Stability of Substances

SOLUBILITY OF SUBSTANCESThe solubility of a substance is the amount of a substance that will dissolve in a given amount of a solvent. Solubility is a quantitative term. Solubility of substance differs greatly. The terms soluble and insoluble are comparative. A compound is termed soluble if more than 0.1g of that compound dissolves in 100 mL of solvent. If less than 0.1 g of the compound dissolves in 100 mL solvent, the compound is said to be insoluble or sparingly soluble. The terms miscible and immiscible liquids that are encountered when describing the solubility of a liquid in another mean: a liquid is miscible when it is soluble beyond measure for instance alcohol is miscible with water. A liquid is immiscible or insoluble means the same thing; oil is said to be immiscible with water, as in oil and vinegar salad dressing.Determination of SolubilityTo determine solubility, a known amount of a solvent such as 100 mL is put in an urn after which the substance whose solubility is to be calculated is added until the substance is unable to dissolve again even when stired vigorously and left to stand for a long period of time. Such a solution is said to be saturated meaning that it contains as much solute that it could dissolve at that temperature.Saturated solutionsA saturated solution is a solution that contains the dissolved solute in equilibrium.Dissolution and precipitation occurs at the same rate, thus satisfying the condition for a dynamic equilibrium. This condition was set forth when discussing the equilibrium between liquid and vapor. We can convey equilibrium condition in sucrose solution for an example with the equation below:Sucrose(s) ----- Sucrose(aq)Two deductions that can be made from the equations are that the two processes of dissolution and precipitation are taking place concurrently and that the number of molecules in the solution remains unchanged.The saturated solution of an ionic compound differs a little from the saturated solution of covalent compounds such as sucrose. The ionic compound melts and stays in solutions as ions while the covalent compound melts and remains in solutions as molecules. The equilibrium of sodium chloride with its ions in a saturated solution is demonstrated with the following equation:NaCl(s) Na+(aq) + Cl-(aq)An unsaturated solution normally has smaller amount of solute than a saturated solution. There is nothing like equilibrium in an unsaturated solution. Such a solution would normally take up and dissolve any additional solute that is added to it. On the contrary when you add an extra amount of solute to a saturated solution, no additional number of solute would be dissolved because it has reached the boundary of its solubility. Any extra solute added to a saturated solution would merely amplify the amount of undissolved solute.It is worth noting that solubility alters with temperature. A solution that is saturated at a particular temperature may be unsaturated at another temperature.Again, dissolution requires interaction between the molecules (or ions) of the solute and the molecules of the solvent. A thinly divided solute will dissolve more quickly than when it is larger due to the fact that it provides a better contact between the dissolving solute and solvent. Constant stirring also increases the rate of dissolution, due to the fact that stirring alters the particular solvent molecules that are come in contact with undissolved solute. Solubility of solids and liquids characteristically increases with increase in temperature, therefore, temperature, solids and liquids are frequently dissolved in warm solvents.Factors affecting solubility of substanceThere are a lot of factors that affect the solubility of a substance in another. Some of them are: temperature, polarity, pressure, and molecular size.1. Forces between particlesThe nature of intermolecular forces in both the solute and the solvent determines the rate of solubility between them. When a substance dissolves in another, it must overcome the attractive forces between two of them. The dissolving solute ought to be able to break up the aggregation of molecules in the solvent. This means overcoming the hydrogen bonds between the molecules or the dispersion forces between molecules in a non-polar solvent. The molecules of the solvent must therefore have enough attraction for the particles of the solute to take them away one after another from their neighbors in the undissolved solute. If the solute is ionic, only an extremely polar solvent such as water provides enough interaction to result to dissolution. If the solute particles are polar molecules, they are easily dissolved in polar solvents like alcohols. Non polar solute on the other hand dissolves in non-polar solvents. The reason is not because polar solvent molecules cannot conquer the weak dispersion forces between the solute molecules, but due to the fact that these dispersion forces are extra ordinarily weak to overcome the dipole-dipole interaction that exists between the solvent molecules.Generally, like dissolves like. Ionic and polar compounds are soluble in polar solvents like water or liquid ammonia. Nonpolar compounds are soluble in non-polar solvents, like carbon tetrachloride, and hydrocarbon solvents like gasoline.The solubility of gases in water depends very much how polar the gas molecules are. Those gases whose molecules are polar are much more soluble in water than non-polar gases. Ammonia, a highly polar molecule, is extremely soluble in water (89.9 g/100 g H2O) and hydrogen chloride (82.3 g/100 g H2O). Helium and nitrogen are nonpolar molecules. Helium is mere partially soluble (1.8 X 10-4 g/100 g H2O), like in nitrogen (2.9 X 10-3 g/100 g H2O)Solubility and inter-particle bonds in various types of compounds and their relative solubilities in water, a polar solvent; in alcohol, a less polar solvent; and in benzene, a non-polar solventKinds of bonds Example Water Alcohol Benzeneionic sodium chloride very soluble slightly soluble insolublepolar covalent sucrose (sugar) very soluble soluble insolublenonpolar covalent napthalene Insoluble soluble very soluble2. TemperatureThe solubility of substances varies at different temperature. Usually, the solubility of solids and liquids rises at higher temperature but the solubility of gases diminishes with an increase in temperature. This property of gases causes is a great concern for the life of fishes in lakes, oceans, and rivers. Fish needs dissolved oxygen to stay alive. If the temperature of their water habitat increases, the concentration of dissolved oxygen is reduced and the life of the fishes is at stake.3. PressureThe pressure on the surface of a solution has less effect on the solubility of solids and liquids but a great effect on the solubility of gases.Solubility Curve:A solubility curve is a measurement shown on a graph that is utilized to establish the mass of a given dissolved item like salt or sugar in 100ml of water. The substance that is that is dissolved in the water is called a solute. If a figure falls under the line on the graph, it denotes an unsaturated solution which has the capacity to take up more solute.Solubility curves permit a scientist to establish the amount of a solute that can dissolve in 100 grams of water at a specific temperature.The Graph below shows the Grams of solute per 100g of water against temperature (°C)A steeper slope shows the increased effect on solubility by temperature rise.Solid Solutes against Gas Solutes: As the temperature rises, the solubility of a solid rises and solubility of gases decrease.A sample question:1. What is the quantity of sodium chloride that can be dissolved in 100 mL of water at 30°C?2. At 20°C, the highest amount of potassium chlorate is dissolved in 100 g of water. When the temperature is increased to 50°C, how much more potassium chlorate can be dissolved in the water?The amount of potassium chloride that can be dissolved in 50 mL of water at 50°C can be estimated as followsApplication of solubility product principle in qualitative analysisThe concepts of solubility of product and the common effect of ion are greatly employed in the process of qualitative analysis to separate essential radicals or cations into various groups.Weak acids and weak bases ionise partially in water leading to an equilibrium situation being attained in their solutions. For instance, in the ionization of a weak base NH4OH shown below:The ionization constant for the base isQualitative analysisThe widespread effect of ion is usually employed in qualitative analysis.The cations of group II elements (Hg2+, Pb2+, Bi3+, Cu2+, As3+, Sb3+, Sn2+) are normally precipitated as their sulphides for example the CuS and PbS through the passage of H2S gas in the presence hydrochloric acid which possesses the common H+ ions.The cations of group III elements are precipitated in their hydroxides forms by NH4OH in the presence of NH 4 Cl.The cations of group V are precipitated in their carbonates forms through the addition of (NH 4 ) 2 CO 3 , in the presence of HCl.In relation to solubility, salts can be classified into three main types:1. Soluble salts ie salts with their solubility greater than 0.1 M2. Slightly soluble salts that is salts with solubility in the range of 0.01 M and 0.1 M3. Sparingly soluble salt ie salts with solubility less than 0.01 MCrystallizationCrystallization is a procedure which chemists employ to purify solid compounds. It is one of the essential procedures every chemist ought to master to become capable in the laboratory. Crystallization is based on the principles of solubility.The difference between crystallization and solubilitySolubility is the capability to dissolve in a solvent. It is measured in units g per 100mL of solvent. Crystallization is the process through which crystals are formed. This could be from a molten substance but is normally from a solvent. As the solvent evaporatesSolutionA solution is a homogenous mixture of two or more substances.There are two components of a solution-Solute and solventSolution = solute + solventSoluteA solute is the constituent of a solution that is in little quantity.SolventA solvent is the constituent of solution that is in greater quantity.Saturated solution-Is a solution that can hold no more of the solute at a specific temperature.An Unsaturated solution-An unsaturated solution is a solution, which contains less amount of solute than is required to saturate it at that temperature.A super saturated solution-Is a solution that is more concentrated than a saturated solution. When an extra crystal of solute is added to the solution, the surplus solute forms crystal.An aqueous solution-Is a solution of any substance in which the solvent is water.