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Properties of Solutions Essay

A solution is a mixture of materials, one of which is usually a fluid. A fluid is a material that flows, such as a liquid or a gas. The fluid of a solution is usually the solvent. The material other than the solvent is the solute. We say that we dissolve the solute into the solvent. Some solutions are so common to us that we give them a unique name. A solution of water and sugar is called syrup. A solution of sodium chloride (common table salt) in water is called brine. A sterilized specific concentration (0.15 molar) of sodium chloride in water is called saline. A solution of carbon dioxide in water is called seltzer, and a solution of ammonia gas in water is called ammonia water. A solution is said to be dilute if there is less of the solute. The process of adding more solvent to a solution or removing some of the solute is called diluting.

A solution is said to be concentrated if it has more solute. The process of adding more solute or removing some of the solvent is called concentrating. The concentration of a solution is some measurement of how much solute there is in the solution. It might initially offend your sensibilities to consider a solution in which the solvent is a gas or a solid. The molecules of a gas do not have much interaction among them, and so do not participate to a large extent in the dissolving process. Solids are difficult to consider as solvents because there is a lack of motion of the particles of a solid relative to each other.

There are, however, some good reasons to view some mixtures of these types as solutions. The molecules of a gas do knock against each other, and the motion of a gas can assist in vaporizing material from a liquid or solid state. The fan in a ‘frost free’ home freezer moves air around inside the freezer to sublimate any exposed ice directly into water vapor, a process clearly akin to dissolving. Solid metals can absorb hydrogen gas in a mixing process in which the metal clearly provides the structure. True solutions with liquid solvents have the following properties:

PROPERTIES OF SOLUTIONS

1.The particles of solute are the size of individual small molecules or individual small ions. One nanometer is about the maximum diameter for a solute particle. 2.The mixture does not separate on standing. In a gravity environment the solution will not come apart due to any difference in density of the materials in the solution. 3.The mixture does not separate by common fiber filter. The entire solution will pass through the filter. 4.Once it is completely mixed, the mixture is homogeneous. If you take a sample of the solution from any point in the solution, the proportions of the materials will be the same. 5.The mixture appears clear rather than cloudy. It may have some color to it, but it seems to be transparent otherwise. The mixture shows no Tyndall effect. Light is not scattered by the solution. If you shine a light into the solution, the pathway of the light through the solution is not revealed to an observer out of the pathway.

6.The solute is completely dissolved into the solvent up to a point characteristic of the solvent, solute, and temperature. At a saturation point the solvent no longer can dissolve any more of the solute. If there is a saturation point, the point is distinct and characteristic of the type of materials and temperature of the solution. 7.The solution of an ionic material into water will result in an electrolyte solution. The ions of solute will separate in water to permit the solution to carry an electric current. 8.The solution shows an increase in osmotic pressure between it and a reference solution as the amount of solute is increased. 9.The solution shows an increase in boiling point as the amount of solute is increased.

10.The solution shows a decrease in melting point as the amount of solute is increased. 11.A solution of a solid non-volatile solute in a liquid solvent shows a decrease in vapor pressure above the solution as the amount of solute is increased. These last four of the properties of solutions collectively are called colligative properties. These characteristics are all dependent only on the number of particles of solute rather than the type of particle or the mass of material in solution.

OTHER TYPES OF MIXTURE

Take a spoonful of dirt and vigorously mix it with a glass of water. As soon as you stop mixing, a portion of the dirt drops to the bottom. Any material that is suspended by the fluid motion alone is only in temporary suspension. A portion of the dirt makes a true solution in the water with all of the properties of the above table, but there are some particles, having a diameter roughly between 1 nm and 500 nm, that are suspended in a more lasting fashion. A suspended mixture of particles of this type is called a colloid, or colloidal suspension, or colloidal dispersion. For colloids or temporary suspensions the phrase dispersed material or the word dispersants describes the material in suspension, analogous to the solute of a solution. The phrase dispersing medium is used for the material of similar function to a solvent in solutions. As with true solutions, it is a bit of a stretch to consider solids as a dispersing medium or gases as forming a large enough particle to be a colloid, but most texts list some such. A sol is a liquid or solid with a solid dispersed through it, such as milk or gelatin. Foams are liquids or solids with a gas dispersed into them.

Emulsions are liquids or solids with liquids dispersed through them, such as butter or gold-tinted glass. Aerosols are colloids with a gas as the dispersing medium and either a solid or liquid dispersant. Fine dust or smoke in the air are good examples of colloidal solid in a gas. Fog and mist are exampes of colloidal liquid in a gas. Liquid dispersion media with solid or liquid dispersants are the most often considered. Homogenized whole milk is a good example of a liquid dispersed into a liquid. The cream does not break down into molecular sized materials to spread through the milk, but collects in small micelles of oily material and proteins with the more ionic or hydrophilic portions on the outside of the globule and the more fatty, or oily, or non-polar, or hydrophobic portions inside the ball-shaped little particle.

Blood carries liquid lipids (fats) in small bundles called lipoproteins with specific proteins making a small package with the fat. Proteins are in a size range to be considered in colloidal suspension in water. Broth or the independent proteins of blood or the casein (an unattached protein) in milk are colloidal. There are many proteins in the cellular fluids of living things that are in colloidal suspension. Colloidal dispersants in water stay in suspension by having a layer of charge on the outside of the particle that is attractive to one end of water molecules.

The common charge of the particles and the water solvation layer keep the particles dispersed. A Cottrel precipitator collects the smoke particles from air by a high voltage charge and collection device. Boiling an egg will denature and coagulate the protein in it. Proteins can be fractionally ‘salted out’ of blood by adding specific amounts of sodium chloride to make the proteins coagulate. The salt adds ions to the liquid that interfere with the dispersion of the colloidal particles. Colloids with liquid as a dispersing agent have the following properties:

PROPERTIES OF COLLOIDS

1.The particles of dispersant are the between about 500 nm to 1 nm in diameter. 2.The mixture does not separate on standing in a standard gravity condition. (One ‘g.’) 3.The mixture does not separate by common fiber filter, but might be filterable by materials with a smaller mesh. 4.The mixture is not necessarily completely homogeneous, but usually close to being so. 5.The mixture may appear cloudy or almost totally transparent, but if you shine a light beam through it, the pathway of the light is visible from any angle. This scattering of light is called the Tyndall effect 6.There usually is not a definite, sharp saturation point at which no more dispersant can be taken by the dispersing agent. 7.The dispersant can be coagulated, or separated by clumping the dispersant particles with heat or an increase in the concentration of ionic particles in solution into the mixture. 8.There is usually only small effect of any of the colligative properties due to the dispersant.

CONCENTRATION

The concentration of a solution is an indication of how much solute there is dissolved into the solvent. There are a number of ways to express concentration of a solution. By far the most used and the most useful of the units of concentration is molarity. You might see ‘6 M HCl’ on a reagent bottle. The ‘M’ is the symbol for molar. One molar is one mol of solute per liter of solution. The reagent bottle has six mols of HCl per liter of acid solution. Since the unit ‘molar’ rarely appears in the math of chemistry other than as a concentration, to do the unit analysis correctly, you will have to insert concentrations into the math as ‘mols per liter’ and change answers of ‘mols per liter’ into molar. Molality is concentration in mols of solute per kilogram of solvent. Mol fraction is the number of mols of solute per number of mols of solution. Weight-weight percent (really mass percent) is the number of grams of solute per grams of solution expressed in the form of a percent.

Mass-volume concentration is the number of grams of solute per milliliter of solution. There are other older units of concentration, such as Baumé, that are still in use, mainly in industrial chemicals. Normality is the number of mols of effective material per liter. In acid-base titrations, the hydroxide ion of bases and the hydrogen (hydronium) ion of acids is the effective material. Sulfuric acid (H2SO4) has two ionizable hydrogens per formla of acid, or one mol of acid has two mols of ionizable hydrogen. 0.6 M H2SO4 is the same concentration as 1.2 N H2SO4. We say that sulfuric acid is diprotic because it has two protons (hydrogen ions) per formula available.

Hydrochloric acid (HCl) is monoprotic, phosphoric acid (H3PO4) is triprotic, and acids with two or more ionizable hydrogens are called polyprotic. Sodium hydroxide (NaOH) is monobasic, calcium hydroxide (Ca(OH)2) is dibasic, and aluminum hydroxide (Al(OH)3) is tribasic. Where ‘X’ is the number of available hydrogen ions or hydroxide ions in an acid or base, N, the normality, is equal to the molarity, M, times X. The normality system can be used for redox reactions, but the effective material is now available electrons or absorption sites for electrons. Consider the following reaction, #43 in the redox section. In a sulfuric acid solution potassium permanganate will titrate with oxalic acid to produce manganese II sulfate, carbon dioxide, water, and potassium sulfate in solution.


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