We see or use many substances on a daily basis, but rarely those substances are pure substances (chemically). Most of the time, we rather have mixtures; natural or man-made.
Questions:
- Explain what a mixture is, in comparison with a pure substance, and illustrate by an example.
- There are two large categories of mixtures:
- Identify those two categories and explain their main properties.
- If those categories have sub-categories, identify them and explain their differences.
Solutions:
- Mixture: a mixture is made of two or more different substances. For example a mixture of water and ethanol (H2O + C2H5OH), alloy of gold and silver (Au + Ag), etc.
Contrary to a pure substance, a mixture doesn’t possess characteristic properties.The properties change with the composition of the mixture. For example:
- a 50% - 50% water-ethanol mixture has different properties than a 75% - 25% water-ethanol mixture;
- gasoline, a mixture of hydrocarbons,boiling point ranges between 40-180oC.
Mixtures can be separated into their respective components (pure substances) by simple physical means such as distillation, filtration, etc.
N.B: In popular and marketing language, a product may be called pure without really qualifying as a chemically pure product. Example, good quality drinking water may be called pure water, yet it is a mixture of water and mineral salts!
- There are two categories of mixtures: homogeneous mixtures and heterogeneous mixtures.
Homogeneous mixtures
In a homogeneous mixture, you cannot distinguish the components of the mixture with your eyes or simple means of observation such as an ordinary microscope. The particles (0.01-1nm) of the different components are completely inter-mixed. They do not separate on standing.
One characteristic of homogeneous mixture is that their composition is homogeneous or uniform in the whole body of the mixture. They cannot be separated by filtration.
There are three categories of homogeneous mixtures:
(i) gaseous: made of gases,
(ii) liquid: made of a liquid solvent, the main component, plus a minor component,
(iii) solid: made of metals, one being the major component, other(s) minor, commonly known as alloys (see Table 1).
Another characteristic that is found in liquid homogeneous mixtures commonly called solutions is that they are transparent, i.e. they let light pass through.
In a liquid solution, the major component is called solvent, whereas the minor component(s) is(are) called solute(s). For example, in a solution of sugar in water, water is the solvent and sugar is the solute.
Water is the most popular solvent commonly used in chemistry laboratories because it has the capacity of dissolving many substances; that is why it is called universal solvent; and it has the advantage of being available almost all over the world and generally at an affordable cost/price.
In chemical equations, solutions in water, or aqueous solutions, are indicated by the symbol (aq) after the chemical formula.
Table 1: Homogeneous mixtures can be gaseous, liquid, and solid
Heterogeneous mixtures
A heterogeneous mixture is a mixture where the components can be distinguished by simple means of observation such as eyes or an ordinary microscope. In these kinds of mixture, the minor component is dispersed, in form of small particles or droplets in a major component called continuous phase or dispersant.
Heterogeneous mixtures can be subdivided into three sub-categories:
(i) Suspensions,
(ii) Colloids, and
(iii) Mixtures of solids.
- Suspensions: contain relatively large particles or droplets (>1000 nm) of an insoluble solid, or droplets of an insoluble liquid dispersed in a continuous liquid phase. If left to rest, the particles or droplets settle out to form two or more phases. Suspensions are opaque, i.e. do not let light pass. They can be separated by filtration.
- Colloids: contain smaller particles or droplets (1-1000 nm) and take various forms: emulsions, sols, gels, aerosols, and foams. They do not separate on standing.The particles or droplets of a colloid are too small to be seen by eyes, but they do scatter light (Tyndall effect). They scatter light, i.e. absorb and re-emit light in all directions (ex: the blue color of the sky). Colloids cannot be separated by filtration.
- Mixtures of solids: although this kind of mixtures is not mentioned in chemistry textbooks, it is one mixture that we deal with in everyday life. It contains large pieces of solid objects that can be hand-picked or separated by different physical means such as sieves, vibration and shaking belts, etc.
Table 2 below shows some examples of Heterogeneous mixtures.
Table 2: Heterogeneous mixtures and their sub-categories
(*) N.B:
-You certainly have seen the word, “homogenized”, on some packets of milk? This is a commercial/marketing description of the product, not a scientific description. Milk is a heterogeneous, not homogeneous mixture.
-Emulsion paints: some mixtures where solid particles are dispersed in water are commonly/commercially called ‘Emulsion’ paints. Example, White ‘emulsion’ paint of TiO2 in water, but they are not in fact true emulsions; they are rather sols.
Summary