Introduction: Matter & Measurement

AP ChemistryChapter 1

(Day 2)

ChemistryWhat is chemistry?What is chemistry?

It is the study of the composition composition of matterof matter and the changes that matter undergoes.

What is matter?What is matter?It is anything that takes up space and has mass.

Elements, Compounds & Mixtures•A substance is matter that

has a definite composition and constant properties

•It can be an element or a compound

Elements, Compounds & Mixtures•An element is the simplest

form of matter.•It cannot be broken down

further by chemical reactions

Elements, Compounds & Mixtures•A compound can be

separated into simpler forms•It is a combination of two or

more elements

Mixtures•A mixture is a physical blend of two

or more substances1. Heterogeneous Mixtures

–Not uniform in composition–Properties indefinite & vary–Can be separated by physical methods

Mixtures2. Homogeneous Mixtures

–Completely uniform in composition

–Properties constant for a given sample

–Cannot be separated by physical methods (need distillation, chromatography, etc)

–Sometimes called solutions

Physical & Chemical Properties•Physical property – characteristics of

a pure substance that we can observe without changing the substance; the chemical composition of the substance does not change

Physical & Chemical Properties

•Chemical property – describes the chemical reaction of a pure substance with another substance; chemical reaction is involved

Physical & Chemical PropertiesPhysical properties• appearance• odor• melting point• boiling point• hardness• density• solubility• conductivity

Chemical properties• reaction with

oxygen (flammability)

• rxn with water• rxn with acid• Etc….

Intensive & Extensive PropertiesIntensive properties• Do not depend on the

amount of sample being examined– temperature– odor– melting point– boiling point– hardness– density

Extensive properties

• Depend on the quantity of the sample– mass– volume

• Etc….

Physical & Chemical ChangesPhysical changes• The composition of the

substance doesn’t change

• Phase changes (like liquid to gas)– Evaporation, freezing,

condensing, subliming, etc.

• Tearing or cutting the substance

Chemical changes• The substance is

transformed into a chemically different substance

• All chemical reactions

Signs of a Chemical Changes

1. permanent color change2. gas produced (odor or

bubbles)3. precipitate (solid) produced 4. light given off 5. heat released (exothermic)

or absorbed (endothermic)

Making MeasurementsMaking Measurements

•A measurement is a A measurement is a number with a unitnumber with a unit

•All measurements, MUST All measurements, MUST have unitshave units

Types of UnitsTypes of Units

MeasurementsMeasurementsThere are two types of measurements: Qualitative measurements are

words, such as heavy or hot Quantitative measurements involve

numbers (quantities), and depend on:

1) The reliability of the measuring instrument

2) The care with which it is read – this is determined by YOU!

Accuracy & PrecisionAccuracy & Precision

AccuracyAccuracy – how close a – how close a measurement is to the true measurement is to the true valuevalue

PrecisionPrecision – how close the – how close the measurements are to each measurements are to each other (reproducibility)other (reproducibility)

Precision and AccuracyPrecision and Accuracy

Neither accurate

nor precise

Precise, but not

accurate

Precise AND

accurate

Our goal!

Which are Precise? Which are Precise? Accurate?Accurate?

Uncertainty in MeasurementsUncertainty in MeasurementsMeasurements are performed with instruments, and no instrument can read to an infinite number of decimal places•Which of the balances below has the greatest uncertainty in measurement?

1 2 3

Significant Figures in Significant Figures in MeasurementsMeasurements

Significant figures in a measurement include all of the digits that are known, plus one more digit that is estimated.

Sig figs help to account for the uncertainty in a measurement

To how many significant figures can you measure this pencil?

What is wrong with this ruler? What is it missing?

Rules for Counting Rules for Counting Significant FiguresSignificant Figures

Non-zerosNon-zeros always count as always count as significant figures:significant figures:

34563456 hashas

44 significant figuressignificant figures

Rules for Counting Rules for Counting Significant FiguresSignificant Figures

ZerosZerosLeading zeroes do not count Leading zeroes do not count as significant figures:as significant figures:

0.0486 0.0486 hashas

3 3 significant figuressignificant figures

Rules for Counting Rules for Counting Significant FiguresSignificant Figures

ZerosZerosCaptive zeroes always count Captive zeroes always count as significant figures:as significant figures:

16.0716.07 hashas

44 significant figures significant figures

Rules for Counting Rules for Counting Significant FiguresSignificant Figures

ZerosZerosTrailing zerosTrailing zeros are significant are significant

only if the number contains only if the number contains a written decimal point:a written decimal point:

9.300 9.300 hashas

4 4 significant figuressignificant figures

Rules for Counting Rules for Counting Significant FiguresSignificant Figures

Two special situationsTwo special situations have an have an unlimitedunlimited (infinite) number (infinite) number of significant figures:of significant figures:

1.1. Counted itemsCounted itemsa)a) 23 people, or 36 desks23 people, or 36 desks

2.2. Exactly defined quantitiesExactly defined quantitiesb)b) 60 minutes = 1 hour60 minutes = 1 hour

Sig Fig Practice #1Sig Fig Practice #1How many significant figures in the following?

1.0070 m 5 sig figs

17.10 kg 4 sig figs

100,890 L 5 sig figs

3.29 x 103 s 3 sig figs

0.0054 cm 2 sig figs

3,200,000 mL 2 sig figs3 cats infinite

These all come from some measurements

This is a counted value

Significant Figures in Significant Figures in CalculationsCalculations

In general a calculated answer cannot be more accurate than the least accurate measurement from which it was calculated.

Sometimes, calculated values need to be rounded off.

Rounding Calculated Rounding Calculated AnswersAnswers

RoundingRounding Decide Decide how manyhow many significant significant

figures are neededfigures are needed Round to that many digits, Round to that many digits,

counting from the counting from the leftleft Is the next digit less than 5? Is the next digit less than 5?

Drop it.Drop it. Next digit 5 or greater? Increase Next digit 5 or greater? Increase

by 1by 1

Rules for Significant Figures in Rules for Significant Figures in Mathematical OperationsMathematical Operations

Addition and SubtractionThe answer should be rounded to the same number of decimal places as the least number of decimal places in the problem.

Rules for Significant Figures in Rules for Significant Figures in Mathematical OperationsMathematical Operations

• Addition and SubtractionAddition and Subtraction: The : The number of decimal places in the number of decimal places in the result equals the number of result equals the number of decimal places in the decimal places in the least least accurate accurate measurement.measurement.

•6.8 + 11.934 =6.8 + 11.934 =•18.734 18.734 18.7 18.7 ((3 sig figs3 sig figs))

Sig Fig Practice #2Sig Fig Practice #2

3.24 m + 7.0 m

Calculation Calculator says: Answer

10.24 m 10.2 m

100.0 g - 23.73 g 76.27 g 76.3 g

0.02 cm + 2.371 cm 2.391 cm 2.39 cm

713.1 L - 3.872 L 709.228 L 709.2 L

1818 lb + 3.37 lb 1821.37 lb 1821 lb

2.030 mL - 1.870 mL 0.16 mL 0.160 mL

*Note the zero that has been added.

Rounding Calculated Rounding Calculated AnswersAnswers

Multiplication and DivisionRound the answer to the same number of significant figures as the least number of significant figures in the problem.

Rules for Significant Figures in Rules for Significant Figures in Mathematical OperationsMathematical Operations

• Multiplication and Division: # sig figs in the result equals the number in the least accurate measurement used in the calculation.

•6.38 x 2.0 =•12.76 13 (2 sig figs)

Other Special CasesOther Special Cases

• What if your answer has less What if your answer has less significant figures than you are significant figures than you are supposed to have?supposed to have?– Calculator Example: 100.00 / 5.00 Calculator Example: 100.00 / 5.00

= 20= 20

• Add zeros!Add zeros!– 20 is 1 sf20 is 1 sf– 20. is 2 sf20. is 2 sf– 20.0 is 3 sf20.0 is 3 sf

Sig Fig Practice #3Sig Fig Practice #3

3.24 m x 7.0 m

Calculation Calculator says: Answer

22.68 m2 23 m2

100.0 g ÷ 23.7 cm3 4.219409283 g/cm3 4.22 g/cm3

0.02 cm x 2.371 cm 0.04742 cm2 0.05 cm2

710 m ÷ 3.0 s 236.6666667 m/s 240 m/s

1818.2 lb x 3.23 ft 5872.786 lb·ft 5870 lb·ft

1.030 g x 2.87 mL 2.9561 g/mL 2.96 g/mL