Jerry Redding, age 12, of Winston‑Salem,
How are colloids held together?
The chemistry of colloids takes us down to particles of matter too small for our eyes to see and explains how they behave when mixed with other substances. These particles carry charges of positive and negative electricity. As in all things, two positive charges repel each other, as do two negative charges. Two opposite charges attract each other and tend to pull small particles together. This principle acts to some extent in the colloids, Another factor is the size of the particles themselves.
Ink, milk, mud, salty water, gelatin and fog are all mixtures of tiny particles suspended in another material. The chemist calls these mixtures dispersions, and they come 3.n three qualities. A pinch of salt in a glass of water dissolves and becomes a solution. The sizable particles of silt in a pail of muddy water will settle to the bottom. When the particles are larger than the dissolved salt particles and smaller than the sizable silt particles, they can form colloid dispersions. Gelatin, protoplasm and sometimes fog are colloids because the right‑sized particles can remain suspended for an indefinite length of time.
In some cases, the particles in a colloid are kept apart because of electric charges. They may be prevented, by electric charges between them and the medium material in which they are suspended or by the charges between each other, from getting together, to form clots large enough to sink or float. Certain chemicals can change this situation. Latex, the milky sap from the rubber tree, is a colloid, When formic acid is added, the electrical picture changes and the rubber particles clot together.
The basic factor in a colloid, however, is the size of the suspended particles. They must be bigger than the atoms, ions and single molecules which form solutions and smaller than those in silty water. By and large, they measure anywhere from 25 million to 2,000 million to an inch. They are often long chains or networks of linked molecule units, Like all particles they are very energetic. In a solid medium, they vibrate, In a gas or liquid they rush at breakneck speeds and collide countless times a second. This speed is governed by size and, at ordinary temperatures, the colloid particles are just the right size to cancel the pull of the earthts gravity.
The chemist calls this surface size the area compared with the volume. The surface area of a one inch cube is six square inches, Cut into colloid particles, this same volume has a total surface area of 200 acres. A greater proportion of atoms are now at the surface and fewer buried inside. This and other factors give the colloid particle just the right amount of energy to remain suspended and separated in the colloid dispersion.