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Introduction: Matter & Measurement

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Introduction: Matter & Measurement

AP Chemistry

Chapter 1

(Day 2)

What is chemistry?

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

What is matter?

It is anything that takes up spaceand has mass.

A substance is matter that has a definite composition and constant properties

It can be an element or a compound

An element is the simplest form of matter.

It cannot be broken down further by chemical reactions

A compound can be separated into simpler forms

It is a combination of two or more elements

A mixture is a physical blend of two or more substances

1. Heterogeneous Mixtures

Not uniform in composition

Properties indefinite & vary

Can be separated by physical methods

2. 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 property – characteristics of a pure substance that we can observe without changing the substance; the chemical composition of the substance does not change

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

Physical 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 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 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

permanent color change

gas produced (odor or bubbles)

precipitate (solid) produced

light given off

heat released (exothermic) or absorbed (endothermic)

A measurement is a number with a unit

All measurements, MUST have units

There are two types of measurements:

Qualitative measurements are words, such as heavy or hot

Quantitative measurements involve numbers (quantities), and depend on:

The reliability of the measuring instrument

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

Accuracy – how close a measurement is to the true value

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

Precise, but not accurate

Neither accurate nor precise

Precise AND accurate

Our goal!

Measurements 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 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

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

Non-zerosalways count as significant figures:

3456has

4significant figures

Zeros

Leading zeroes do not count as significant figures:

0.0486 has

3 significant figures

Zeros

Captive zeroes always count as significant figures:

16.07has

4 significant figures

Zeros

Trailing zerosare significant only if the number contains a written decimal point:

9.300 has

4 significant figures

Two special situationshave an unlimited (infinite) number of significant figures:

Counted items

23 people, or 36 desks

Exactly defined quantities

60 minutes = 1 hour

How many significant figures in the following?

1.0070 m

5 sig figs

17.10 kg

4 sig figs

These all come from some measurements

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 figs

This is a counted value

3 cats

infinite

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

Decide how many significant figures are needed

Round to that many digits, counting from the left

Is the next digit less than 5? Drop it.

Next digit 5 or greater? Increase by 1

Addition and Subtraction

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

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

6.8 + 11.934 =

18.734 18.7 (3 sig figs)

Calculation

Calculator says:

Answer

10.24 m

3.24 m + 7.0 m

10.2 m

100.0 g - 23.73 g

76.3 g

76.27 g

0.02 cm + 2.371 cm

2.39 cm

2.391 cm

713.1 L - 3.872 L

709.228 L

709.2 L

1821 lb

1818 lb + 3.37 lb

1821.37 lb

0.160 mL

0.16 mL

2.030 mL - 1.870 mL

*Note the zero that has been added.

Multiplication and Division

Round the answer to the same number of significant figures as the least number of significant figures in the problem.

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)

What if your answer has less significant figures than you are supposed to have?

Calculator Example: 100.00 / 5.00 = 20

Add zeros!

20 is 1 sf

20. is 2 sf

20.0 is 3 sf

Calculation

Calculator says:

Answer

22.68 m2

3.24 m x 7.0 m

23 m2

100.0 g ÷ 23.7 cm3

4.22 g/cm3

4.219409283 g/cm3

0.02 cm x 2.371 cm

0.05 cm2

0.04742 cm2

710 m ÷ 3.0 s

236.6666667 m/s

240 m/s

5870 lb·ft

1818.2 lb x 3.23 ft

5872.786 lb·ft

2.9561 g/mL

2.96 g/mL

1.030 g x 2.87 mL