Chapter 15: Solutions

3/30/2011

What does “dissolve” mean?

• When a compound dissolves, the atoms in thecompound break apart and spread out.

• Substance 1 breaks apart the crystal lattice ofsubstance 2 by pulling on it:

Substance 1

Substance 2

• Substance 1 is called the solvent. It does the breaking.

• Substance 2 is called the solute. It gets broken apart.

Substance 1 Solvent

Substance 2 Solute

• Dissolving is NOT a chemical reaction.

• The atoms are NOT rearranged into new substances, they are just separated temporarily.

• If you remove the solvent, the solute will goback to its crystal lattice.

Dissolving:

• Dissolving is when two substances mix andone compound is broken apart by the other.– The solute is the substance that breaks apart

(usually a solid)

– The solvent is the substance that does thebreaking (usually a liquid)

• Dissolving is NOT a chemical reaction, so there are no reactants or products.

Think!

• A teaspoon of dry coffee crystals dissolves when mixed in a cup of hot water. The original crystals are classified as a

A. solute.B. solvent.C. reactant.D. product.

What is a solution?

• When a substance dissolves in anothersubstance, the whole thing is called a solution.

Ocean water is a solution of salt

dissolved in waterSteel is a solution

of carbon dissolved in iron

Air is a solution of oxygen dissolved in

nitrogen

May exist in all states of matter (gas, liquid, solid)• Examples:

• Oxygen and Nitrogen gas (gas/gas)• Carbon Dioxide in water (gas/liquid)• Water in air (Liquid/gas)• Alcohol in water (Liquid/Liquid)• Mercury in Silver (Liquid/Solid)• Sugar in water (solid/liquid)• Copper in Nickel (Solid/Solid)

Types of Mixtures

• Solutions are homogeneous mixtures of a solute anda solvent. Looks the same

throughout

Solutions:

• Solution – a homogeneous mixture of a soluteand a solvent

• Homogeneous means that it looks the same throughout.

• Ex: Sugar dissolves in water to make sugar water

Solute Solvent Solution (homogeneous

mixture)

Think!

• Label the picture using the vocabulary words:

homogeneous, heterogeneous

Heterogeneous

Homogeneous

Think!

• Label the picture using the vocabulary words:

solution, solvent, solute

Solute

Solution

Solvent

Chapter 15, WS #1

Dissolving & Solutions

3/30/2011

Why do substances dissolve?

Start today’s notes on theFRONT of a NEW PAGE

Why do substances dissolve?

Remember:

• Particles are in constant, random motion– Gases move quickly and freely

– Liquids flow

– Solids vibrate in place

• So all particles bump into each other (whetherthey are solid, liquid, or gas)

Even though they bump into each other, compounds hold together because of:– Intramolecular forces (within compounds)

– Intermolecular forces (between compounds)

• But what if they bump into a different substance that they are more attracted to?

Na+ Cl–

H+ O–2

H+

Attraction!

• If the intermolecular attraction (between compounds) is stronger than the intramolecular attraction (within the compound), the compound will dissolve (break apart)

Na+

H+ O–2

H+

Cl–

Stronger Intermolecular

Attraction!

Weaker Intramolecular

Attraction

Polar dissolves polar

Why do substances dissolve?

• Particles are in constant, random motion.

• Particles are bumping into each other.

• If a particle bumps into another particle that it is more attracted to, it can break apart from its crystal lattice and dissolve.

3/30/2011

Which particles will be mostattracted to each other?

• Some compounds do not share their electrons evenly.

• They are called polar molecules.

• Other molecules share their electrons evenly.

• They are called nonpolar molecules.

Dissolving Rule:

• Like dissolves like– Nonpolar solvents will dissolve nonpolar solutes– Polar solvents will dissolve polar solutes

• Ex: water is polar and NaCl is polar, so NaCl will dissolve in water

• Ex. chalk is nonpolar and ethanol is nonpolar, sochalk will dissolve in ethanol

Polar dissolves polar

Water and NaCl with Audio

• Label each solute as polar or nonpolar:

– NH4Cl

– C10H8

– C2H5OH– CO(NH2)2

Think!Polar Nonpolar

Polar

Nonpolar

½ is Polar & ½ is Nonpolar

Polar

Chapter 15, WS #2

Like Dissolves Like

How fast will something dissolve?

• Three things affect how quickly a substancewill dissolve:– Temperature

– Pressure

– Surface Area

Which dissolves faster?

• Sugar in ice water

• Sugar in hot water

• Heat makes things dissolve faster!

• Heat makes things dissolve faster– Heat makes particles move faster

– Then they bump into each other more

– This makes them dissolve faster

Temperature

Pressure

• Higher pressure makes things dissolve faster– When pressure increases, particles are forced

closer together

– Then they bump into each other more

– This makes them dissolve faster

When you want something todissolve faster, what do you do?

• Drop it in water and leave it alone

• Drop it in water and crush it up

• More surface area makes things dissolve faster

Surface Area

• More surface area makes things dissolve faster– If particles have more surface area, they will have

more places to get bumped

– More surface to bump into means more chances to dissolve

Chapter 15, WS #3

Dissolving Rates

Which cup of tea has the mosttea dissolved in it?

Concentration

• The concentration of a solution is how much solute is dissolved in a specific amount of solvent.

• Levels of concentration:– Dilute (not much solute dissolved)

– Concentrated (lots of solute dissolved)

– Saturated (the maximum amount of solute dissolved)

3/30/2011

Think!

• Label each beaker as dilute, concentrated, orsaturated.

Is there a limit to how muchsolute will dissolve?

• Will 1 teaspoon of salt dissolve in a cup of hot water?

YES

• Will 1 tablespoon of salt dissolve in a cup of hot water?

YES

• Will 1 cup of salt dissolve in a cup of hot water?

MAYBE

• Will 1 container of salt dissolve in a cup of hot water?

NO

Yes, there is a limit!

• The limit is called the solubility• It is a number with a unit• It is different for every solute• It depends on the temperature and pressure

• Example:– The most sugar that will dissolve in 100 grams of

water at 25°C is 67.9 grams.

– The solubility of sugar in water at 25°C is 67.9 g/100 g H2O.

Solubility

• The solubility is the maximum amount of solute that can be dissolved in a given amount of solute.

• Example:– The most salt that will dissolve in 100 grams of

water at 25°C is 36.2 grams.

– The solubility of NaCl in water at 25°C is 36.2 g/100 g H2O.

Think!

• If the solubility of NaCl at 25°C is 36.2 g/100g H2O, what mass of NaCl can be dissolved in50.0 g of H2O?

½ as much solvent

½ as much solute

Solute Solvent Temperature

36.2 g NaCl 100 g H2O 25°C18.1 g NaCl 50 g H2O 25°C

Chapter 15, WS #4

Concentration

• Molarity (M) = moles of solute

• Example: How many moles of HNO3 are needed to prepare 5.0 L of a 2.0 M solution of HNO3?

liters of solution

3/30/2011

Molarity (M) = moles of solute liters of solution

2.0 M = moles of solute 5.0 L

(2.0 M) × (5.0 L) = moles of solute

10 moles of solute (HNO3)

Chapter 15, WS #7

Molarity