Chapter 7: Solutions

Section 1:Solutions VS Mixtures

Sodium Chloride Liquid

Solution

Air is a Gaseous Solution

•Mixtures of different substances exist all around us.–Many of these mixtures are invisible, and we never notice them.

–However, some mixtures can easily be identified.

What is a mixture?

•All matter is either a pure substance or a mixture of other substances.–Definition: pure substance – matter that has a fixed chemical composition.

•For example:

What is a mixture?

Water is a pure substance. It is ALWAYS H2O.

Kool-Aid is a mixture of water and other substances.

•Mixtures can be categorized into two groups:–Heterogeneous and Homogeneous

–Definition: heterogeneous mixture – a mixture made of visibly different substances.•The particles in a het. Mixture are not spread evenly.

•Orange juice with pulp is an example of a heterogeneous mixture.

Types of Mixtures

Heterogeneous Mixture

Clear Orange Juice

Orange Juice Pulp

Heterogeneous Mixtures

Different fragments make

up this rock.

This rock is a heterogeneous mixture.

•Homogeneous mixtures look the same all over.–Definition: homogeneous mixture – a mixture in which the particles are spread evenly.

–Sweet tea is an example of a homogenous mixture.

–There are several different substances (tea, water, and sugar) but you cannot see them. The tea looks uniform (the same) throughout.

Homogeneous Mixtures

+

=

+

The tea looks the same throughout

the pitcher.

•We can call homogeneous mixtures by another name: solutions.–Definition: solution - a homogenous mixture of 2 or more substances that are evenly dispersed.

Solutions

• Many solutions are formed by dissolving one substance into another substance.

– These tablets are dissolving in the water to form a

solution.

Solutions

• It is important to know what makes up a solution.

• All solutions are made of solutes and solvents.

– Definition: solute - a substance that dissolves into another substance.

– Definition: solvent - a substance that a solute is dissolved into.

What is in a Solution?

•For example:–When you stir sugar into water, the sugar dissolves.

–The water is the solvent.–The sugar is the solute.

Solvents

Solute Solvent Solution

+ =

•Water is very good at dissolving things.–Water is known as “The Universal Solvent”

The Universal Solvent

•Not all solutions contain water!•Other states of matter can be

solutions.–Gases & Solids can form solutions also.

–Air is an example of a gaseous solution.

–Air is composed of lots of different gases that we cannot see.

Other types of Solutions

•2 or more solids can form solutions also.–Metal alloys are homogenous mixtures that contain a metal mixed with another substance.

–Some examples are:•Steel – iron and carbon•Brass – copper and zinc•Bronze – copper and tin

Metal Alloys

•In order to make an alloy, the metals must be melted.–While melted, the metals are mixed to form a solution.

Metal Alloys

+Tin Copper

Bronze

Chapter 7: Solutions

Section 2: How Substances Dissolve

• Water can dissolve ionic compounds because of its structure.– The electrons in the hydrogen atoms

are pulled toward the oxygen atom.

– This gives the oxygen atom a slight negative charge.

– The hydrogen atoms gain a slight positive charge.

How do things dissolve?

The Water Molecule

Oxygen

H H

• Because water’s + and – charges are not spread out evenly, it becomes “polar”.

– Definition: polar compound – a molecule that has a positive side and a negative side.

•Because water is a polar compound, it is a good solvent.

Polar Compound

– In chemistry, a rule of thumb is that “like dissolves like.”

–Water is a polar compound, so it can dissolve other polar compounds.

– If water cannot dissolve a substance, then that substance is “nonpolar”.

Like dissolves Like

– Definition: nonpolar compound – a compound that has no charge on its molecules..

•Nonpolar compounds can only dissolve other nonpolar substances.

•Example – oil-based paint will not dissolve in water. A nonpolar solvent must be used.

Nonpolar Compound

Polar vs. Nonpolar

Oil is Nonpolar

They cannot mix.

Water is Polar

• We have all seen solutes dissolve into solvents before.

• And we all *probably* know some ways to speed up the process.

The Dissolving Process

• Making a solute smaller makes it dissolve faster.– By crushing up a solute, you increase the

surface area.

The Dissolving Process

Rock Salt

Crushed Salt

• Stirring or shaking will make a solute dissolve faster.

The Dissolving Process

• Increasing the solvent’s temperature will make the solute dissolve faster.

The Dissolving Process

Which one willDissolve sugar

Fastest?

• Did you know that liquids can dissolve gases?

• Fish and other aquatic life breath oxygen that has dissolved into water.

• Liquids dissolve gases best when they are cold.

The Dissolving Process: Gases

• Don’t believe it? Well, consider this…

– Which makes a louder “whoosh” sound when opened… a hot soda or a cold one?

The Dissolving Process: Gases

• The amount of solute dissolved in a solvent affects its concentration.

– Think of concentration as being how “strong” a solution is.

– Definition: concentration – the amount of a substance in a certain amount of solution.

Concentration

•“Concentrated” substances have lots of solute.

•“Diluted” substances only have a little solute.

Concentration

• When a solvent can no longer hold any more solute, we called it “saturated”.

– Definition: saturated solution – a solution that cannot dissolve any more of a given solute.

– Definition: unsaturated – a solution that CAN hold more solute.

Saturated / Unsaturated

• Sometimes, a solvent can be made to hold more solute than normal.– The solution is called “supersaturated”.

– Definition: supersaturated – a solution that has more solute than normal.

Super Saturated

•Supersaturated solutions are unstable.–The extra solute can “fall out” at any time.

Super Saturated