Experimental cookery

Solutions and suspensions. When a solid substance is added to a liquid one of three types of mixtures may be formed. ( 1 ) A true solution, which is a homogeneous mixture. (2) A colloidal solution, which appears to be homogeneous. The dispersed particles have a size of 1 m/x to 0.1/x and can be separated by a sufficiently fine filter. (3) A suspension in which the dispersed particles are greater than OA[x. The dispersed particles can be separated from the liquid by filtration or sedimentation. Gortner states, "Inasmuch as fine suspensions possess, to a large degree, certain characteristic properties of colloidal systems, there has been a general tend- ency of recent years to raise the upper limit to 0.5/x," instead of OA/x. It should be understood that the above classification is an arbitrary one and that in nature there are no abrupt transitions. There can be no definite division made between true and colloidal solutions for the transition is gradual. There are some colloidal solutions known in which the dispersed particles are less than 1 m/x. There is also no line of demarcation between colloidal solutions and suspensions. In true solutions the dispersed phase consists of particles of molecular or ionic size, whereas the colloidal solutions contain particles of larger size, and the suspension contains particles large enough for mechanical separation. The smaller the particles with a definite quantity of material the more dispersed the substance; the larger the particles the less dispersed. A true solution can be reproduced if the temperature, pressure, and concentration are known, since the degree of dispersion is constant. But in colloidal solu- tions it is necessary to know the degree of dispersion as well as the tem- perature, pressure, and concentration to reproduce the system. The prop- erties of solutions like gelatin also depend upon the method of preparation and their previous history; the properties of a true solution are independent of the method of preparation or their previous history. The systems whose properties are dependent upon their previous history are said to show hysteresis. Colloidal systems are heterogeneous, so that it is necessary to distinguish which is the dispersed phase and which is the dispersing medium. Some of SOLUTIONS AND SUSPENSIONS the terms used are dispersed phase and dispersing medium* discontinuous phase and continuous phase ; internal phase and external phase ; and micelles and intermicellar liquid. The usage of the last group appears to be in- creasing at the present time. TABLE 1 Characteristic Differences of True solutions Colloidal solutions In molecular subdivision Particles are not visible with ultra-microscope Particles less than 1 m/x Formation of gels is not characteristic Transparent Particles pass through parchment membranes Intense kinetic movement Systems show high os- motic pressure In colloidal subdivision Refracted light of parti- cles is visible with ultra- microscope Particles from 1 m^i toO. 1 m Formation of gels is characteristic Transparent Particles or micelles pass through high-grade filter paper, but not parchment Less kinetic movement, more Brownian move- ment Systems show low os- motic pressure Suspensions In mechanical subdivision Particles visible with or- dinary microscope or naked eye Particles greater than 0. 1 /z Formation of gels is not characteristic Generally opaque