Chemistry 20 – Solutions

What is a solution?

A solution is a mixture of a minor component (solute), evenly distributed (homogeneous) in a major component (solvent). An example would be salt (NaCl) in water (H2O). The H2O is in a HUGE excess and is called the solvent and the NaCl is present in a minor amount and is termed a solute.

Solutions can also be called homogeneous mixtures. Liquid solutions (liquid solvent) are most common:

Gas in a Liquid Oxygen in water
Liquid in a Liquid Alcohol (ethanol) in water
Solid in a Liquid Sugar (glucose) in water
Salt (NaCl) in water

To get your mind thinking, alcohol in water is very common in drinks (wine, beer, etc.). Oxygen in water answers an interesting question – how do fish breathe? (their gills filter the dissolved oxygen from the water!)

Solid and gas solutions:

Gas in a Solid Hydrogen dissolves well in some metals (e.g., palladium)
Liquid in a Solid Mercury can dissolve in gold (called an amalgam)
Solid in a Solid Carbon can be distributed in iron – this makes steel, an alloy

Gaseous solvents can only contain gaseous solutes. (e.g., air is composed of molecular oxygen and other gases dissolved in molecular nitrogen). Gaseous solutions are called mixtures.

Mixtures:

Heterogeneous mixtures are mixtures that are not evenly distributed and are NOT solutions. An example would be particles of dirt in water – they are NOT evenly mixed.

There is a third type of solution called a colloid where particles are SO small they can be distributed evenly. An example is water in the air forming fog. Another example is fat suspended in store bought milk.

In Chemistry 20, we talk about homogeneous liquid solutions of water only.

Solutions with Water:

In water, according to Chem 20, there are three main types of dissolving events. They are all based on the type of compound that enters solution.

Examples of the three compounds:

Soluble Covalent Compound Reactive (Acidic or Basic) Covalent Compound Ionic Compound
CH3CH2OH CH3COOH or HCl NaCl
ethanol – an alcohol ethanoic acid or hydrochloric acid sodium chloride
COVALENT COVALENT IONIC

Dissolution or Solvation:

Ethanol has intermolecular forces identical to water and seamlessly goes into solution without  a chemical reaction:

$$CH_3CH_2OH_{(l)} \xrightarrow{H_2O_{(l)}} CH_3CH_2OH_{(aq)}$$

Ethanol’s state changes from liquid to aqueous (think aqua) – this means it’s dissolved in water.

Dissociation:

Many ionic compounds (salts) are soluble in water. They break up into ions in solution:

$$NaCl_{(aq)} \xrightarrow{H_2O_{(l)}} Na^+_{(aq)}+Cl^-_{(aq)}$$

To dissociate means to disconnect or separate. Because the compound is already ionic and fell apart into its constituent ions we call this dissociation.

Ionization:

When a covalent compound is reactive enough with water it can turn into ions:

$$HCl_{(aq)}+H_2O_{(l)} \rightarrow H_3O^+_{(aq)}+Cl^-_{(aq)}$$

Typically this happens with acids or bases and the products include hydronium (H3O+) or hydroxide (OH) respectively. When a covalent compound turns into ions, we call it ionization. Because the compound became ionic.

Properties of solutions:

When a solution has ions it can move electric charge (ions are negative and positive). That means ionization and dissociation can produce electrically conductive solutions!

 Type of Process  Example of Process  Electrical Conductivity
Solvation  $$C_6H_{12}O_{6(s)} \xrightarrow{H_2O_{(l)}} C_6H_{12}O_{6(aq)}$$ None
Dissociation $$ScF_{3(aq)} \xrightarrow{H_2O_{(l)}} Sc^{3+}_{(aq)}+3 F^-_{(aq)}$$ Excellent
Ionization $$HClO_{4(aq)}+H_2O_{(l)} \rightarrow H_3O^+_{(aq)}+ClO^-_{4(aq)}$$ Excellent
Ionization $$CH_3COOH_{(aq)}+H_2O_{(l)} \rightleftharpoons H_3O^+_{(aq)}+CH_3COO^-_{(aq)}$$ Poor

Notice above that any solution that forms ions is electrically conductive. Covalent compounds ionize ( turn into ions) but to different degrees. You have to know that the strong acids (HCl, HBr, HI, H2SO4, HNO3, HClO4) all ionize 100% – leading to a very conductive solution. Anything with an NH3 or COOH is considered to ionize poorly or partially and hence doesn’t create as many ions – leading to a poorly conducting solution.

Summary:

If a covalent compound dissolves, there is a non-conductive solution (no ions).

Covalently bonded acids or bases ionize. The solution can conduct poorly or excellent depending on percent ionization.

Ionic compounds dissociate. Assuming the salt is very soluble, the solution will be an excellent conductor. Rocks are also ionic compounds but they don’t dissolve, right?

Comments

So empty here ... leave a comment!

Leave a Reply

Your email address will not be published. Required fields are marked *

Sidebar