Solution

A solution is a homogeneous mixture of two or more substances. The term homogeneous means "the same throughout." For example, suppose that you make a solution of sugar in water. If you were to take a drop of the sugar solution from anywhere in the solution, it would always have the same composition.

Terminology

A number of specialized terms are used in talking about solutions. The solvent in a solution is the substance that does the dissolving. The solute is the substance that is dissolved. In the sugar solution described above, the water is the solvent and the sugar is the solute.

Although that definition is neat, it does not always make a lot of sense. For example, one can make a solution of two gases. In fact, the air around us is a solution consisting of oxygen, nitrogen, argon, carbon dioxide, and other gases. In this case, it is difficult to say which gas "does the dissolving" and which gas (or gases) "is dissolved."

An alternative method of defining solvent and solute is to say that the component of the solution present in the largest amount is the solvent while the components present in lesser amounts are solute. According to that definition, nitrogen is the solvent in atmospheric air because it is present in the largest amount. Oxygen, argon, carbon dioxide, and other gases, then, are the solutes.

The term miscible is often used to describe how well two substances—generally, two liquids—mix with each other. For example, if you try to mix oil with water, you will find that the two do not mix very well at all. They are said to be immiscible—incapable of mixing. In contrast, ethyl alcohol and water are completely miscible because they mix with each other in all proportions.

Words to Know

Concentration: The amount of a substance (solute) present in a given volume of solvent or solution.

Homogeneous: The same throughout.

Miscibility: The extent to which some substance will mix with some other substance.

Saturated: In referring to solutions, a solution that contains the maximum amount of solute for a given amount of solvent at a given temperature.

Solubility: The tendency of a substance to dissolve in some other substance.

Solute: The substance that is "dissolved" or that exists in the least amount in a solution.

Solvent: The substance that "does the dissolving" or that exists in the largest amount in a solution.

Supersaturated: In referring to solutions, a solution that contains more than the maximum amount of solvent that can normally be dissolved in a given amount of solvent at a given temperature.

Unsaturated: In referring to solutions, a solution that contains less than the maximum amount of solvent that can be dissolved in a given amount of solvent at a given temperature.

Solubility is a term similar to miscibility but more exact. The solubility of a substance is the amount of the substance that will dissolve in a given amount of solvent. For example, the solubility of sugar in water is approximately 90 grams of sugar per 100 grams of water. That statement means that one can dissolve up to 90 grams of sugar in 100 grams of water.

The solubility of a substance is dependent on the temperature. The statement in the previous paragraph, for example, should have been that 90 grams of sugar will dissolve in 100 grams of water at some specific temperature. That temperature happens to be 0°C.

Generally speaking, the solubility of substances increases with temperature. The graph in Figure 1 illustrates this point. Notice that the solubility of sugar increases to a little over 100 grams per 100 grams of water at 25°C and to 130 grams per 100 grams of water at 50°C.

An important exception to this rule concerns gases. All gases become less soluble in water as the temperature increases.

Concentration of solutions

Solutions are mixtures whose composition can vary widely. One can make a water solution of sodium chloride by dissolving 1 gram of sodium chloride in 100 grams of water; 5 grams in 100 grams of water; 10 grams in 100 grams of water; and so on. The amount of solute for any given amount of solvent is defined as the concentration of the solution.

One way of expressing the concentration of a solution is with the terms dilute and concentrated. These terms are not very specific. For example, a solution containing 1 gram of sodium chloride in 100 grams of water and a second solution containing 2 grams of sodium chloride in 100 grams of water are both dilute. But the term is appropriate because, at room temperature, nearly 40 grams of sodium chloride can be dissolved in 100 grams of water. Thus, a solution containing 35 grams of sodium chloride in 100 grams of water could be called a concentrated solution.

Solutions can also be classified as saturated, unsaturated, or supersaturated. A saturated solution is one that holds all the solute it possibly can at any given temperature. For example, the solubility of sodium chloride in water is 37 grams per 100 grams of water. If you make a solution containing 37 grams of sodium chloride in 100 grams of water, the solution is said to be saturated; it can't hold any more sodium chloride.

Any solution containing less than the maximum possible amount of solute is said to be unsaturated. A solution with 5 grams of sodium chloride (or 10 grams or 20 grams or 30 grams) in 100 grams of water is unsaturated.

Finally, supersaturated solutions are also possible. As bizarre as it sounds, a supersaturated solution is one that holds more solute than is possible at some given temperature. The way to make a supersaturated solution is to make a saturated solution at some higher temperature and then let the solution cool very carefully.

For example, one could make a saturated solution of sugar in water at 50°C by adding 130 grams of sugar to 100 grams of water. That solution would be saturated. But then, one could allow the solution to cool down very slowly. Under those circumstances, it might happen that all of the sugar would remain in solution even at a temperature of 25°C. But at that temperature, the solubility of sugar is normally a little over 100 grams per 100 grams of water. Therefore, the cooled solution would be supersaturated. Supersaturated solutions are normally very unstable. The slightest movement in the solution, such as simply shaking it, can cause the excess solute to settle out of the solution.

solution in chemistry, homogeneous mixture of two or more substances. The dissolving medium is called the solvent , and the dissolved material is called the solute. A solution is distinct from a colloid or a suspension .

In most common solutions, the solvent is a liquid, often water, and the solute may be a solid, gas, or liquid. For example, syrups are solutions of sugar, a solid, in water, a liquid; household ammonia is a solution of ammonia gas in water; and vinegar is a solution of acetic acid, a liquid, in water. When two liquids, e.g., water and ethanol, can be mixed in any proportions, the solvent is commonly considered to be the one present in greater proportion. Some alloys are solutions of one solid in another, as are many rocks. A mixture of gases, such as air, is usually not thought of as a solution.

Characteristics of Solutions

The solute particles in a solution are generally of molecular size or smaller, much smaller than those in a colloid or a suspension. The solute particles cannot be observed even with an ultramicroscope. They do not settle out from the solvent on standing, and they cannot be separated from the solvent by physical means, such as filtration or centrifugation. On the other hand, a solution differs from a compound in that its components can occur in continuously varying proportions, within certain limits (although within a given solution they are present in the same proportions throughout the solution), while the components of a compound can occur only in certain fixed proportions.

The addition of solute affects the boiling point, freezing point, and vapor pressure of the solution, in general raising the boiling point, depressing the freezing point, and lowering the vapor pressure (see Raoult's law ). A number of substances (acids, bases, and salts) exhibit characteristic behavior in aqueous solution. These substances dissociate in water to form positive and negative ions that enable the solution to conduct electricity. Such solutions are called electrolytic (see electrolyte ).

The proportion of solute to solvent in a given solution is expressed by the concentration of the solution. Concentrations may be stated in a number of ways, such as giving the amount of solute contained in a given volume of solution or the amount dissolved in a given mass of solvent. A solution having a relatively high concentration is said to be concentrated, and a solution having a low concentration is said to be dilute.

In many solutions the concentration has a maximum limit that depends on various factors, such as temperature, pressure, and the nature of the solvent. The maximum concentration is called the solubility of the solute under those conditions. When a solution contains the maximum amount of solute, it is said to be saturated; if it contains less than that amount, it is unsaturated.

The most obvious factor affecting solubility is the nature of the solvent. Ordinary table salt (sodium chloride) is soluble in water, but only slightly soluble in ethanol, and insoluble in diethyl ether. Temperature is also important in determining solubility. Solids are usually more soluble at higher temperatures; more salt will dissolve in warm water than in an equal amount of cold water. Graphs showing the solubility of different solids as a function of temperature are called solubility curves and are very useful in chemical analysis. Solubility also depends on pressure, especially in the case of gases, which are more soluble at higher pressures.

Under certain conditions a solution may be made to contain more solute than a saturated solution at the same temperature and pressure; such a solution is called supersaturated. If even a single crystal of undissolved solute is added to a supersaturated solution, all the excess solute above the normal solubility concentration will immediately crystallize out of the solution.

Heat of Solution

The addition of some solutes to a solvent will raise the temperature of the solution, while others may lower the temperature and still others will have no noticeable effect. This behavior depends on the heat of solution of the solute in the given solvent. The heat of solution, i.e., the amount of heat given off or absorbed during the process of solution, is equal to the difference between the energy that must be supplied to break up the crystals of the solute and the energy that is released when the solute particles are taken into solution by the solvent (see enthalpy ). If the heat of solution is negative (i.e., more energy is required to break up the crystal than is released in forming the solution), then the temperature will decrease; if the heat of solution is positive, the temperature will increase.

solution
1. A physically homogeneous mixture of two or more substances in which solid, liquid, or gaseous phases may combine in one of those phases. A constituent of a solution can be separated out by changing its phase, e.g. boiling, condensing, or freezing. Where a solution is formed by dissolving a quantity of one substance in a larger quantity of another, the smaller quantity is called the ‘solute’, the larger quantity, the ‘solvent’. Compare COLLOID.

2. A weathering process by which weakly bonded ionic components of minerals are detached through the attraction of water molecules (which carry a positive electrical charge at one end and a negative charge at the other, although they are neutral overall), and then carried away from the weathering environment. Halites and the sulphates and carbonates of magnesium and calcium are especially vulnerable. Solution is usually the first stage of chemical weathering.

so·lu·tion / səˈloōshən/ • n. 1. a means of solving a problem or dealing with a difficult situation: there are no easy solutions to financial and marital problems. ∎  the correct answer to a puzzle: the solution to this month's crossword. 2. a liquid mixture in which the minor component (the solute) is uniformly distributed within the major component (the solvent). ∎  the process or state of being dissolved in a solvent. 3. archaic the action of separating or breaking down; dissolution: the solution of British supremacy in South Africa.

solution A homogeneous mixture of a liquid (the solvent) with a gas or solid (the solute). In a solution, the molecules of the solute are discrete and mixed with the molecules of solvent. There is usually some interaction between the solvent and solute molecules. For example, when sodium chloride dissolves in water the sodium ions attract polar water molecules, with the negative oxygen ion pointing towards the positive Na⁺ ion. This is called hydration.

solution Liquid (the solvent) into which another substance (the solute) is dissolved, or a liquid consisting of two or more chemically distinct compounds, inseparable by filtering. The amount of a solute dissolved in a solvent is called the concentration. See also mixture; saturated solution

solute Gaseous, liquid or solid substance that dissolves in a solvent to form a solution. Many solids dissolve in water. Liquids can dissolve in liquids, and some gases, such as hydrogen chloride (HCl), are soluble in water.

solution A word appended onto a computer term as a marketing ploy. It implies that a product can solve certain commercial problems: for example, ‘ECOMMERCE solution’.

solution (sŏ-loo-shŏn) n. a homogeneous mixture of two or more dissimilar substances, usually of a liquid (the solvent) in which a solid (the solute) is dissolved.

sol·ute / ˈsälˌyoōt/ • n. the minor component in a solution, dissolved in the solvent.

solute The substance dissolved in a solvent in forming a solution.

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