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Encyclopedia > Solubility equilibrium
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Solubility equilibrium is any chemical equilibrium between solid and dissolved states of a compound at saturation. Wikipedia does not have an article with this exact name. ... Chemical equilibrium is the state in which a chemical reaction proceeds at the same rate as its reverse reaction; the rates of the forward and reverse reactions are equal, and the concentration of the reactants and products stop changing. ... In chemistry, saturation has three different meanings: In physical chemistry, saturation is the point at which a solution of a substance can dissolve no more of that substance. ...

The substance that is dissolved can be an organic solid such as sugar or an ionic solid such as table salt. The main difference is that ionic solids dissociate into constituent ions when they dissolve in water. Most commonly water is the solvent of interest, although the same basic principles apply with any solvent. Magnified view of refined sugar crystals. ... Edible salt is a mineral, one of the few rocks people eat. ... A solvent is a liquid that dissolves a solid, liquid, or gaseous solute, resulting in a solution. ...

## Contents

Dissolution of an organic solid can be described as an equilibrium between the substance in its solid and dissolved forms:

$mathrm{{C}_{12}{H}_{22}{O}_{11}}(s) overrightarrow{leftarrow} mathrm{{C}_{12}{H}_{22}{O}_{11}}(aq),$

An equilibrium expression for this reaction can be written, as for any chemical reaction (products over reactants):

$K = frac{left[mathrm{{C}_{12}{H}_{22}{O}_{11}}right](aq)}{ left {mathrm{{C}_{12}{H}_{22}{O}_{11}}right}(s)}$

where K is called the equilibrium constant (or solubility constant) and the square brackets mean molar concentration in mol/L (sometimes called molarity with symbol M). Because a notion of concentration for a solid doesn't make sense, curly brackets are used, which mean activity, around the solid. Luckily, the activity of a solid is almost always equal to one. So, a very simple expression suffices: In chemistry, the equilibrium constant is a theoretically-calculated number associated to a reaction which is a useful tool to determine the concentration of various reactants or products in a system where chemical equilibrium occurs. ... The mole and its simple conversions into different units of measurements. ... The liter (spelled liter in American English and litre in Commonwealth English) is a unit of volume. ... This page refers to concentration in the chemical sense. ...

$K = left[mathrm{{C}_{12}{H}_{22}{O}_{11}}right](aq),$

This statement says that water at equilibrium with solid sugar contains a concentration equal to K. For table sugar (sucrose) at 25 °C, K = 1.971 mol/L. (This solution is very concentrated; sucrose is extremely soluble in water.) This is the maximum amount of sugar that can dissolve at 25 °C; the solution is saturated. If the concentration is below saturation, more sugar dissolves until the solution reaches saturation, or all the solid is consumed. If more sugar is present than is allowed by the solubility expression then the solution is supersaturated and solid will precipitate until the saturation concentration is reached. This process can be slow; the equilibrium expression describes concentrations when the system reaches equilibrium, not how fast it gets there. Sucrose (common name: table sugar, also called saccharose) is a disaccharide with the molecular formula C12H22O11. ...

## Ionic compounds

Ionic compounds normally dissociate into their constituent ions when they dissolve in water. For example, for calcium sulfate: Calcium sulfate is a common laboratory and industrial chemical. ...

$mbox{CaSO}_4(s) overrightarrow{leftarrow} mbox{Ca}^{2+}(aq) + mbox{SO}_4^{2-}(aq),$

As for the previous example, the equilibrium expression is:

$K_c = frac{left[mbox{Ca} ^{2+}(aq)right]left[mbox{SO}_4^{2-}(aq)right]}{ left{mbox{CaSO}_4(s)right}}$

where K is called the equilibrium (or solubility) constant, the square brackets mean molar concentration (M, or mol/L), and curly brackets mean activity. Since the activity of a pure solid is equal to one, this expression reduces to the solubility product expression:

$K_{sp} = left[mbox{Ca}^{2+}(aq)right]left[mbox{SO}_4^{2-}(aq)right].,$

This expression says that an aqueous solution in equilibrium with (saturated with) solid calcium sulfate has concentrations of these two ions such that their product equals Ksp; for calcium sulfate Ksp = 4.93×10−5. If the solution contains only calcium sulfate the concentration of each ion (and the overall solubility of calcium sulfate) is A substance is soluble in a fluid if it dissolves in that fluid. ...

$sqrt{ K_{sp}}=sqrt{4.93times10^{-5}}=7.02times10^{-3}=left[mbox{Ca}^{2+}right]=left[mbox{SO}_4^{2-}right].,$

## Solubility constants

Solubility constants have been experimentally determined for a large number of compounds and tables are readily available. For ionic compounds the constants are called solubility products. Concentration units are assumed to be molar (moles per liter) unless otherwise stated. Solubility is sometimes listed in mass units such as grams dissolved per liter of water. // An ion is an atom or group of atoms with a net electric charge. ... This page refers to concentration in the chemical sense. ...

Solubility (and equilibrium) constants themselves are dimensionless (however, they may have units). The lack of units in the constant may look inconsistent, but it comes about because the use of molar concentration in the solubility expression is only an approximation to activity, a unitless quantity that is approximately equal to molarity at low concentrations. In the physical sciences, a dimensionless number (or more precisely, a number with the dimensions of 1) is a quantity which describes a certain physical system and which is a pure number without any physical units; it does not change if one alters ones system of units of measurement...

The common ion effect refers to the fact that solubility equilibria shift in response to Le Chatelier's Principle. In the above example, addition of sulfate ions to a saturated solution of calcium sulfate causes CaSO4 to precipitate until the ions in solution again satisfy the solubility expression. (Addition of sulfate ions could be accomplished by adding a very soluble salt, such as Na2SO4.) The common-ion effect is a term used to describe the effect on a solution of two dissolved solutes that contain the same ion. ... In chemistry, Le Chateliers principle can be used to predict the effect of a change in conditions on a chemical equilibrium. ... In inorganic chemistry, a sulfate (IUPAC-recommended spelling; also sulphate in British English) is a polyatomic anion or a compound containing this group. ...

Solubility is sensitive to temperature. For example, sugar is more soluble in hot water than cool water. It occurs because solubility constants, like other types of equilibrium constant, are functions of temperature. A thermodynamic approach is required to predict how much and in what direction a particular constant changes. Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... Thermodynamics (Greek: thermos = heat and dynamic = change) is the physics of energy, heat, work, entropy and the spontaneity of processes. ...

Some values [1] at 25°C:

Barium carbonate (BaCO3), also known as witherite, is a chemical compound used in rat poison, bricks, and cement. ... Copper(I) chloride (quite commonly called cuprous chloride), is the lower chloride of copper, with the formula CuCl. ... Lead (II) sulfate (PbSO4) is a white crystal or powder. ... Magnesium carbonate, also known as the mineral magnesite when found in nature, is a chemical compound with a variety of uses. ... Silver chloride (also called silver(I) chloride) is a chemical compound with chemical formula AgCl and is composed of one silver and one chlorine molecule. ... Silver bromide (Chemical formula: AgBr) is a photosensitive substance. ... Calcium hydroxide Calcium hydroxide is a chemical compound with the chemical formula Ca(OH)2. ...

## References

1.   CRC Handbook of Chemistry and Physics H.P.R. Frederikse (Editor), David R. Lide (Editor) ISBN 0849304784

Results from FactBites:

 Solubility equilibrium - Wikipedia, the free encyclopedia (684 words) Solubility equilibrium is any chemical equilibrium between solid and dissolved states of a compound at saturation. Solubility is sometimes listed in mass units such as grams dissolved per liter of water. Solubility (and equilibrium) constants themselves are dimensionless (however, they may have units).
 Equilibrium constant - definition of Equilibrium constant in Encyclopedia (420 words) In chemistry, the equilibrium constant is a theoretically-based number that helps chemists determine the concentration of various reactants or products in a reaction where chemical equilibrium exists. For simplicity in this example all reactants are aqueus, in solution, because solids and liquids to not enter the equation for the equilibrium constant as discussed in the article on solubility equilibrium (however gases do). The equation of the equilibrium constant is equal to the product of the product concentrations to the power their respective stoichiometric coeffecients divided by the product of the reactant concentrations to the power of their stoicheometric coefficients.
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