Water, Electrolytes, and Solutions Courtesy: Ahajokes.com

Aqueous solutions: water is the dissolving medium, or solvent. One of most important properties of water is its ability to dissolve many different substances.

Water is Polar Covalent bonds in water Polarity gives water its great ability to dissolve many substances.

Hydration (Dissolving) of Ionic Compounds “Positive ends” of water molecules are attracted to the negatively charged anions and the “negative ends” are attracted to the positively charged cations. This process is called hydration. The hydration of its ions tends to cause a salt to “fall apart” in the water or dissolve.

Very Important!! When ionic substances (salts) dissolve in water, they break up into the individual cations and anions. Example: ammonium nitrate (NH4NO3) dissolved in water

Polar covalent compounds will also dissolve in water. Polar and ionic substances are more soluble in water than nonpolar substances (animal fat).

Many substances will not dissolve in water. Nonpolar substances (like animal fat) do not interact with water (polar substance). “Like dissolves like” is a general rule for predicting solubility.

Strong and Weak electrolytes Solute – what is being dissolved Solvent – what is doing the dissolving Electrical conductivity (ability to conduct an electric current) – useful for characterizing solutions

Weak Electrolyte Nonelectrolyte Electrical Conductivity Strong Electrolyte Nonelectrolyte

Strong Electrolytes Completely ionized in H2O Soluble salts (ionic) Strong Acids Strong Bases

Strong Acids Arrhenius proposed that an acid is a substance that produces H+ ions (protons) when it dissolves in water (sour taste). Strong acids are strong electrolytes because when placed in water virtually every molecules ionizes (breaks apart into ions). These are in aqueous solutions and should be written in chemical equations with the (aq) symbol. HCl (hydrochloric acid) HNO3 (nitric acid) HClO4 (perchloric acid) H2SO4 (sulfuric acid) HBr (hydrobromic acid) HI (hydroiodic acid) HClO3 (chloric acid)

Strong Bases Strong bases: soluble ionic compounds containing the hydroxide (OH-) ion (bitter taste and a slippery feel). When dissolved in water, the cations and OH- ions separate and move independently. NaOH (sodium hydroxide) KOH (potassium hydroxide) LiOH (lithium hydroxide) RbOH (rubidium hydroxide) CsOH (cesium hydroxide) Ca(OH)2 (calcium hydroxide) Ba(OH)2 (barium hydroxide) Sr(OH)2 (strontium hydroxide)

Weak Electrolytes Produce few ions when dissolved in water. Most common are weak acids and weak bases. Acetic acid (common weak acid) Weak electrolyte = weak acid = ionizes to a small extent

Ammonia (NH3) is the most common weak base. NH3 (aq) + H2O (l) → NH4+ (aq) + OH- (aq) The solution is basic because OH- ions are produced. Ammonia is a weak base because the resulting solution is a weak electrolyte (very few ions are formed).

Nonelectrolytes Dissolve in water but do not produce any ions therefore do not conduct an electric current. Example: table sugar (sucrose, C12H22O11) Very soluble in water but does not produce any ions (Figure c).

Concentration Most commonly used in chemistry is molarity. Molarity (M) is defined as moles of solute per volume of solution in liters. A solution that is 1.0 molar (1.0 M) contains 1.0 mole of solute per liter of solution. Molarity can be used as a conversion factor.

Standard Solution Solution whose concentration is accurately known.

Dilution M1V1 = M2V2 Preparing solutions from stock solutions. Example: HCl is purchased as a concentrated solution (12 M) and is diluted as needed. M1V1 = M2V2 M1V1 = mol solute before dilution = mol solute after dilution = M2V2