CRYSTALLOIDS AND COLLOIDS 59 



PRACTICAL WORK. 



A. BEHAVIOUR OF CRYSTALLOIDS AND COLLOIDS WITH WATER. 



(1) Place some crystals of potassium bichromate at the bottom 

 of a test tube and just cover with hot water. The crystals 

 quickly dissolve and disappear and there is no expansion on 

 solution. Examine a drop of the solution under the microscope 

 it is clear, no particles are to be seen. This is a true (crystalloid) 

 solution. Leave the tube of solution and examine later : as the 

 solution cools and the solvent evaporates, crystals of the salt 

 reappear. 



(2) Warm some powdered glue in a test tube with a little water : 

 the glue disappears gradually, taking up water and forming a 

 colloid sol. On cooling and standing a solid jelly (gel) is formed. 



(3) Place a rectangular strip of dry gelatine on a sheet of glass 

 over squared paper. Mark the edges of the gelatine with Indian 

 Ink opposite the divisions of the paper. Allow the ink to dry, 

 then dip the gelatine in water and float it on the glass in a little 

 water. Note the gradual expansion of the gelatine as it absorbs 

 water. After a time measure the increase in area of the gelatine 

 strip by again placing the glass over squared paper. Now 

 transfer the gelatine to a piece of fine muslin and hang it up to 

 dry. It contracts, and gradually decreases in area owing to loss 

 of water. The gelatine is a colloid gel. 



(4) Put one drop of Indian Ink in a test tube and dilute with 

 water till you can see through the mixture. Filter, and examine 

 a drop of the filtrate under the high power of the microscope. 

 It is full of fine particles. This is a coarse suspensoid sol. The 

 particles show Brownian movement. The particles do not settle 

 if the tube is left standing. 



(5) Place a very little solid Congo Red in water. It apparently 

 dissolves, the solution looks clear and the microscope reveals 

 no particles. This is a fine suspensoid sol of which the disperse 

 particles are ultramicroscopic. 



(6) Examine the demonstration in which the three liquids 

 (a) potassium bichromate solution, (b) Congo Red sol, (c) Indian 

 Ink sol are illuminated laterally by a strong beam (Tyndall's 

 beam), (a) is clear, (b) and (c) cloudy; (b) looks more cloudy 

 when thus lighted than when looked at against the light. 



B. DIFFERENCE IN THE RATES OF DIFFUSION OF CRYSTALLOIDS 



AND COLLOIDS. 



(7) The two test tubes provided contain at the bottom orange 

 and red jellies respectively. The orange jelly is impregnated 



