Which mixtures are classified as solutions?

KEY IDEAS
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A simple definition of “solution” is a mixture of substances which is homogeneous: This means that ....

• we cannot distinguish the individual component substances by eye
• the composition is the same in all parts of the mixture, and so
• the properties of the mixtures (such as density, colour hue, colour intensity, and refractive index) are the same everywhere.

Solutions may be gaseous, liquid or solid. Examples of solutions are:

• Salty water (water and sodium chloride)
• Air (nitrogen, oxygen, argon, carbon dioxide, and lesser components)
• Brass (copper and zinc metals)
• Wines (water, alcohol, and substances that contribute to the aroma, flavour and colour)

​Liquid solutions in water, for example, can result from dissolving liquids, solids, or gases:

• liquids (eg, ethanol);
• solids (eg, sugar); or
• gases (eg, carbon dioxide).

 

​Many mixtures are not solutions because they are not homogeneous: that is, they are heterogeneous: Examples include:

• water and sand.
• air with dust particles.
• liquid (not yet set) concrete (sand, gravel and cement – each component identifiable).

These are called suspensions. Particles of one substance are suspended within the other, but over time will settle out due to gravity.
 

As is often the case, artificial classifications have ambiguity.
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Firstly, if we have a glass vessel containing only air, we can presume that the composition of the mixture is the same from location to location in the sample: that is, it is homogeneous. So we would have no trouble declaring this to be a solution.

However, the composition of earth’s atmosphere at an altitude of 10 km is different from that at the earth’s surface. So the whole of the atmosphere does not technically satisfy the criteria of a solution.

The same reasoning applies when we consider a confined sample of seawater, and (say) the Pacific Ocean.
 

And there is another, more important issue …..

In some mixtures, the particles of one component are borderline in size between those of a solution and a suspension. They are bigger than simple molecules, and their size is in the range about 10–1000 nanometres (1 nm = 1 E-9 m, or 0.000 000 001 metre).
 
These cannot be seen by eye, but those above about 200 nm are visible in an optical microscope, and the smaller ones can be “seen” by electron microscope. A diameter of a helium atom is about 0.1 nm.

These particles remain homogeneously dispersed throughout the other substance, and do not settle under gravity.
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They are called colloidal dispersions, and have a whole set of characteristic properties, the most obvious often being the ability to scatter a beam of light.
 

Milk is a colloidal dispersion: Tiny butterfat globules are dispersed throughout an aqueous medium.
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Other examples include

• in liquid medium: mayonnaise, creams, blood
• in solid medium: jellies, agar
• in gaseous medium: mist, aerosol sprays, fog, clouds, and smoke.

 

Blood is all three of (i) a solution, (ii) a suspension, and (iii) a colloidal suspension:

(i) It is a solution because very small molecules (< 2 nm) of, for example, glucose and amino acids are indistinguishable in the plasma.

(ii) Blood cells are at least 6000 nm, so they are in blood as a suspension.

(iii) The plasma also contains proteins whose size is in the range 5-500 nm, and these remain distributed through the plasma as a colloidal dispersion.
 

SELF CHECK
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1.        Are the following statements correct or incorrect?
(a)       A mixture of copper, sulfure and oxygen is a heterogeneous mixture.
(b)       A sample of copper sulfate is a heterogeneos mixture.
(c)       A sample of copper sulfate is a homogeneous mixture.
(d)       A mixture of a little copper sulfate in lots of water forms a homogeneous mixture when shaken.
(e)        Typical of all solutions, the colour hue and colour intensity of a red wine is the same at all locations.
(f)        Dusty air is a suspension because it is possible to see the dust particles, and they settle out over time.
g)        Dusty air is a homogeneous mixture because it looks the same everywhere.

2. The text below has some missing terms. Insert the following terms into the appropriate places in the text.

• dense
• suspension
• heterogeneous
• colloidal dispersions
• refractive index
• solution
• mud
• homogeneous
• vinegar

 
​If you cannot recognise all of the individual components of a mixture, and the composition of the mixture is the same at all locations, that mixture is said to be …………………... At all places in a homogeneous mixture, properties such as ……………………. have the same value. Homogeneous mixtures are classified (by people) as ………………... An example of a solution is …………………….. Mixtures in which individual components can be recognised are said to be ……………………... Heterogeneous mixtures are called …………………….. An example of a heterogeneous mixture is …………………….. Over time, the more ……………………. components of a suspension will settle out. Mixtures in which the size of the particles of at least one component are intermediate between those of a solution and a suspension are called ……………………..
 

3. Disregarding the social comment, evaluate the technical correctness of the label in the pic below.

ANSWERS
1.         (a) Correrct
            (b) Incorrect
            (c) Incorrect
            (d) Correct
            (e) Correct
            (f) Correct
            (g) Incorrect
 
​2.         If you cannot recognise all of the individual components of a mixture, and the composition of the mixture is the same at all locations, that mixture is said to be homogeneous. At all places in a homogeneous mixture, properties such as refractive index have the same value. Homogeneous mixtures are classified (by people) as solutions. An example of a solution is vinegar. Mixtures in which individual components can be recognised are said to be heterogeneous. Heterogeneous mixtures are called suspensions. An example of a heterogeneous mixture is mud. Over time, the more dense components of a suspension will settle out. Mixtures in which the size of the particles of at least one component are intermediate between those of a solution and a suspension are called colloidal dispersions.

3. Alcohol (ethanol) is a pure substance, so technically the statement is a nonsense. [There are, of course many alcohol-containing solutions which give some people pleasure or comfort.]

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