Unit 8: Solutions

Chapter 7 and 8

Section 1: Solutions and Other Mixtures Objectives

• Distinguish between heterogeneous mixtures and homogeneous mixtures.

• Compare the properties of suspensions, colloids, and solutions.

• Give examples of solutions that contain solids or gases.

Review

• Recall from our discussion of matter that substances can be classified as either pure substances or mixtures.

• Elements and compounds are pure substances. Almost everything else is a mixture of some sort.

• Within the mixture category, there are two sub-categories: heterogeneous and homogeneous.

• What do those words mean?

Heterogenous Mixtures

• Remember that a heterogenous is not very well mixed and the parts of it can be separated by relatively simple means.

• Examples include salad, orange juice, granite, concrete, oil and vinegar, paint, pizza, etc.

Sub-groups of Heterogeneous Mixtures

• Suspensions – these are mixtures in which the particles settle out when the mixture is allowed to stand. Orange juice is in this sub-category.

• Colloids – these are mixtures in which the particles are too small to settle out. Paint is an example.

Homogeneous Mixtures

• A homogenous mixture is pretty well mixed and it’s more difficult to separate the parts of it.

• A solution of water and sugar is an example. So are stainless steel, air, saltwater, rubbing alcohol, etc.

• Remember that some solutions don’t contain water – metal alloys, crude oil, and vinegar are examples.

Section 2: How Substances DissolveObjectives

• Explain how the polarity of water enables it to dissolve many different substances.

• Relate the ability of a solvent to dissolve a solute to the relative strengths of forces between molecules.

• Describe three ways to increase the rate at which a solute dissolves in a solvent.

• Explain how a solute affects the freezing point and boiling point of a solution.

Water: The Universal Solvent

• Water is often referred to as the universal solvent because it is able to dissolve so many different substance.

• The reason for this is a property of water called polarity. Polarity refers to the fact that water has a positive end (the Hs) and a negative end (the O).

• Polar molecules attract other polar molecules, just like magnetic materials attract other magnetic materials.

• Ionic compounds are polar molecules (remember, they are ionic because one part of the compound is a positive ion and the other is a negative ion).

• This is why water will easily dissolve NaCl, a common ionic compound.

• The (-) end of water attracts to the (+) end of salt, and vise versa. This force of attraction pulls the molecules in closely and clings to them, so that they become very well mixed together and hard to separate.

Increasing Solubility

• By increasing the surface area of the solute, it will dissolve faster. This can be done by breaking the solute into small pieces.

• Stirring or shaking will speed up the dissolving process. This is because it allows more water to come into contact with more of the solute.

• Heating the substances up will also speed up the process. This is because molecules move faster when heated, so they’re mixing with the solvent more quickly.

Properties Affected by Solubility

• The boiling and freezing points of a substance are both affected when that substance is part of a solution.

• For example, the boiling point of water is normally 100o C. However, by adding salt to the water, the boiling point is raised about 2o C. the freezing point is lowered to about -7o C.

• This is helpful as we use this property when we use antifreeze in our cars and add salt to ice to help it melt more quickly.

Section 3: Solubility and ConcentrationObjectives

• Explain the meaning of solubility and compare the solubilities of various substances.

• Describe dilute, concentrated, saturated, unsaturated, and supersaturated solutions.

• Relate changes in temperature and pressure to changes in solubility of solid and gaseous solutes.

• Express the concentration of a solution as molarity, and calculate the molarity of a solution given the amount of solute and the volume of the solution.

Solubility

• As you know, solubility is a substance’s ability to dissolve in another substance, usually water.

• Different substances have different solubilities, meaning that some substances dissolve better in water than others.

• Concentration refers to the amount of solute that is dissolved in a given amount of solution. Something with a low concentration does not have have much solute (and is called diluted), which a high concentration has a lot of solute (and is called concentrated).

Saturation

• A saturated solution is one that can dissolve no more solute and that has reached its maximum concentration

• An unsaturated solution is one that can still dissolve more solute and has not reached its maximum concentration.

• A supersaturated solution is one that has more solutes than it can normally hold, which sometimes happens when a solution is heated, then cooled.

Molarity

• As we learned earlier, molarity is a measure of concentration of a substance.

• To calculate the molarity of a solution:

molarity = moles of solute liters of solution

Or, M = mol L

Example• Calculate the molarity of Na2CO3 in a solution of 38.6g of solute in 0.500 L

of solution.

Moles Na2CO3 = mass Na2CO3

molar mass Na2CO3

Molarity = moles Na2CO3

0.500 L solution

Molar mass Na2CO3 = 106 g

Moles Na2CO3 = 38.6 g = 0.364 mol Na2CO3

106 gMolarity = 0.364 mol Na2CO3 = 0.728 M

0.500 L solution

Section 4: Reactions of Acids with Bases Objectives

• Write ionic equations for neutralization reactions.

• Identify the products of a neutralization reaction.

• Describe the composition of a salt.

Acid-Base Reactions

• A neutralization reaction is a reaction between an acid and a base.

• A strong acid and a strong base react to form water and salt.

• Acid Reaction: HCl + H2O H3O+ + Cl-

• Base Reaction: NaOH Na+ + OH-

• Combined Reaction: H3O+ + Cl- + Na+ + OH- NaCl+ 2H2O

• When a strong acid reacts with a weak base, the solution will still be acidic.

• When a weak acid reacts with a strong base, the solution will be basic.

• The final pH of the solution is determined by the pH of the acid and base involved in the reaction.