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Main article: Solution.
Solution is a homogeneous (single-phase) mixture of atoms, molecules and ions (particles) of two or more substances, and products of their reaction. Usually people mean solution of some solid substance in a liquid solvent, but there also such things as gas-liquid, solid-solid and gas-solid solutions.
Solvent - a component of this mixture which has the largest number of particles. Other components are named soluble substances.
Making a solution
Making a solution is a two-step process:
- Physical interaction between solvent and soluble substance (e.g. destruction of crystal lattice).
- Chemical reactions between solvent and soluble compounds (e.g. dissociation, solvatation, hydratation, hydrolisys).
Destruction of crystal lattice
Particles of a solvent penetrate the crystal lattice of soluble substance (due diffusion effect), and destroy it. Particles of soluble substance (molecules, atoms) diffusing into the solvent, as solvent is diffused into the crystal.
The destruction of crystal lattice increases the entropy, in the cost of energy from the environment. So this process leads to decreasing the temperature of the solution, sometimes quite significant. For example, dissolving ammonium nitrate in boiling water takes so much energy that final solution is actually a room temperature of even less.
Dissociation
Main article: Dissociation.
If both solvent and soluble substance have ionic crystal lattice or covalent polar bond, after the substance is dissolved, its particles start interact with particles of solvent producing positive and negative ions - cations and anions, respectively. Ions are electrically changed, and solution may conduct electricity. The original compound and final solution are called electrolytes. Most of salts, acids and bases are electrolytes.
If soluble substance is neither ionic nor covalent polar, then dissociation doesn't occur, and this substance is called non-electrolyte. Such solutions don't conduct electricity, e.g. sugar solution.
Dissociation process is caused by electrostatic attraction of changed parts of solvent molecule and opposite changes on molecules of soluble substance, leading to separating substance's molecule to ions. Let's see how it works while dissolving table salt in water.
In neutrally charged molecule of table salt NaCl (ionic crystal lattice) all valent electrons are bound to chlorine atom, so it is negatively charged, and sodium ion is positively changed. In neutrally charged water molecule H2O (covalent bonds) electrons are concentrated near the oxygen ion, making it partially negatively changed, and hydrogen is partially positively charged. During dissociation process, molecules of salt and molecules of water are approaching each other. Sodium ions are attracted by oxygen ions, chlorine ions are attracted by hydrogen ions. This attraction makes sodium-chlorine ions bond weaker, the distance between these ions is increasing (because they are attracted by more and more molecules of water), until these ions are separated completely.
Dissociation process is releasing energy contained in destroyed chemical bonds. Sometimes this energy may be higher than energy required to destroy the crystal lattice, so the final solution may heat. For example, dissolving strong bases like sodium hydroxide in cold water produces boiling hot solution (depending on the amount of substance and water).
Dissolving the substance with increasing the temperature is called exothermic, and with decreasing the temperature is called endothermic.
Dissociation process may have multiple steps. For example, dissolving tribasic acid like phosphoric acid H3PO4 in a polar solvent (like water) firstly removes one hydrogen ion [H2PO4]-, then two [HPO4]2-, and then tree [PO4]3-. But removing each next hydrogen ion from this molecule is harder because the rest of molecule becoming more and more negatively charged, so in most of solution there are ions [H2PO4]- and much less [PO4]3-.
If all the molecules of the soluble substance are dissociated into ions, then the electrolyte is called strong. These are salts of alkaline metals, bases and some inorganic compounds.
If only some of molecules are dissociated into ions, then electrolyte is called weak. These are salts and bases of transitional metals and some organic compounds.
Solvolysis and hydrolysis
Main article: Solvolysis.
In some cases, soluble substance and a solvent may react with producing new products. This process is called solvolysis, and if solvent is water it's called hydrolysis.
This is mostly the case for salts of transitional metals or anions of weak acids, like cobalt(II) acetate. During dissociation phase there more cations like Co2+ are formed than anions like [CH3COO]-, because latter don't dissociate much. Because if this, unbonded cations start reacting with a solvent, producing other salts - for example, in case of water these are hydroxides and hydroxosalts, like cobalt(II) hydroxide or copper(II) hydroxicarbonate.
This can be prevented by introducing additional anions into the solution, e.g. by adding more acid.
Solvation and hydration
Main article: Solvation.
After the substance is partly of fully dissociated into ions, the latter are freely moving in the solution. But they are electrically charged, so they can attract oppositely charged molecules or ions of the solvent, via electrostatic forces. Depending on the exact substance and solvent, these forces may be quite strong, and solvent molecules may surround an ion of a substance, forming a complex ion (inner ion of substance and outer "shell" of solvent particles). This process is called solvation, and the complex ion is called solvate. In case of using water as a solvent, these are called hydration and hydrates, respectively.
These complex ions (e.g. hexaaquacopper(II) ions [Cu(6H2O)2+) may have different properties than original substance ions have (e.g. copper(II) ions Cu2+). More details can be found in water of crystallization article.
Solubility
Main article: Solubility.
It should be mentioned, that there is a limit of how much of a substance may be dissolved in specific amount of solvent in specific conditions. This is caused by a fact, that solvation/hydration require a lot of particles of solvent per one particle of the substance, and they can't destroy the crystal lattice anymore. So the process of dissolving takes progressively more time if most of solvent particles already bound with some particles of a substance, until the process is stopped.
Then substance stops dissolving, this point is called saturation. The amount of substance per amount of solvent is called solubility, this is maximum amount of substance which can be dissolved under certain conditions.
Factors
Solubility value depends of several factors:
- Temperature - increasing the temperature of a solvent increases the rate of molecules bumping into each other, so solubility increases. This is not the case for gas-liquid solutions, and some specific salts.
- Pressure - increasing the pressure decreases the distance between molecules of substance and solvent, and they interact more often.
- The type of a substance and a solvent. The common principle is "like dissolves like", e.g. polar solvents are effective for dissolving ionic substances or substances with polar covalent bonds. And vice versa, non-polar solvents are good at dissolving substances with non-polar covalent bonds.
Concentrations
There are several concentrations the solution could reach:
- Non-saturated - solutions which still can dissolve some amount of soluble substance, and not yet reached the saturation point.
- Saturated - solution have reached the saturation point, it can't dissolve more substance.
- Supersaturated - solution somehow has more amount of soluble substance than saturation point allows.
Actually, the solubility point is an equilibrium between dissolving a substance and the substance precipitation from the solution.
For example, if solution is saturated at 20°C, it may become non saturated at 25°C. This leads to another portion of a substance starting to dissolve into the solution, until the new saturation point is reached.
Decreasing solubility (by dropping the temperature fro 25°C to 20°C) will decrease solubility point, and the solution become supersaturated. If environment conditions don't change rapidly, this state could last for a long time. But then some disturbance occurs (fast temperature decrease, introducing a crystallization center), the excess of a substance will precipitate as new crystal. Solution become saturated again.
Solubility value
Reference data: Solubility/Characteristics.
Concentrations describe the amount of dissolved substance in a solution relative to max solubility. But until the saturation point is reached, no crystals will form from this solution, so we need to know where the saturation point is.
This site uses these levels of solubility:
- very high solubility - minimal solubility is more than 100g of substance per 100g of solvent;
- highly soluble - more than 10g of substance per 100g of solvent;
- moderately soluble - more than 1g of substance per 100g of solvent;
- slightly soluble - more than 0.01g of substance per 100g of solvent;
- insoluble - less than 0.01g per 100g of solvent.
According to this scale, someone can reach the highest possible crystal growth rate by using substance+solvent combinations with very high solubility. This is mostly true for growing polycrystals, but not much for single crystals. If crystallization rate is very high, a lot of different crystal seeds will form in the solution, which may cover the single crystal and ruin it. In worst cases, the entire solution may crystallize as one large blob.
On the other size, using substance+solvent combination with low solubility may lead to forming crystal seeds only at the surface of the solution, and a bunch of tiny crystals will form instead of a single one.
So the optimum is to use moderately soluble substances, as the rate of diffusion or convection is about the same as the rate of evaporating the solvent from a solution.
Examples
More detailed information cab be found on each substance page, as well as these articles:
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