Aqueous Solutions

Student Expectations

The student is expected to describe the unique role of water in chemical and biological systems AND develop and use general rules regarding solubility through investigations with aqueous solutions.

Key Concepts

    • Water has several unique properties. It is amphoteric, is highly polar, and has a higher boiling and melting point than other molecules of the same structure due to the hydrogen bonding between water molecules. This strong attraction between water molecules accounts for properties such as cohesion and surface tension.

    • Life on Earth is highly dependent on the unique properties of water, as most of the metabolic processes in biological organisms take place in aqueous solutions. For example, water is an essential component of the energy transformation processes of photosynthesis and cellular respiration. Also, water’s high specific heat plays a critical role in the Earth’s relatively moderate temperature variations.

    • Ionic compounds that are soluble in water with no exceptions include salts containing Group 1A cations and compounds containing the ions of acetate, nitrate, cyanide, perchlorate, chlorate, chlorite, hypochlorite, and ammonium. Compounds containing the sulfate ion are soluble with the exception of compounds that also contain the ions of strontium, barium, lead (II), and mercury (I). Compounds containing the ions of bromine, chlorine, and iodine are soluble with the exception of compounds that also contain the ions of silver, lead (II), and mercury (I).

    • Ionic compounds that are insoluble in water include compounds containing the ions of carbonates, phosphates, chromates, and dichromates, all with the exception of compounds that also contain the ammonium ion and the alkali metal ions. The hydroxide and the sulfite ions are also insoluble in water with the exception of compounds that also contain the ammonium ion, the alkali metal ions, and the ions of calcium, strontium, and barium.

AQUEOUS SOLUTIONS

Properties of Water

Water is known as the universal solvent as so many compounds are water soluble, which means that they dissolve in water. A solvent is a substance in which another substance, called a solute, dissolves. This is a result of water’s several unique properties:

    • Amphoteric: Water molecules can, under the right conditions, release either a free hydrogen ion (H+) or a free hydroxide ion (OH-). Water is composed of a hydrogen ion and a hydroxide ion, so you can create reactions that go either way. Therefore, under the appropriate conditions water acts like an acid, while under other conditions it acts as a base.

Water acts as an acid, releasing H+ ions, when it reacts with a base stronger than itself:

H-OH + NH3 → NH4+ + OH-

Water acts as a base, releasing OH- ions, when it reacts with an acid stronger than itself:

H-OH + HCl → H3O+ + Cl-

    • Polarity: Water has an uneven, or asymmetrical, distribution of electron density. Water has a partial negative charge near the oxygen atom due the unshared pairs of electrons, and partial positive charges near the hydrogen atoms. The polar nature of water causes strong attractive forces between water molecules. More specifically, these strong attractive forces result in hydrogen bonds, which form between water molecules.

    • Hydrogen Bonding: A strong interaction between a partially positive hydrogen atom of one water molecule and the partially negative oxygen atom of another water molecule, called “hydrogen bonding”, causes the following additional other properties of water: cohesion, high surface tension, high boiling point, and high freezing point.

    • Cohesion: Cohesion of water is a property that results from the strong forces of attraction between particles within a substance. Cohesion is not observed if these forces of attraction are absent or weak. The attractive force between water molecules is high, which accounts for the ability of water to bulge up over the rim of a glass as pennies are dropped in. Cohesion also accounts for the beading of water droplets on a surface.

    • Surface Tension: Surface tension of water is a property that results from the strong attraction between water molecules.

    • Higher Boiling and Melting point: Water has a higher boiling and melting point than other molecules of the same structure due to the hydrogen bonding between water molecules.

Importance of Water

Life on Earth is highly dependent on the unique properties of water as most of the metabolic processes in biological organisms take place in aqueous solutions. Water also plays a critical role in Earth’s relatively moderate temperature variations.

In addition, it is an essential component of the energy transformation processes of photosynthesis and cellular respiration. Plants make use of solar energy, carbon dioxide, and water to produce glucose and oxygen. Photosynthesis is critically important to all species. It stores the energy from the sun in the form of glucose, the most important source of energy at the base of the food chain. The oxygen produced during photosynthesis is also vital to many organisms. The metabolic process of cellular respiration is the reverse of photosynthesis. It produces energy in the form of ATP, maintains metabolic reactions, and releases carbon dioxide as waste.

Water is a good solvent due to the attractive forces between the polarity of water molecules and the solute particles. For example, table salt, or sodium chloride (NaCl), is an ionic compound consisting of positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). When you dissolve table salt in water, the solid compound breaks apart. Each individual ion becomes associated with water molecules through strong attractive forces. The diagram below shows how a sodium ion with a 1+ charge associates with polar water molecules in one orientation, and a chloride ion with a 1- charge associates with polar water molecules in the opposite orientation.

Soluble Ionic Compounds

Ionic compounds, linked together by the lattice structures by ionic bonds, are useful solutes. Ionic compounds that are soluble in water, with no exceptions, include salts containing Group 1A cations, and compounds containing the ions of acetate, nitrate, cyanide, perchlorate, chlorate, chlorite, hypochlorite, and ammonium. Compounds containing the sulfate ion are soluble, with the exception of compounds that also contain the ions of strontium, barium, lead (II), and mercury (I). Compounds containing the ions of bromine, chlorine, and iodine are soluble, with the exception of compounds that also contain the ions of silver, lead (II), and mercury (I).

Insoluble Ionic Compounds

Some ionic compounds are insoluble in water. These ionic compounds include compounds containing the ions of carbonates, phosphates, chromates, and dichromate, with the exception of compounds that also contain the ammonium ion and the alkali metal ions. The hydroxide and sulfite ions are also insoluble in water, with the exception of compounds that also contain the ammonium ion, alkali metal ions, and the ions of calcium, strontium, and barium.

Note, there are no absolute insoluble ionic compounds. The term insolubility means that the number of ionic compounds ions that could exist independently in solvent is extremely close to zero. However, a few are still distributed in the water.