- Properties of aqueous solutions
- Why do some solids dissolve in water?
- Solubility rules
- Examples of solubility in aqueous solutions
- Example 1: What happens when Ba (NO
- Example 2: What happens when Pb (NO
- References
The aqueous solutions are solutions that use water to decompose a substance. For example, mud or sugar water. When a chemical species has dissolved in water, this is denoted by writing (aq) after the chemical name.
Hydrophilic (water-loving) substances and many ionic compounds dissolve or dissociate in water. For example, when table salt or sodium chloride dissolves in water, it dissociates into its ions to form Na + (aq) and Cl- (aq).
Figure 1: aqueous solution of potassium dichromate.
Hydrophobic (water-afraid) substances generally do not dissolve in water or form aqueous solutions. For example, mixing oil and water does not lead to dissolution or dissociation.
Many organic compounds are hydrophobic. Non-electrolytes can dissolve in water, but they do not dissociate into ions and maintain their integrity as molecules. Examples of non-electrolytes include sugar, glycerol, urea, and methylsulfonylmethane (MSM).
Properties of aqueous solutions
Aqueous solutions often conduct electricity. Solutions containing strong electrolytes tend to be good electrical conductors (eg seawater), while solutions containing weak electrolytes tend to be poor conductors (eg tap water).
The reason is that strong electrolytes completely dissociate into ions in water, while weak electrolytes dissociate incompletely.
When chemical reactions between species occur in an aqueous solution, the reactions are usually double displacement reactions (also called metathesis or double substitution).
In this type of reaction, the cation in one reagent takes the place of the cation in the other reagent, typically forming an ionic bond. Another way to think of it is that reactive ions "switch partners."
Reactions in aqueous solution can result in products that are soluble in water or can produce a precipitate.
A precipitate is a compound with low solubility that often falls out of solution as a solid.
The terms acid, base, and pH only apply to aqueous solutions. For example, you can measure the pH of lemon juice or vinegar (two aqueous solutions) and they are weak acids, but you cannot get any meaningful information from the vegetable oil test with pH paper.
Why do some solids dissolve in water?
The sugar we use to sweeten coffee or tea is a molecular solid, in which individual molecules are held together by relatively weak intermolecular forces.
When sugar dissolves in water, the weak bonds between individual sucrose molecules are broken, and these C12H22O11 molecules are released into solution.
Figure 1: dissolution of sugar in water.
It takes energy to break the bonds between the C12H22O11 molecules in sucrose. It also takes energy to break the hydrogen bonds in water which must be broken to insert one of these sucrose molecules into solution.
Sugar dissolves in water because energy is released when slightly polar sucrose molecules form intermolecular bonds with polar water molecules.
The weak bonds that form between the solute and the solvent compensate for the energy required to alter the structure of both the pure solute and the solvent.
In the case of sugar and water, this process works so well that up to 1800 grams of sucrose can be dissolved in one liter of water.
Ionic solids (or salts) contain positive and negative ions, which are held together thanks to the great force of attraction between particles with opposite charges.
When one of these solids dissolves in water, the ions that make up the solid are released into solution, where they associate with the polar solvent molecules.
Figure 2: Dissolution of sodium chloride in water.
NaCl (s) »Na + (aq) + Cl- (aq)
We can generally assume that salts dissociate into their ions when dissolved in water.
Ionic compounds dissolve in water if the energy released when ions interact with water molecules outweighs the energy required to break ionic bonds in the solid and the energy required to separate water molecules so that ions can be inserted into the solution.
Solubility rules
Depending on the solubility of a solute, there are three possible outcomes:
1) If the solution has less solute than the maximum amount that it is capable of dissolving (its solubility), it is a diluted solution;
2) If the amount of solute is exactly the same amount as its solubility, it is saturated;
3) If there is more solute than is capable of dissolving, the excess solute separates from the solution.
If this separation process includes crystallization, it forms a precipitate. Precipitation reduces the concentration of the solute to saturation in order to increase the stability of the solution.
The following are the solubility rules for common ionic solids. If two rules appear to contradict each other, the preceding one takes precedence.
1- The salts that contain Group I elements (Li +, Na +, K +, Cs +, Rb +) are soluble. There are few exceptions to this rule. The salts containing the ammonium ion (NH 4 +) are also soluble.
2- Salts containing nitrate (NO 3 -) are generally soluble.
3- The salts that contain Cl -, Br - or I - are generally soluble. Important exceptions to this rule are the halide salts of Ag +, Pb2 +, and (Hg2) 2+. Thus, AgCl, PbBr 2 and Hg 2 Cl 2 are insoluble.
4- Most of the silver salts are insoluble. AgNO 3 and Ag (C 2 H 3 O 2) are common soluble salts of silver; Virtually all the others are insoluble.
5- Most of the sulfate salts are soluble. The major exceptions to this rule include CaSO 4, BaSO 4, PbSO 4, Ag 2 SO 4 and SrSO 4.
6- Most of the hydroxide salts are only slightly soluble. The hydroxide salts of Group I elements are soluble. The hydroxide salts of the Group II elements (Ca, Sr and Ba) are slightly soluble.
The hydroxide salts of transition metals and Al 3 + are insoluble. Thus, Fe (OH) 3, Al (OH) 3, Co (OH) 2 are not soluble.
7- Most of the transition metal sulfides are highly insoluble, including CdS, FeS, ZnS and Ag 2 S. The sulfides of arsenic, antimony, bismuth and lead are also insoluble.
8- Carbonates are frequently insoluble. Group II carbonates (CaCO 3, SrCO 3 and BaCO 3) are insoluble, as are FeCO 3 and PbCO 3.
9- Chromates are frequently insoluble. Examples include PbCrO 4 and BaCrO 4.
10- Phosphates such as Ca 3 (PO 4) 2 and Ag 3 PO 4 are frequently insoluble.
11- Fluorides such as BaF 2, MgF 2 and PbF 2 are frequently insoluble.
Examples of solubility in aqueous solutions
Cola, salt water, rain, acid solutions, base solutions, and salt solutions are examples of aqueous solutions. When you have an aqueous solution, you can induce a precipitate by precipitation reactions.
Precipitation reactions are sometimes called "double displacement" reactions. To determine if a precipitate will form when aqueous solutions of two compounds are mixed:
- Record all ions in solution.
- Combine them (cation and anion) to get all the potential precipitates.
- Use the solubility rules to determine which (if any) combination (s) is insoluble and will precipitate.
Example 1: What happens when Ba (NO
Ions present in solution: Ba 2+, NO 3 -, Na +, CO 3 2-
Potential precipitates: BaCO 3, NaNO3
Solubility Rules: BaCO 3 is insoluble (rule 5), NaNO 3 is soluble (rule 1).
Complete chemical equation:
Ba (NO 3) 2 (aq) + Na 2 CO 3 (aq) »BaCO 3 (s) + 2NaNO 3 (aq)
Net ionic equation:
Ba 2+ (aq) + CO 3 2- (aq) »BaCO 3 (s)
Example 2: What happens when Pb (NO
Ions present in solution: Pb 2+, NO 3 -, NH 4 +, I -
Potential precipitates: PbI 2, NH 4 NO 3
Solubility rules: PbI 2 is insoluble (rule 3), NH 4 NO 3 is soluble (rule 1).
Complete chemical equation: Pb (NO 3) 2 (aq) + 2NH 4 I (aq) »PbI 2 (s) + 2NH 4 NO 3 (aq)
Net ionic equation: Pb 2+ (aq) + 2I - (aq) »PbI 2 (s).
References
- Anne Marie Helmenstine. (2017, May 10). Aqueous Definition (Aqueous Solution). Recovered from thoughtco.com.
- Anne Marie Helmenstine. (2017, May 14). Aqueous Solution Definition in Chemistry. Recovered from thoughtco.com.
- Antoinette Mursa, KW (2017, May 14). Solubility Rules. Recovered from chem.libretexts.org.
- Aqueous Solutions. (SF). Recovered from saylordotorg.github.io.
- Berkey, M. (2011, November 11). Aqueous Solutions: Definition & Examples. Recovered from youtube.com.
- Reactions in Aqueous Solution. (SF). Recovered from chemistry.bd.psu.edu.
- Reid, D. (SF). Aqueous Solution: Definition, Reaction & Example. Recovered from study.com.
- Solubility. (SF). Recovered from chemed.chem.purdue.edu.