Chemistry in Our Lives

Chemistry and Chemicals

Chemistry is the study of substances in terms of

Composition What a material it made of

Structure How the elementary particles are put together; atoms and molecules

Properties The characteristics of the material

Reactions How it behave with other substances

What is chemistry?

Chemical reactions happen when

• a car is started• fertilizer is added to help

plants grow• food is digested• electricity is produced from

burning natural gas• rust is formed on iron nails

Everything in our lives from materials to life involve chemistry

• metal alloys• chemically treated water• plastics and polymers• drugs• foods • fertilizers and pesticides• living beings

Chemicals in Toothpaste

The Scientific Method

The scientific method is the process used to explain observations in nature.

The method involves:• making observations• forming a hypothesis• doing experiments to test

the hypothesis

Everyday Scientific Thinking• Observation: The sound from a CD in a CD

player skips.• Hypothesis 1: The CD or player is faulty.• Experiment 1: When the CD is replaced with another

one, the sound from the second CD is OK.

• Hypothesis 2: The original CD has a defect.• Experiment 2: When the original CD is played in another

player, the sound still skips. • Theory: The experimental results suggest that

the original CD has a defect.

Units of Measurement

In chemistry or in medicine: quantities are measuredexperiments are performedresults are often calculatedWe often use numbers to report measurements, and ourresults are usually compared to standards.

In a measurement of the thickness of the skin fold at the waist, calipers are used.A measuring tool is used to compare some dimension of an object to a standard.

In every measurement, a number must be followed by a unit to have any meaning.

Observe the following examples of measurements:Number and Unit

35 m (meter) measure of length0.25 L (liter) measure of volume225 lb (pound) measure of ?3.4 h (hour) measure of time

Does anyone know what the units associated with blood pressure are?

The Metric System

The metric system is the standard system of measurement

units are based on the number 10used in most of the worldused everywhere by scientists

Length is measured using a meter stickuses the unit meter

The unit of an inch is equal to exactly 2.54 centimeters in the metric system

1 in. = 2.54 cm

Volume is the space occupied by a substancethe unit of volume is the liter (L) in the metric system

1 L = 1.06 qt

The mass of an object is a measure of the quantity of material it containsthe unit gram (g) or kilogram (1000 g) is used

What is the difference between mass and weight?

Weight is the result of the action of gravity on mass. Your weight on the moon would be a lot less even though your mass would remain the same

Despite this important difference, we will use these two terms interchangeably

The temperatureindicates how hot or cold a substance isthe Celsius (°C) scale is used in the metric system the Kelvin (K) scale is also used18 °C is 64 °F on this thermometer

On the °C scale, the melting point of ice is 0 °C and boiling point of water is 100 °C

What is heat or cold? What does temperature really measure?

Time measurementthe unit second (s) is used in the metric system.Time is based on an atomic clock that uses a frequency emitted by cesium atoms

Scientific notation is used to write very large or very small numbersthe width of a human hair (0.000 008 m) is written

8 x 10-6 ma large number such as 4 500 000 s is written

4.5 x 106 s

Scientific NotationA number in scientific notation contains a coefficient and a power of 10.

coefficient power of ten coefficient power of ten1.5 x 102 7.35 x 10-4

To write a number in scientific notation, the decimal point is usually but not necessarily placed after the first digit. The spaces moved are shown as a power of ten.52 000. = 5.2 x 104 0.00378 = 3.78 x 10-3

4 spaces left 3 spaces right

10-3/105 =

10-3*105 =

10-3 +105 =

10-8

102

105 100000 + 0.001 = 100000.001

Measurements

What is the length of this piece of wood?

What is the first digit? Any uncertainty in the digit? 4

What is the second digit? Any uncertainty in this digit? 4.5

What is the third digit? Any uncertainty in this digit? 4.56

Definition of a significant figure:

Significant digits include all digits with no uncertainty plus one estimation

• . l8. . . . l . . . . l9. . . . l . . . . l10. . cm

• What is the length of the red line?• 1) 9.38 cm • 2) 9.39 cm • 3) 9.40 cm

9.38, or 9.39, 9.40 is less likely

Measurement

38.15 cm

5.6 ft

120.55 m

0.0055 in

1200 m

Number of Significant Figures

4

2

5

2

2

A. Exact numbers are obtained by1. counting2. definition; they are known exactly

B. Measured numbers are obtained by3. using some measuring tool; they have some associated uncertainty

Classify each of the following as exact (E) or measured (M) numbers. Explain your answer.

A. __ Gold melts at 1064 °C.

B. __ 1 yard = 3 feet

C. __ The diameter of a red blood cell is 6 x 10-4 cm.

D. __ There are 6 hats on the shelf.

E. __ The atom sodium has 11 protons and 12 neutrons.

Significant FiguresIn calculations:

Answers must have the same number of significant figures as the measured numbers. Calculator answers must often be rounded off.Rounding rules are used to obtain the correct number of significant figures.

Rounding Off

When the first digit dropped is 4 or less, the retained numbers remain the same.

To round 45.832 to 3 significant figuresdrop the digits 32 = 45.8

When the first digit dropped is 5 or greater, the last retained digit is increased by 1.

To round 2.4884 to 2 significant figuresdrop the digits 884 = 2.5 (increase by 0.1)

– To round 2.4884 to 3 significant figures 2.49

Multiplication and Division

When multiplying or dividing usethe same number of significant figures (SF) as the measurement with the fewest significant figures

Example:110.5 x 0.048 = 5.304 = 5.3 4SFs 2SFs calculator 2SFs

Addition and Subtraction

• Keep as many significant figure as the number of fewest digits after the decimal

For each calculation, round the answer to give thecorrect number of decimal places if all numbers are measured numbersA. 235.05 + 19.6 + 2 =

1) 2572) 256.73) 256.65

B. 58.925 – 18.2 =1) 40.7252) 40.733) 40.7

1m/100cm = 1; 1m/1000mm = 1

An equalitystates the same measurement in two different unitscan be written using the relationships between two metric units

Example: 1 meter is the same as 100 cm and 1000 mm.1 m = 100 cm1 m = 1000 mm

1 = 100cm/1m; 1 = 1000mm/1m

volume has the dimensions of length cubed

Several equalities can be written for mass

1 kg = 1000 g1 g = 1000 mg1 mg = 0.001 g

Some Common Equalities

• An injured person loses 0.30 pints of blood. How many milliliters of blood would that be?

0.30pt*1qt/2pt = 0.15qt; 0.15qt*946mL/qt = 141.9 mL; 140 mL

0.30pt*2pt/1qt = 0.60pt2/qt

• If a person weighs 200 pounds, how many kiograms does the person weight?

• 200 lb*1 kg/2.2 lb = 90.9 kg• 200 lb*2.2 lb/1 kg = 440 lb2/kg

If the thickness of the skin fold at the waist indicates an 11% body fat, how much fat is in a person with a mass of 86 kg?

11 % fat means 11kg/100kg body weight

86 kg x 11 kg fat = 9.5 kg of fat 100 kg

Density

Densitycompares the mass of an object to its volumeis the mass of a substance divided by its volume

Density expression:D = mass = g or g = g/cm3

volume mL cm3

Osmium is a very dense metal. What is its density in g/cm3 if 50.0 g of osmium has a volume of 2.22 cm3?

1) 2.25 g/cm3

2) 22.5 g/cm3

3) 111 g/cm3

• The density of the zinc object can be calculated from its mass and volume.

d = 68.6g/(45.0-35.5)mL; 68.6g/9.5 mL

d = 7.2 g/mL