Solutions are homogeneous mixtures, which contain dissolved substance, solute and solvent. The size of particles in the true solutions is less than 10–9 m (or <1 nm), which means the simplest small molecules or ions are present in a true solution, in contrast to colloidal dispersions. (For example, glucose solution contains molecules, sodium chloride solution — ions).
There are three states of matter, thus solutions may be solid (alloys), liquid (biological fluids) and gaseous (air). The component which does not change the aggregative state during the preparation of the solution is called a solvent. (For example, water is a solvent when we prepare the solution of sugar in water). If both components have the same aggregative state, we consider that the solvent is present in greater amount (e.g., acetic acid dissolved in water forms vinegar. In this case acetic acid is a solute, and water is a solvent).
Actually, solutions take an intermediate position between the barely mixtures and compounds, and the following effects may prove that an interaction between the molecules or ions of the solution takes place:
1) heat effect (when we dissolve ammonium nitrate, the beaker with freshly-prepared solution becomes cold, in case of sulphuric acid — vice versa);
2) change in volume (when we prepare the solution of alcohol in water, the resulting volume of solution is always less than expected; some substances, vice versa, may increase the final volume);
3) change in colour (compare the colour of CuSO4, white if no water is present, and the solution of this salt, which is blue).
Interaction of solvent with solute leads to the formation of solvates (or hydrates in water solution) due to electrostatic forces, donor-acceptor and hydrogen bonds. Some hydrates are rather stable and popular in medicine. For example, we use the crystalline hydrate MgSO4 · 7H2O to prepare solutions which are used as vasodilators (hypotensive effect), anticonvulsants, decongestants (anti-edematous action), for the labor pain relief; analgetic, sedative, laxative drugs and the part of polarizing mixtures for the various groups of patients.
Solubility of a substance depends on the nature of solvent, temperature, pressure (for gases). Solubility coefficient (k) shows how many grams of matter may be dissolved in 1 L of water to reach the maximum degree of saturation at constant temperature (visual effect — the sediment of substance). We know 3 groups of substances by solubility:
1) soluble (k >10 gr/L, like CaCl2, sugar, table salt);
2) slightly soluble (k = 0,01–10 gr/L, like CaSO4, silver sulfate);
3) insoluble (k <0,01 gr/L, like Ca3(PO4)2, chalk).
Lots of solids increase solubility with temperature raise. Solubility of gases, vice versa, decreases at high temperatures. That is why, in cold sea water animals have a much greater supply of oxygen. Solubility of gases increases with pressure raise, and this is the reason for aeroembolism (caisson disease; the bends): solubility of nitrogen in the blood increases very sharply at high pressure, but when the conditions return to normal, excess of gas forms bubbles and blood-vessel occlusion follows.
The concentration is a measure of the amount of solute in a given amount of solution or solvent. A solution is unsaturated when more solute will dissolve in it. It is saturated when no more solute will dissolve (e.g., when solid sugar is added to a saturated solution of sugar, it falls to the bottom and no more seems to dissolve even if you stir it intensively). Thus, the term ‘saturated’ means ‘full’, whereas ‘unsaturated’ means that ‘the solution could hold more’. There are also supersaturated solutions, but they are quite unstable and we need some special conditions to prepare them. A concentrated solution contains a large amount of solute per given amount of solvent or solution. A dilute solution contains a small amount of solute per given amount of solvent or solution (compare 50% and 1% H2SO4).
The qualitative terms dilute versus concentrated or unsaturated versus saturated are descriptively useful. However, quantitative measures of concentration are needed for many purposes in chemical laboratories, hospitals and pharmacies. For example, the effective administration of medicine usually requires a prescribed amount of the therapeutic agent.
Concentration can be expressed in various ways.
1. Percent by mass, С%, means the number of grams of solute in 100 gr of solution:
С% = (msolute / msolution) · 100%. (1.1)
A 10% NaCl solution may contain 10 grams of NaCl (and 90 grams of water) in 100 grams of solution or any other ratio of NaCl to solution for which the mass of NaCl is 10% of the total mass, for example, 20 milligrams of NaCl in 200 mg of the solution, 1.5 g of NaCl in 15 g of solution, or 7.4 kg of NaCl in 74 kg of solution. When using percent concentration by mass in calculations, it is convenient to use grams of solute per 100 grams of solution, since the number of grams of solute in 100 g of the solution is equal to the percent by mass of solute. To calculate the mass of solute in a given volume of solution, you must know the specific gravity (or density) of the solution (d). This is the mass of 1 ml of the solution:
msolution = d · V. (1.2)
In case when you need percent by volume you are supposed to use «V» instead of «m» in formula (1.1) given above.
2. Molarity, СМ, is defined as the number of moles of solute per 1 L of solution (1 L =1 dm³ = 1000 mL).
In accordance with this definition, СМ = ν/V, where the number of moles ν = m/M, and the volume V is put in liters. Finally, we get formula:
СМ = m / (М · V), (1.3)