by boiling 5 gms. with 100 c.c. 

 of distilled water. Note that a sticky or viscous solution is formed which froths on 

 shaking, (c) Protein. Weigh out 10 gms. of white flour and add 100 c.c. of distilled 

 water. Let the mixture stand for 2 or 3 hours and then filter. The extract contains 



