Unit 8: Solutions Mathematicians have Problems, But Chemists have Solutions

Water  Water is a polar molecule.  This means the oxygen side is negatively charged and the hydrogen side is positively charged.  This charge allows water molecules to be attracted to other water molecules through intermolecular forces. H H O

 The forces that hold water molecules together are called hydrogen bonding.  Because of water’s strong hydrogen bonding, it is a liquid at normal temperatures even though is has such a low molecular weight.  It is also the reason it has such an unusually high boiling point (100 o C) and high specific heat capacity.  (this means it takes a lot of energy (4.18 J/g o C )to raise the temperature of liquid water!)

Physical Properties of Water Surface Tension—The resistance of a liquid to increase in surface area   The Result : Water beads as a drop instead of spreading out   --Cohesive forces are those holding the liquid together to present the minimum of surface area. Water has a very high surface tension due to strong hydrogen bonding!

  Capillary Action—The spontaneous rising of liquid in a narrow tube due to two forces   Cohesive and Adhesive forces.   Adhesive forces are those connecting a liquid and its container.   For Water- Adhesive forces are stronger than the cohesive forces, the liquid is higher nearer the container and lower in the middle (the meniscus is concave.)

Solutions are mixtures…  SOLUTIONS are HOMOGENOUS MIXTURES. They cannot be separated by filtration or settling. They cannot be separated by filtration or settling.  Solutions have two components-  1. SOLUTE- the solid that gets dissolved, or the liquid that is present in smaller amount.  2. SOLVENT- the liquid that does the dissolving. It is always present in a larger amount.  Solvation – The dissolving process where solute particles are surrounded by solvent molecules.

 Example: Salt water What is the solute?___________ What is the solute?___________ What is the solvent?___________ What is the solvent?___________  Example – 70% ethanol solution. Solute _______________ Solute _______________ Solvent________________ Solvent________________ Salt water Water ethanol

Solubility Principle   Solutes will dissolve in solvents with similar polarity. “Like Dissolves Like”  Polar substances will dissolve in polar solvents and nonpolar substances will only dissolve in nonpolar substances.  Water is considered a universal solvent because it can dissolve both ionic and polar covalent compounds   Example – Alcohol will dissolve in water because both are polar molecules.

Like Dissolves Like  Q: Why does salad dressing separate into its oil and vinegar components? The polarity of vinegar and oil are not the same. Oil is non-polar and vinegar is polar. Therefore, they do not mix.

Ionic Compounds  In an aqueous solution, water is the solvent. HCl (aq), NaCl(aq), CuSO 4 (aq)   Ionic compounds are considered polar Ionic compounds will separate into its ionic parts when dissolved in water.   NaCl + water    K 2 SO 4 + water  Na + + Cl - 2K + + SO 4 2-

Solvation  Water dissolves ionic solutes through a process called solvation.  Steps to solvation:  1. An ionic solute is added to water.  2. Water molecules bombard the ionic compound until it breaks into its ions.  3. The positive “H’s” in water molecules surround the negative solute ion.  4. The negative “O” in water molecules surround the positive solute ion.  Ions that are completely surrounded by water molecules form hydration spheres and are said to be “hydrated”.

Solvation Animation  ssentialchemistry/flash/molvie1.swf ssentialchemistry/flash/molvie1.swf ssentialchemistry/flash/molvie1.swf

Hydration Spheres O H H O H H O H H O H H O H H O H H Na + Cl