Solutions

There are three main ways that stuff can be mixed with liquids:

  1) Suspensions: Heterogeneous

mixtures where small particles of a solid are floating around in a liquid.

  How you can tell if something is a

suspension: over time, these particles settle to the bottom of the liquid. For example, if mud is left alone for a long period of time, the particles settle to the bottom.

  Examples of solutions: Mud, peanut butter,

ketchup, etc.

2) Colloids: Heterogeneous mixtures in which mixtures consist of two phases of matter that are stable and don’t settle out no matter how much time you give them.

  Why they don’t settle out: Brownian motion

 

Brownian motion is when the particles of the thing that’s dispersed is hit from all directions by the particles of the thing it’s dispersed in. Because the particles of the thing that is dispersed are so small, they keep it from settling.

 

How you can tell if something is a colloid: It experiences the Tyndall effect: When light shines through a colloid, you can see the rays of light as they travel though the solid because the suspended particles reflect/scatter it. For example, smoke is colloidal because you can see light beams pass through it, and the smoke particles don’t settle to the bottom. This causes it to appear “milky” or “foggy.”

Examples of colloids: Butter, milk, smoke, aerosols.

3) Solutions: Homogeneous mixtures, in which one material completely dissolves in another.

  All solutions have two parts: 

Solvent: The thing that does the dissolving.

  Solute: The thing that gets dissolved.

  How you can tell if something is a

solution: If you let it sit for a long time, nothing settles to the bottom. Additionally, it doesn’t experience the Tyndall effect (i.e. it doesn’t appear “cloudy” and light beams are invisible as they pass through it. 

Concentration:  It’s frequently handy to know how much

stuff is dissolved in a solution.

concentration: Any measurement of how much solute is dissolved in a solution.

Qualitative ways of measuring concentration:

Unsaturated: more solute can be dissolved

Saturated: no more solute can be dissolved

Supersaturated: more solute is dissolved than the solvent can handle Two ways of making them:

Cool a saturated solution. Evaporate solvent from a saturated solution. 

Supersaturated solutions are never stable; when disturbed they will recrystallize.

Quantitative method of measuring concentration

Molarity (M): moles of solute / liter of solution.

Sample problems:  

What’s the molarity of a solution that contains 0.5 moles of NaCl dissolved to make 1.5 L of solution?  

If I have 30 g LiOH dissolved in 300 mL of solution, what’s the molarity?

Quantitative method of measuring concentration

Molarity (M): moles of solute / liter of solution.

Sample problems:  

What’s the molarity of a solution that contains 0.5 moles of NaCl dissolved to make 1.5 L of solution?  

0.5 mol/1.5L = 0.33M If I have 30 g LiOH dissolved in 300 mL of

solution, what’s the molarity?

Quantitative method of measuring concentration

Molarity (M): moles of solute / liter of solution.

Sample problems:  

What’s the molarity of a solution that contains 0.5 moles of NaCl dissolved to make 1.5 L of solution?  

0.5 mol/1.5L = 0.33M If I have 30 g LiOH dissolved in 300 mL of

solution, what’s the molarity?

(30g/24g/mol)÷ 0.3L = 4.2M

How to make a solution: 

Using the equation M = mol/L, determine how much solute will be required.

 

Add solvent until the solution is at the desired volume.

 

Stir until the solute is completely dissolved. 

Add more solvent until the solution is at the desired volume (this will require more solvent because the volume of the solution will decrease when the solute dissolves – explain this).

How NOT to make a solution:

 

Using the equation M = mol/L, determine how much solute will be required.

 

Add a volume of solvent equal to the volume of the solution you’re trying to make.

 

Stir and dissolve. 

The reason this doesn’t work well is that the solute has a volume of its own, so the final volume of the solution will be greater than desired, making it slightly less concentrated that you’d like.