Basic Concepts: What are Solutions?

In my last Basic Concept post when discussing why reactions occur, I left it rather vague as to what happens when substances dissolve. I suggested that no reaction had taken place because the chemical structure of the solutes had not really changed. Today I want to go into a little more detail as to what happens when solutes and solvents mix and discuss the difference between solution and liquid. We typically think of solutions as being a liquid mixture. But are all liquids solutions and all solutions liquids?

This question is a bit like the rhetoric around the series of statements:

My cat is gray. Are all cats gray?

To be able to answer the question as to the relationship between liquids and solutions we need to define what we mean by liquid and solutions. Janet in a post on phase changes describes:

A liquid has more intermolecular attractions holding the molecules to each other than the gas phase, but fewer than the solid phase. As a result, while the resulting clump of molecules is still nowhere near as compressible as the gas phase of the same substance would be, liquids don’t have rigid shapes, and they take on the shape of the containers you pour them into. The intermediate number of intermolecular associations (between gas and solid) means that the molecules in the liquid are sticking together but have some “wiggle room”.

Chambers Dictionary of Science and Technology defines liquid as:

A state of matter between a solid and a gas, in which the shape of a given mass depends on the containing vessel, the volume being independent. Liquids are almost as incompressible as solids.

The dictionary defines a solution (chem) as a:

Homogeneous mixture of two or more components in a single phase. Often refers specifically to a solution in water.

From this it would imply that solutions are special cases of liquids in the same way gray cats are a special case of cat. I quibble with the dictionary definition of solution (and fortunately so does this site for intro chemistry at OKState) as solutions can be solids in solids, liquids n solids, gases in solids, solids in liquids, liquids in liquids, gases in liquids, and gases in gases.

So having established that solutions are:

1. A mixture of two or more components
2. Typically but not always liquid
3. Homogeneous

When discussing how components become a solutions, it is for convenience sake that the major component of a solution is termed the solvent and the other component(s) is(are) termed the solute(s). This helps chemists discuss what is going on when components of a solution are mixed. These are defined by the dictionary as:

Solvent: The component of a solution which is present in excess, or whose physical state is the same as that of the solution.

Solute: A substance that is dissolved into another.

Solutions typically occur by the solute being dissolved into the solvent. In the case of an aqueous solution of sodium chloride (table salt, NaCl), water is the solvent (aqueous is just the chemistry fancy way of saying in water) and NaCl is the solute.

Interestingly, different solutes dissolve in different ways. As I have posted about before, sodium chloride (NaCl) becomes ions of Na(+) and Cl(-) when mixed with water. This allows the salt to stay in solution because the bipolar nature of water means that water molecules surround the ions:

Note that the partially positive hydrogen atoms of water face the negatively charged Cl ions and the partially negative oxygen atoms face the positively charged Na ions.

Organic molecules such as glucose, sucrose (table sugar) and vitamin C (ascorbic acid) are soluble because they have hydroxyl (OH) side chains. These form hydrogen bonds with water molecules.

The way that water molecules are arranged around the solute is known as solvation. This is defined as the association or combination of molecules of solvent with solute ions or molecules.

Not all organic molecules are able to dissolve in water. The familiar example being oil and water when making a salad dressing. Some organic compounds are able to partially dissolve into water. This makes them sparingly soluble and they partition into the water phase or form micelles with hydrophilic (water loving) groups facing the water molecule and hydrophobic (water hating) groups facing the inside of the micelle. This is how solutions such as milk are formed as suspended in water solvent are proteins and lipids.

Solutions are endlessly fascinating for food scientists and I will discuss more about colloids and suspensions in another post.

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