Solution
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In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. Usually, the substance present in a greater amount is considered as the solvent. Solutions may have multiple solvents. Gases may dissolvein liquids, for example, carbon dioxide or oxygen in water. Liquids may dissolve in other liquids. Gases can combine with other gases to form mixtures, rather than solutions.[1]
All solutions are characterized by interactions between the solvent phase and solute molecules or ions that result in a net decrease in free energy. Under such a definition, gases typically cannot function as solvents, since in the gas phase interactions between molecules are minimal due to the large distances between the molecules. This lack of interaction is the reason gases can expand freely and the presence of these interactions is the reason liquids do not expand.
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[edit]Introduction
Solutions, solutes and solvents may be combinations of solids, liquids or gases. The solution that forms has the same physical state as the solvent. For example, carbonated beverages are formed by dissolving CO2 (or carbon dioxide) gas in water. The air we breathe is a solution of oxygen and nitrogen gases. When we make solutions of coffee or tea, we use hot water to dissolve substances from coffee beans or tea leaves. In a hospital, the antiseptic tincture of iodine is a solution of iodine dissolved in alcohol. The ocean is an aqueous solution of many salts such as NaCl dissolved in water.
Body fluids are electrolyte solutions containing solute ions (e.g. potassium and sodium) as well as molecular sugar (or glucose) and urea dissolved in polar solvents such as blood plasma. Oxygen and carbon dioxide are also essential components of blood chemistry, where significant changes in their concentrations can be a sign of illness or injury.
Other examples of solid solutions are alloys and certain minerals and polymers containing plasticizers. Stainless steel, for example, is a solid solution of carbon atoms in a crystalline matrix of iron atoms. The ability of one compound to dissolve in another compound is called solubility. The physical properties of compounds such as melting point and boiling point change when other compounds are added. Together they are called colligative properties. There are several ways to quantify the amount of one compound dissolved in the other compounds collectively called concentration. Examples include molarity, mole fraction, and parts per million (ppm).
Solutions should be distinguished from non-homogeneous mixtures such as colloids and suspensions. When a liquid is able to completely dissolve in another liquid the two liquids are miscible. Two substances that can never mix to form a solution are called immiscible.
[edit]Properties of solutions
[edit]Types of bonding
Consider what happens when a Group I element combines with a Group VII element to form a chemical compound. The Group I element has one valence electron in its outermost shell, while the Group VII element has seven. In order for both of them to be satsified, one of two things has to occur. They will either share the entire set of eight valence electrons between them equally, or one atom will give up (or transfer) its valence electrons to the other.
In the first case, if the valence electrons are shared equally, the distribution of electrical charge will be fairly uniform in the region in between the two atoms. This type of bonding results in what is known as a covalent bond (or ”sharing of strength”).
In the second case, if the valence electrons are not shared equally, then one atom will need to give up its valence electrons to the other. In the case of sodium chloride, the sodium atom gives up its single valence electron ot become a sodium cation, and the chlorine atom takes the valence electron to become a chlorine anion. The forces responsible for the formation of this ionic compound are electrical in nature, and much stronger than those evidenced in covalently bonded materials. Thus, the ionic bond is characterized by such physical properties as a highmelting point and high boiling point (i.e. it takes more thermal energy to break apart the ionic bonds).
Let us also consider the size of these atoms. The sodium atom, with atomic number Z = 11, started with 11 electrons orbiting around its nucleus. The chlorine atom started with 17. In NaCl, the sodium cation has only 10, while the chlorine anion has a total of 18. The result is that the chlorine ion is much larger than the sodium ion. The result is evident in the crystal lattice structure.
In covalent compounds, the valence electrons are shared equally between the atoms. Mosthydrogen compounds such as water H2O are covalently bonded. 70% of earths surface is covered by water. The human body is about 60-65% water. Organic (C, H, O) compounds include carbohydrates, fats, proteins and nucleic acids such as DNA. Nearly all organic compounds are held together by covalent bonds. Since covalent bonds are not electrical in nature, the forces holding these compounds together are weaker than those evidenced in ionic compounds. Therefore organic compounds tend to exhibit lower melting and boiling points.
[edit]Bond polarity
There are many intermediate cases where valence electrons are not shared equally, but they are also not completely transferred from one atom to another. In this case, it can be said that the electrons will spend more time in the vicinity of one atom than the other. The result is that one end of the molecule will have a net negative charge, while the other end will have a net positive charge. When electrons in a covalent bond are not equally shared, the molecule is said to be polar.
A polar molecule contains an electrical dipole, meaning that it has a (+) and (-) end. In the depicted case of the hydrogen fluoride molecule, covalently bond electrons are displaced towards the more electronegative fluorine atom.
Thus, bond polarity is a simple result of the fact that electrons are not shared equally. In a polar covalent bond, the atom with the stronger affinity for electrons may be shown with a partial negative charge. The atom with the lower affinity for electrons is farther from the electron pair and is shown with a partial positive charge. The illustration depicts the electrical dipole resulting from the formation of a hydrogen fluoride (HF) molecule. The dipole is symbolized by a cross (+ end) pointing in the direction of the negative end with an arrowhead. Note the larger size of the molecule on the negative end. This is because the unequally shared electrons are spending more time there. This is known as a polar covalent bond -- a compromise between ionic and covalent bonding.
[edit]Aqeous solutions
It is the polar structure of the water molecule that is responsible for many of the unique physical properties of water. In the water molecule, the oxygen atom, with its eight (+) protons, has a much greater attraction for the shared valence electrons than do either of the hydrogen atoms with a single proton. Therefore, the shared electrons spend more time around the oxygen part of the molecule than they do around the hydrogen part. This results in the oxygen end being more negative than the hydrogen end.
Polar molecules of any substance have attractions between the positive end of one molecule and the negative end of another molecule. When the polar molecule has hydrogen at one end and fluorine, oxygen or nitrogen at the other end, the attractions are strong enough to qualify as type of chemical bonding called hydrogen bonding. Hydrogen bonds are weaker than either covalent bonds or ionic bonds.
A salt such as sodium chloride will form a solution with water because the positively charged sodium cations and the negatively charged chlorine anions are attracted to the positive and negative ends of the water molecule, respectively. The sodium and chlorine ions thus become dissolved (or hydrated) as part of the aqueous saltwater solution. Hydration occurs when the charged solute ions become surrounded by the polar solvent, or water molecules.
Thus, sodium chloride (NaCl) will form a solution with water because the Na+ and Cl- ions in the salt are attracted to the positive and negative parts of water molecules. When ionic compounds dissolve, the resulting solution contains separated ions. The conduction of electricity provides evidence for these ions in solution. Charged ions in solution act as mobile charge carriers (like electrons in metals). Sodium chloride and water together form a strong electrolyte aqueous solution.
Ethanol is one example of a non-ionic solute that is very soluble in water. All alcoholic beverages are aqueous solutions of ethanol. Why is ethanol so soluble in water? The answer lies in the structure of the ethanol molecule. The molecule contains a polar O-H bond like those found in water, which makes it very compatible with water. Just as hydrogen bonds form among water molecules in pure water, ethanol molecules can form hydrogen bonds with water molecules. Table sugar can also be dissolved in water because the glucose molecule has many polar OH groups which will attract the polar water molecules.
[edit]Non-polar solutions
Whether or not two given liquids form solutions depends to some degree on the similarity (or lack thereof) between their respective molecular structures. In general, the forces of attraction between molecules or ions of the solvent and solute will determine the limits of solubility. The water molecule, for example, is a polar molecule with a negative end and a positive end. Polar molecules (such as water) therefore require polar solvents.
On the other hand, covalently bonded substances such as elemental gases, long chain hydrocarbon fuels, oil or grease do not dissolve in water or other polar solvents. Most grease and oil molecules have no polarity. They are virtually electrically neutral, and are therefore non-polar compounds. In general, “like dissolves like” and thus water will not dissolve oil and grease. Non-polar solutes require non-polar solvents.
A molecule may be polar either as a result of polar bonds due to differences inelectronegativity as described above, or as a result of an asymmetric arrangement of non-polar covalent bonds and non-bonding pairs of electrons known as a full molecular orbital. In a similar manner, a molecule may be non-polar either because there is (almost) no polarity in the bonds or because of the symmetrical arrangement of polar bonds. For example, in the methane, CH4 molecule the four C–H bonds are arranged tetrahedrally around the carbon atom. Each bond has polarity (though not very strong). However, the bonds are arranged symmetrically so there is no overall dipole in the molecule.
Many substances do not dissolve in water. When petroleum leaks from a damaged tanker, it does not disperse uniformly in the water (does not dissolve) but rather floats on the surface because its density is less than that of water.
Petroleum is a mixture of molecules like the one illustrated in this slide. Since carbon and hydrogen have similar affinities for electrons, the bonding electrons are shared almost equally and the bonds are essentially non-polar. Covalently bonded substances such as long chain hydrocarbon fuels, oil or grease do not dissolve in water or other polar solvents. Most grease and oil molecules have no polarity. They are virtually electrically neutral, and are therefore non-polar compounds.
In general, “like dissolves like” and thus water will not dissolve oil and grease. Similarly, non-polar solutes require non-polar solvents.
[edit]Solubility
When no more of a solute can be dissolved into a solvent, the solution is said to be saturated. However, the point at which a solution can become saturated can change significantly with different environmental factors, such as temperature, pressure, and contamination. For some solute-solvent combinations a supersaturated solution can be prepared by raising the solubility (for example by increasing the temperature) to dissolve more solute, and then lowering it (for example by cooling).
Usually, the greater the temperature of the solvent, the more of a given solid solute it can dissolve. However, most gases and some compounds exhibit solubility that decrease with increased temperature. Such behavior is a result of an exothermic enthalpy of solution. Some surfactantsexhibit this behaviour. The solubility of liquids in liquids is generally less temperature-sensitive than that of solids or gases.
The properties ideal solutions can be calculated by the linear combination of the properties of its components. If both solute and solvent exist in equal quantities (such as in a 50% ethanol, 50% water solution), the concepts of "solute" and "solvent" become less relevant, but the substance that is more often used as a solvent is normally designated as the solvent (in this example, water).
[edit]Examples
Many types of solutions exist, as solids, liquids and gases can be both solvent and solute:
| Examples of solutions | Solute | |||
|---|---|---|---|---|
| Gas | Liquid | Solid | ||
| Solvent | Gas | |||
| Liquid |
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| Solid |
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