Unit 1: B.1-B.2

In which you will learn about:•Physical v. chemical properties•Physical v. chemical changes

•Density

B.1 Physical B.1 Physical Properties of Properties of WaterWaterMatterMatter: anything that occupies : anything that occupies

spaced and has massspaced and has massMatter can be distinguished by its Matter can be distinguished by its

physical propertiesphysical propertiesPhysical propertyPhysical property: a property that : a property that

can be observed/or measured can be observed/or measured without changing the chemical without changing the chemical makeup of the substancemakeup of the substance

What are some physical properties?What are some physical properties?• colorcolor• melting and boiling pointmelting and boiling point• odorodor

Other Physical Properties

• Density: the mass of a material within a given volume– The density of liquid water is usually given as 1 g/mL,

but it’s actually temperature dependent– 1 cm3 = 1 mL (this is super useful for the rest of the

year so MEMORIZE it now!)• Freezing point: the temperature at which a

substance changes from a liquid to a solid– For water, it is of course, 0°C

• What others can you think of?

GraphiteGraphite — layer — layer structure of structure of carbon atoms carbon atoms reflects physical reflects physical properties.properties.

This allows layers This allows layers to easily be to easily be removed.removed.

This easy transfer This easy transfer of layers is why of layers is why we use it in we use it in pencils!pencils!

Water Is Never Pure

• Water is the only ordinary liquid found in naturally in our environment– Because so many substances dissolve readily in water,

quite a few liquids are actually water solutions– A water-based solution is an aqueous solution

• BTW, what’s a chemical property? A property that can only be observed and/or measured if the substance is chemically altered (Example: flammability)

Physical ChangesPhysical Changes– can be observed without

changing the identity of the substance

Some Some physical changes physical changes would would bebe

• boiling of a liquidboiling of a liquid• melting of a solidmelting of a solid• dissolving a solid in a liquid dissolving a solid in a liquid

to give a homogeneous to give a homogeneous mixture — a SOLUTION.mixture — a SOLUTION.

Chemical Properties and Chemical Properties and Chemical ChangeChemical Change

• Chemical change Chemical change or or chemical reactionchemical reaction — — transformation of one or transformation of one or more atoms or molecules more atoms or molecules into one or more different into one or more different molecules.molecules.

•Burning hydrogen (HBurning hydrogen (H22) in ) in oxygen (Ooxygen (O22) gives H) gives H22O.O.

Sure Signs of a Chemical Sure Signs of a Chemical ChangeChange

• HeatHeat• Odor changeOdor change• Gas Produced Gas Produced

(not from boiling!)(not from boiling!)• Precipitate – a Precipitate – a

solid formed by solid formed by mixing two liquids mixing two liquids togethertogether

• Color changeColor change

http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA0/MOVIES/S1047.MOV

Physical vs. Chemical Properties

• Examples:

– melting point

– flammable

– density

– magnetic

– tarnishes in air

physical

chemical

physical

physical

chemical

Physical vs. Chemical Changes

• Examples:

– rusting iron

– dissolving in water

– burning a log

– melting ice

– grinding spices

Chemical

Physical

Chemical

Physical

Physical

Most of Chemistry Concerns Chemical Properties &Changes

• BUT, physical properties & changes are important, too!

• ALL mixtures can be separated physically.

• They can be separated based on their PHYSICAL properties.

B.2 DENSITYB.2 DENSITY - an important and - an important and useful physical propertyuseful physical property

Density mass (g)volume (cm3)

Density mass (g)volume (cm3)

Mercury

13.6 g/cm13.6 g/cm33 21.5 g/cm21.5 g/cm33

Aluminum

2.7 g/cm2.7 g/cm33

Platinum

ProblemProblem A piece of copper has a mass of 57.54 g. It is 9.36 cm long, 7.23 cm wide, and 0.95 mm thick. Calculate density (g/cm3).

Density mass (g)volume (cm3)

Density mass (g)volume (cm3)

StrategyStrategy

1. Get dimensions in common units.1. Get dimensions in common units.

2.2. Calculate volume in cubic centimeters. Calculate volume in cubic centimeters.

3. Calculate the density.3. Calculate the density.

SOLUTIONSOLUTION

1. Get ALL dimensions in common units.1. Get ALL dimensions in common units.

2.2. Calculate volume in cubic centimeters. Calculate volume in cubic centimeters.

3. Calculate the density.3. Calculate the density.

0.95 mm • 1cm

10 mm = 0.095 cm

57.54 g

6.4 cm3 = 9.0 g / cm3

(9.36 cm)(7.23 cm)(0.095 cm) = 6.4 cm(9.36 cm)(7.23 cm)(0.095 cm) = 6.4 cm33

Note only 2 significant figures in the answer!Note only 2 significant figures in the answer!

PROBLEM: Mercury (Hg) has a density of 13.6 g/cmPROBLEM: Mercury (Hg) has a density of 13.6 g/cm33. . What is the mass of 95 mL of Hg in grams? In pounds?What is the mass of 95 mL of Hg in grams? In pounds?

PROBLEM: Mercury (Hg) has a density of 13.6 g/cmPROBLEM: Mercury (Hg) has a density of 13.6 g/cm33. . What is the mass of 95 mL of Hg in grams? In pounds?What is the mass of 95 mL of Hg in grams? In pounds?

Strategy1. Use density to calc. mass (g) from volume.2. Convert mass (g) to mass (lb)

Need to know conversion factor= 454 g / 1 lb= 454 g / 1 lb

PROBLEM: Mercury (Hg) has a density of 13.6 g/cmPROBLEM: Mercury (Hg) has a density of 13.6 g/cm33. . What is the mass of 95 mL of Hg?What is the mass of 95 mL of Hg?

PROBLEM: Mercury (Hg) has a density of 13.6 g/cmPROBLEM: Mercury (Hg) has a density of 13.6 g/cm33. . What is the mass of 95 mL of Hg?What is the mass of 95 mL of Hg?

First, note that First, note that 1 cm1 cm33 = 1 mL = 1 mL

1.1. Convert volume to massConvert volume to mass

PROBLEM: Mercury (Hg) has a density of 13.6 g/cmPROBLEM: Mercury (Hg) has a density of 13.6 g/cm33. . What is the mass of 95 mL of Hg?What is the mass of 95 mL of Hg?

PROBLEM: Mercury (Hg) has a density of 13.6 g/cmPROBLEM: Mercury (Hg) has a density of 13.6 g/cm33. . What is the mass of 95 mL of Hg?What is the mass of 95 mL of Hg?

95 cm3 • 13.6 g

cm3 = 1.3 x 103 g

1.3 x 103 g • 1 lb

454 g = 2.8 lb

2.2. Convert mass (g) to mass (lb)Convert mass (g) to mass (lb)

Learning CheckLearning Check

Osmium is a very dense metal. What is its Osmium is a very dense metal. What is its density in g/cmdensity in g/cm3 3 if 50.00 g of the metal occupiesif 50.00 g of the metal occupiesa volume of 2.22cma volume of 2.22cm33??

1) 2.25 g/cm1) 2.25 g/cm33

2)2) 22.5 g/cm22.5 g/cm33

3)3) 111 g/cm111 g/cm33

Solution

2) Placing the mass and volume of the osmium metal 2) Placing the mass and volume of the osmium metal into the density setup, we obtaininto the density setup, we obtain

D = D = massmass = = 50.00 g 50.00 g = = volumevolume 2.22 cm2.22 cm33

= 22.522522 g/cm= 22.522522 g/cm3 3 = = 22.5 g/cm22.5 g/cm33

Volume DisplacementVolume Displacement

A solid displaces a matching volume of water when A solid displaces a matching volume of water when the solid is placed in water.the solid is placed in water.

33 mL33 mL25 mL 25 mL

Density Depends on Temperature

• Most density tables are given with a specific temperature because substances expand when heated.

Direct vs. Inverse Proportions

• Directly proportional – the relationship between two variables can be expressed as y/x = k where k is a constant.

• Graphs of directly proportional variables are linear.

How do mass and volume relate?

• If mass is your y variable, and volume is your x variable, y/x = k! (m/V = D)

•The graph is linear, showing a directly proportional relationship between mass and volume.• Notice that the slope = density, a CONSTANT!

Mass

Volume

Inverse proportions will come later

• In inversely proportional relationships, yx = k• This type of graph is curved.• We will see this a lot more when we get to the

gas laws later in the year.

HOMEWORK EXERCISES• 1) What is a physical property?• 2) Identify three physical properties of water.• 3) How does the density of solid water compare to the density of

liquid water?• 4) Describe a setting where you might observe water as a solid, a

liquid, and a gas all at the same time.• 5) Distinguish between physical changes and chemical changes.• 6) A star is estimated to have a mass of 2 x 1036 kg. Assuming it to

be a sphere of average radius 7.0 x 105 km, calculate the average density of the star in units of grams per cubic centimeter.

• CONTINUED…

HOMEWORK END

• 7) Classify the following as physical or chemical changes.– a) Moth balls gradually vaporize in a closet.– b) Hydrofluoric acid attacks glass, and is used to etch

calibration marks on glass laboratory utensils.– c) A French chef making a sauce with brandy is able to

burn off the alcohol from the brandy, leaving just the brandy flavoring.

– d) Chemistry majors sometimes get holes in the cotton jeans they wear to lab because of acid spills.