3. Water as a Solvent

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3. Water is an important solvent

3.1 - Explain changes, if any, to particles and account for those changes when the following types of chemicals interact with water: a soluble ionic compound such as sodium chloride, a soluble molecular compound such as sucrose, a soluble or partially soluble molecular element or compound such as iodine, oxygen or hydrogen chloride, a covalent network structure such as silicon dioxide, a substance with large molecules, such as cellulose or polyethylene

Soluble Ionic Compounds

Many ionic compounds, such as sodium chloride, are readily soluble in water but many others, such as silver chloride, are insoluble

The insolubility of some ionic compounds in water occurs due to the particular stability of the crystal lattices of these compounds

The lattice breaks up into free ions which are able to freely move within the solution

This occurs because dipoles on water molecules cause them to attach onto free ions due to the strong electrostatic attraction

The attraction between the water molecules and ions are stronger than the forces between the ions in the ionic lattice, causing the ionic compound to dissolve

Within the solution, the cations surrounding the water molecules are oriented towards the negatively charged end of the water dipole, while the anions oriented towards the positive end of the water dipole

Aqueous solutions of ionic compounds produce ions in solution by the process known as dissociation

Most ionic solutions are good electrical conductors due to the formation of ions allowing a current to flow

Substances that produce ions in solution are called electrolytes

Soluble Molecular Compounds

Some soluble molecular compounds include simple sugars (glucose, sucrose, fructose), alcohols (methanol, ethanol, propenol) and fats (glycerols)

When soluble molecular compounds dissolve in water, the crystal breaks up into individual molecules that form hydrogen bonds with individual water molecules

These substances all contain hyrdoxyl (OH) groups that can form strong hydrogen bonds with water molecules

When these substances dissolve, intermolecular forces within the substance are overcome by stronger hydrogen bonding with water molecules

Individual molecules are then carried away by water molecules as they dissolve

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Soluble or partially soluble molecular element or compound

Non-polar molecular substances such as oxygen gas, nitrogen gas, bromine and iodine are slightly soluble in water

These only interact through dispersion forces, as they do not have net dipoles and are unable to form dipole-dipole forces or hydrogen bonds

These weak dispersion forces are enough to allow water to dissolve small amounts of non-polar substances like oxygen and nitrogen

However the solubility of these substance are much lower than substances that can form hydrogen or dipole-dipole bonds

Polar molecular compounds are able to form dipole-dipole forces with water molecules, and have greater solubilities

Covalent Network Structures

Covalent network solids such as silicon dioxide and diamond are insoluble in water and most other solvents

This occurs because of the very strong covalent bonds that exist within these crystal lattices, which cannot be broken by the weaker intermolecular forces that could be formed with water molecules

These substances do not react or dissolve as their internal bonding is stronger than all intermolecular forces, even hydrogen bonding

Substances with large molecules

Substances with large molecules, such as cellulose or polyethylene also tend to not dissolve or react with water

They have large molecular weights in the hundreds and thousands, and are not soluble as these molecules are strongly held together to each other in an orderly fashion by strong intermolecular forces

For cellulose, long cellulose chains are strongly held together by hydrogen bonding which water will not be able to overcome

For polyethylene, individual molecules are long hydrocarbon chains that are non-polar, which water cannot dissolve. At high molecular weights, dispersion forces can also get strong enough to resist water's hydrogen bonding

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3.2 - Analyse the relationship between the solubility of substances in water and the polar nature of the water molecule

Like dissolves like

Polar solvents (water, ammonia) are good at dissolving polar or ionic substances (glucose, ethanol, NaCl)

Non-polar solvents (oil, petrol, kerosene) are good at dissolving non-polar substances (plastics, dirt)

Therefore polar substances will dissolve in water, and non-polar substances will not

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3.2P - Process information from secondary sources to visualise the dissolution in water of various tyoes of substances and solve problems by using models to show the changes that occur in particle arrangement as dissolution occurs