Unit 6: Solution Chemistry

Content Outline: Basic Solution Chemistry (6.1)

  1. Solution
  1. Term referring to a homogenous (same)mixture of two or more substances in a single phase of matter. ( One substance dissolves into the other substance.)

1. Each sample of a homogenous solution would possess the exact same chemical composition and same chemical properties…hence homogenous (same).

  1. Most solutions tend to be of the liquid state, but they can also be mixtures of gases (such as O2 and N2 in the atmosphere) or solids such as 14 K gold. (Gold, silver and Copper blended together while inthe molten state…remember the previous unit?)
  1. Solvent
  1. This term refers to the substance or medium (liquid base) that will “perform the dissolving” (separating of atoms/molecules)of the other substance.

For example: water – the universal solvent as it can dissolve most substances.

1.The solvent for a solution is usually water, but it can be other substances such as ethanol (an alcohol) or a gas such as Nitrogen in the atmosphere.

  1. Soluble
  1. Refers to a substance that is capable of being dissolved (separated apart at the atomic/molecular level) into another substance.

For example: salt or sugar in water OR water in ethanol.

  1. Solute
  1. This is the term referring to the substance that is “being dissolved” (separated).
  2. Terms referring to the amount of solute in a solution:
  1. Unsaturated
  1. The solution contains less solute than the maximum amount at that temperature.
  2. Basically, you can add more to the solution.
  1. Saturated
  1. The solution contains the maximum amount of solute at that temperature.
  2. Basically, you cannot add more, unless you change the temperature.

For example: you add sugar to cold unsweet tea, but you can see the sugar sitting in the bottom of the glass.

  1. Solution equilibrium
  1. This is the state of a solution when dissolving rate = re-crystallizationrate.
  1. Supersaturated
  1. A solution that contains more than the maximumamount of solute at that temperature.
  2. This is achieved by heating a solution, so that more can be dissolved into it. Then it is allowed to cool.
  1. The heating breaks the intermolecular bonds (in water…think hydrogen bonds breaking) so that there are more free solvent molecules to grab and hold separated solute molecules.

For example: To make sweet tea, you first brew (hot) tea. Then add in the sugar. Stir. And then cool down and add ice to enjoy on a hot afternoon.

  1. The solute tends to stay suspended in the solvent as long as the substance is not disturbed (shaken).
  2. Once shaken, the solute recrystallizes and “falls out” of the solution.

This is how rock candy pops (sugar on a stick) are made. You heat up water. Add sugar until you can not dissolve anymore. Let it cool in an undisturbed area. Then come back in about 10 minutes and stick a clean wooden stick in the container and wait. Also, honey is a super saturated solution. If you put honey in the refrigerator it will crystallize.

  1. Solubility
  1. The amount of a substance required to form a saturated solution with a specific amount of solvent at a specific temperature.
  2. These values are usually represented in a Solubility Table showing various temperatures.
  1. Warmer temperatures usually hold more solute.
  1. More “free solvent molecules” to grab solute molecules because of broken intermolecular bonds.
  1. Colder temperaturesusually hold less solute.

a. Less “free solvent molecules” to grab solute because they are bound inter-molecularly.

For example, cold tea is much harder to sweeten than hot tea.

  1. Gases also require a specified pressure…remember that temperaturedirectly affects pressure.

For example, cold water holds more dissolved Oxygen gas than warm water. The amount of dissolved Oxygen gas determines the types of life forms that can live in that environment. Also, a cold Coke has less fizz when opened that a hot Coke.

  1. Factors that affect the rate of dissolving:
  1. Increasing the Surface Area of a solid by “crushing it”.

Take two ice cubes and two glasses of warm water (same temperature). Put one in a paper towel. Wrap the cube so it doesn’t go all over your kitchen. Crush it with a hammer or mallet. Now put the crushed in one cup and the intact cube in the other. Which one melted first?

  1. “Frees up” solute molecules to have greater interaction with the solvent molecules more collisions.
  1. Agitation (stirring the solution)

Repeat the above, but do not crush one cube. Put one cube in each glass of warm water. Stir the water of one glass with a spoon. Which melted first?

  1. Stirring the solvent increases the Kinetic Energy of the solvent molecules…more collisions quicker.
  1. Heating the solvent
  1. Heating causes the intermolecular bonds of the solvent to break and “free up” the molecules to grab solute.
  2. In other words, there are more collisions between solvent molecules and solute molecules.