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Water molecules are polar. This is because they have two polar O-H bonds and a non-symmetrical v-shaped or bent shape, leading to the water molecules having a net dipole.
For a molecule of such as low mass (18 g/mol) water has higher melting and boiling points than expected. The melting point of water is 0 degrees Celsius and the boiling point is 100 degrees Celsius.
There are relatively strong attractive forces between water molecules, this is due to the relatively large electronegativity difference between the hydrogen and oxygen in the polar O-H bonds, and the small size of the water molecule. The relatively strong attractive forces between molecules (intermolecular forces) mean that more energy is needed to overcome these forces of attraction and separate the molecules. This explain the relatively high melting and boiling points. Even though the strength of the intermolecular forces of attraction are quite high they are much less than the strength of covalent or ionic bonds.
Some substances are able to dissolve in water, this is due to the polarity of water molecules. This will be explained in more detail later.
The dotted green lines represent strong attractive forces between the H and O atoms of neighbouring water molecules. The positively charge H side of the polar water molecule is attracted to the negatively charges O side of a neighbouring water molecule. These intermolecular forces of attraction are called hydrogen bonds. The strong covalent bonds (intramolecular forces) inside the water molecule are shown in white.
We know that molecules in liquids have mobility, they can move.
When polar molecules, such as water are placed near an electrically charged object the polar molecules align and attracted to the charged object.
You can try this at home by charging up an inflated balloon, charging up an acetate rod, or even a plastic ruler, by rubbing it on a woollen jersey/jumper and then placing it near a stream of water from the tap. You will see that the stream of polar water is deflected towards the charged balloon.
This provides a test to find out which liquids are polar and which are not.
Polar liquids are deflected towards a charged object. Non polar liquids are not deflected towards the charged object
The video below shows this.
First of all we should make sure that we understand the meaning of some key terms in Chemistry
Solvent - a solvent is a liquid in which as solute is dissolved to make a solution. Water is the most common solvent, but in Chemistry we use other organic solvents as well to dissolve solutes.
Solute - solutes can be solid or liquid and are the substances that are dissolved in solvents.
Solution - a solution is made when a solute dissolves in a solution. If water is the solvent then we call the solution an aqueous solution.
The term 'like dissolves like' describes how polar molecules such as ethanol, ethanoic acid, and methanol will dissolve in polar solvents and non-polar molecules such as cyclohexane and turpentine will dissolve in non-polar solvents.
When two liquids dissolve in each other and form a single continuous layer we say they are miscible.
Insoluble molecules are immiscible.
Polar molecules will be miscible in (dissolve in) polar water, but non-polar molecules will be immiscible in polar water.
TASK 1:- Answer the questions on page 64 & 65 on your SciPAD book.
Sometimes there is a question on the exam paper that relates to solubility and polarity of molecules. In order to answer the question you need to be able to discuss the polarity of both molecules before you can discuss whether the two molecules are miscibles.
In order to justify the miscibility of two substances you can follow the 4 steps outlined below:
Explain the shape of the two molecules (see the 'Shapes of Molecules' section on the page.
Explain the polarity of the two molecules (use the writing frame in the 'Electronegativity & Polarity of Molecules' section on this page).
State that polar substances dissolve in polar solvents or that non-polar substances dissolve in non-polar solvents.
State whether the two molecules given are miscible (soluble).
TASK 2:- Answer the questions on page 66-69 on your SciPAD book.
Compare the solubilities of iodine, I2 (s), in water, H2O (l) - a polar solvent, and in cyclohexane, C6H12 (l) - a non-polar solvent.
Use your knowledge of structure and bonding to explain the solubility of iodine in these two solvents.
Iodine is a non-polar (covalent) molecular made up of I2 molecules which are held together by weak intermolecular forces of attraction. The iodine molecules are non-polar because there is no difference in electronegativity of the atoms in the molecule. The weak intermolecular forces of attraction between iodine molecules are relatively easy to break. In these examples the iodine molecules are the solute.
Iodine is soluble in cyclohexane, but not in water. This is because like dissolves like. By looking at the diagram of the cyclohexane molecule you can see that the cyclohexane molecule is non-polar. Non-polar solutes e.g. iodine can dissolve in non-polar solvents e.g. cyclohexane. In order to dissolve then the solute-solvent forces of attraction (in this case iodine-cyclohexane attractions) are strong enough to overcome the solute-solute (iodine-iodine) forces of attraction and the solvent-solvent (cyclohexane-cyclohexane) attractions because all the attractive forces are similar (non-polar).
Iodine will not dissolve in water, because iodine is non-polar and water is polar. The iodine-water attractions are not strong enough to overcome the solute-solute (iodine-iodine) forces of attraction and the solvent-solvent (cyclohexane-cyclohexane) attractions.
Dissolving of ionic compounds can be explained in two steps.
Step 1) The strong ionic bonds that hold the ions together in a lattice are overcome by attraction between the ions and the oppositely charged ends of the polar water molecules. The ions then separate.
Step 2) The separated ions are then surrounded by water molecules to form hydrated ions. These hydrated ions are written with (aq) after their symbol.
When the ionic solid magnesium chloride is placed in water, the charged ends of the polar water molecules are attracted to the positive and negative ions in the magnesium chloride crystal lattice. The partially negative (delta negative) oxygen side of the polar water molecule is attracted to the positively charged magnesium ions (magnesium cations) and the partially positive (delta positive) hydrogen side of the polar water molecule is attracted to the negatively charged chloride ions. This attractive force allows the ions to break away from the lattice and become surrounded by water molecules. The hydrated ions can then move independently through the solution.