Water – as a solvent
Almost all biological processed, and a lot of industrial processes, occur in an aqueous environment. When
substances are dissolved in water, the particles move freely so interactions between reactants are very effective.
A solution is a evenly distributed mixture of atoms, molecules or ions. Our atmosphere is a solution of gases. Salt
water is a solution of salt in water. When a solid, liquid or gas dissolves in water, an aqueous solution is formed.
The dissolved solid/liquid/gas is called the solute and the water is the solvent.
Solutions have the following important characteristics:
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They are homogenous; the solute and solvent cannot be distinguished from one another
The dissolved particles are too small to see
The proportion of dissolved solute varies from one solution to another
Water is such an excellent solvent that it is rarely found pure in nature. Not all substances dissolve in water, and
most gases have very low solubility in water.
When one substance dissolves in another, the following occurs:
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The particles of the solute are separated from one another
The particles of the solvent are separated from one another
The solute and solvent particles attract each other
A solute will dissolve if the attraction between the solute and solvent particles is strong enough to compete with
the solute-solute and solvent-solvent forces of attraction.
Substances that dissolve readily in water have ionic or polar covalent bonding. They fall into one of the following
three groups:
1) Polar covalent compounds that can form hydrogen bonds with water
2) Polar covalent molecular compounds that can ionise
3) Ionic compounds
1. Polar covalent compounds that can form hydrogen bonds with water
Sugar and ethanol dissolve easily in water because
they are polar in character, usually due to the
presence of an OH group. When ethanol is added to
water, hydrogen bonds form between the ethanol
and water molecules and the ethanol dissolves. The
water and ethanol molecules mix freely together, held together in solution by hydrogen
bonding.
The more polar a substance is, the more likely a substance is to dissolve in water. Non-polar substances are
insoluble in water because water molecules form hydrogen bonds with each other in preference to weak
attractions with non-polar molecules.
Vitamin A is insoluble because it is largely non-polar (except for
the OH group) whereas Vitamin C is soluble as there are many
polar OH groups that hydrogen bond with water molecules.
2. Polar covalent molecular compounds that ionise
Some compounds have covalent bonds that are so highly polarised that they break when the compound is
placed in water. The bonds break because of the electrostatic forces of attraction between the solute
molecules and water molecules.
For example, when HCl is placed in water the hydrogen – chlorine bond breaks with both bonding electrons
going with the Cl atom. The H+ atom covalently bonds with the water molecule to form a H3O+ ion (known as a
hydronium ion) and a Cl- ion. This process is known as ionisation. The HCl molecules have ionised in water. The
chloride and hydronium ions will be surrounded by water – they are hydrated ions.
In summary: Polar covalent bonds within
molecules are broken producing Hydrogen
ions and anions. A covalent bond forms
between H+ and H2O to form a H3O+ ions.
Ion-dipole attractions between the newly formed ions and the polar water molecules are formed.
Examples: HNO3, H2SO4, CH3COOH
3. Ionic compounds
Ionic solids contain positive and negative ions held
together in a 3D lattice by strong electrostatic
forces.
When an ionic solid (eg NaCl) is placed in water, the
positive ends of the water molecules are attracted
to the negative ends of the water molecules are
attracted to the negative Chloride ions and the negative ends of the water
molecules are attracted to the positive Sodium ions. Because water is constantly moving, they pull the sodium
and chloride ions on the outer part of the crystal out of the lattice and into the solution.
When the NaCl has dissolved in water, the Na+ and Cl- are surrounded by water molecules and are hydrated.
The attraction between an ion and a polar molecule is described as an iondipole attraction.
When an ionic compound dissolves in water, the positive and negative ions are
separated from one another. This process is known an dissociation.
****When writing equations for dissociation of ionic compounds, the rules for balancing equations apply.
In summary:
 Ionic bonds within the solid are broken
 Hydrogen bonds with the water molecule are broken
 Ion-dipole attractions between ions and polar water molecules are formed
Not all ionic compounds are water soluble. For insoluble substances, the energy required to separate the ions
from the lattice is much greater than the energy released when the ions are hydrated so the ions tend to stay in
their lattice.
Note: Substances that are described as soluble dissolve to varying extents. Substances that are insoluble tend
to dissolve sparingly. In addition, solubility varies with temperature. As temperature increases, gases become
less soluble and solids become more soluble.
Chapter 10 Review Questions:
Q6, Q7, Q8, Q9, Q10, Q11, Q12, Q17, Q19, Q22, Q24,
Q25, Q28, Q29, Q31, Q32
Handout: Water is wonderful
Q1.
Use the data in Table 10.2 (page 193) to answer the following questions:
a How much energy is needed to raise the temperature of 500 g of water from 23C to 90C?
b How much energy would be required to raise the temperature of an equal mass of lead by the same number
of degrees? SHC of Pb = 0.13
Q2.
Methanol (CH3OH) and glucose (C6H12O6) are compounds that can form hydrogen bonds with water. They will
dissolve in water without ionising. Write chemical equations to represent the dissolving process for each of these
compounds.