Solutions

This kind…

Section 15.1

Forming Solutions

1. To understand the process of dissolving2. To learn why certain substances dissolve in water3. To learn qualitative terms describing the

concentration of a solution 4. To understand the factors that affect how fast a

solid dissolves   

Objectives

Section 15.1

Forming Solutions

What is a solution?

Solution – homogeneous mixture Solvent – substance present in largest amount Solutes – other substances in the solution Aqueous solution – solution with water as the

solvent

Section 15.1

Forming Solutions

ExampleState of Solution

Original State of Solute

State of Solvent

Air, natural gas Gas Gas Gas

Antifreeze in water

Liquid Liquid Liquid

Brass Solid Solid Solid

Carbonated water (soda)

Liquid Gas Liquid

Seawater, sugar solution

Liquid Solid liquid

Section 15.1

Forming Solutions

A. Solubility – Ionic Substances

Ionic substances breakup into individual cations and anions.

Section 15.1

Forming Solutions

A. Solubility of Ionic Substances

Polar water molecules interact with the positive and negative ions of a salt.

Section 15.1

Forming Solutions

Solubility of Polar Substances

Ethanol is soluble in water because of the polar OH bond.

Polar water

molecule

Polar ethanolMolecule

Section 15.1

Forming Solutions

A. Solubility – Polar Substances

Why is solid sugar soluble in water?Polar substances are hydrophilic.

Polar OH bonds in

sugar molecule

Section 15.1

Forming Solutions

A. Solubility – Substances Insoluble in Water

Nonpolar oil does not interact with polar water nonpolar substances are hydrophobic.

Water-water hydrogen bonds keep the water from mixing with the nonpolar molecules.

Section 15.1

Forming Solutions

A. Solubility – How Substances Dissolve

A “hole” must be made in the water structure for each solute particle.

The lost water-water interactions must be replaced by water-solute interactions.

“like dissolves like” – polar solutes dissolve in polar solvents, nonpolar solutes dissolve in nonpolar solvents

Section 15.1

Forming Solutions

What kind of bear is hydrophilic?

A Polar Bear!

Section 15.1

Forming Solutions

Solution Composition: Concentration

• Amounts of substances can vary in different solutions. – Concentration - amount of solute in relation to solvent– Qualitative measures of concentration

• concentrated – relatively large amount of solute

• dilute – relatively small amount of solute

Section 15.1

Forming Solutions

Solution Composition: Concentration

Which solution is more concentrated? B

Section 15.1

Forming Solutions

Solution Composition: Concentration

Which solution is more concentrated? Neither

Section 15.1

Forming Solutions

The solubility of a solute is limited. Saturated solution – contains as much solute as will dissolve at that temperature

Unsaturated solution – has not reached the limit of solute that will dissolve at that temperature

Solution Composition: How much solute can you dissolve in the solvent?

Section 15.1

Forming Solutions

Interpreting Solubility Curves

Section 15.1

Forming Solutions

Supersaturated solution – occurs when a solution is saturated at an elevated temperature and then allowed to cool but all of the solid remains dissolved

• Contains more dissolved solid than a saturated solution at that temperature

• Unstable – adding a crystal causes precipitation

B. Solution Composition: An Introduction

Section 15.1

Forming Solutions

Making a Supersaturated Solution

At 20oC, a solution containing ~19g Na2SO4 dissolved in it would be

saturated. To dissolve more solute, the temperature has to

be increased.

If we dissolve more solute at a higher

temperature, then cool the solution slowly so the

extra solute stays dissolved in it, the

solution will be supersaturated.

Section 15.1

Forming Solutions

Supersaturated SolutionYouTube Video

Section 15.1

Forming Solutions

Factors Affecting the Rate of Dissolving Increasing the number of collisions between solute and

solvent particles increases the rate (speed) at which a solid dissolves.

Three factors that speed up dissolving:– Increasing Surface area – dissolving occurs at the

surface the greater the surface area exposed to solvent, the faster the dissolving processEx.: A sugar cube will dissolve faster if it is ground to small crystals, increasing the total surface area.

– Stirring – removes newly dissolved particles from the surface of the solid and continuously exposes the surface to fresh solvent.

– Increasing Temperature – higher temperatures cause molecules to move more rapidly; in addition, most solids are more soluble at higher temperatures*.Ex.: More sugar will dissolve in hot water than in cold water.

*For gases dissolved in water, the opposite is true: the solubility of gases in water typically decreases as the temperature increases.

Got problems?Don’t Worry…

Chemistry has SOLUTIONS!