Chapter 2
Scientific Method
The scientific method is a logical approach to solving problems:
Stating the problem
Formulating hypotheses
Formulating Hypotheses
Scientists use
generalizations about the
data to formulate a
Testing Hypotheses
Testing a hypothesis requiresexperimentation that provides data to support or refute the hypothesis
If the tests do not support the
Theorizing
A theory is a broad generalization that explains a body of facts or phenomena.
Theories are successful if they can predict the results of many new experiments.
When data from experiments show that the predictions of the hypothesis are successful, scientists try to explain the phenomena by constructing a model.
MEASUREMENTS
Measurements in science are very important. Measurements can fall into 2 different catagories:
1)Qualitative measurements: These are measurements that are descriptive. Examples: the sky is blue, the ground is hard, the penny is shiny
Section 2.2 Units of Measure
SI Units – Le Systeme International d’Units The seven base SI units:
Mass – kilogram (kg) Length – meter (m)
Temperature – Kelvin (K)
Amount of substance – mole (mol) Time- seconds (s)
Electric current – ampere (A)
Derived Units
Derived SI Units: Many SI units are combinations of the quantities
shown in table 2-1 of your book pg 36.
Derived units are produced by
multiplying or dividing standard units.
MASS AND WEIGHT ARE NOT THE SAME!!
Weight is the measure of the gravitational pull on an object.
Mass measures the amount of matter in an object
Volume
Volume is the amount of space occupied by an object.
The derived SI unit of volume is cubic meters or m3
Chemists often use cm3 or mL interchangeable since:
IN SCIENCE WE USUALLY USE
THE METRIC SYSTEM
The base units in the metric system are: For mass the gram (g)
PREFIXES USED FOR METRIC
UNITS
Kilo hecta deca Base unit deci centi milli
WHAT IS DENSITY?
Density is defined as the mass per unit volume – what this means is how much matter is in a
particular amount of space. If a substance is dense like lead, it has atoms packed close together, while a
Density
Density is the ratio of mass to volume, or mass divided by volume
D= m/V
Where m is mass, V is volume, and D is density Units can include kg/m3, g/cm3, g/mL or g/L
Density is a physical property
Practice Density Problems
1. What is the density of a block of marble that occupies 310 cm3 and has a mass of
853 g?
Answer = 2.8 g/cm3
2. What is the density of a diamond that has a mass of 1.14 grams and volume of 0.350 mL?
•
Scientific Notation
Scientific notation is used to express very large or very small numbers in an easier format. For example:
250000000 g = 2.5 x 108 g
or
When writing numbers in
scientific notation the number
that comes before the power of
10 must be between 1 and 10.
Example:
2500 kg = 2.500 x 10
3not 25.00 x 10
2or 0.2500 x 10
4Section 2.3 Using Scientific
Measurements
Scientists distinguish between
accuracy and precision
Accuracy is how close a measurement is
to the correct or accepted value of the quantity measured.
Precision is how close a number of
Precision vs. Accuracy
Percent Error
The accuracy can be compared to the correct value by calculating the percent error. Percentage error is and
Example of Percent Error
A student measures the mass and volume of a
substance and calculates its density as 1.40 g/mL. The correct, or accepted value of the density is 1.30 g/mL. What is the percent error of the student’s measurement? Percent error = IA – EI X 100
A
Percent error = IA – E I X 100 A
Practice % Error Problems
1. What is the percent error for a mass measurement of 17.7 g, given that the correct value is 21.2 g?
Significant Figures
Significant Figures in a measurement consist of all the digits known with
Rules for Sig. Figs.
1. Non-zero numbers are always significant!
Example: 62.1 g has 3 sig. figs
2. Zeros appearing between nonzero digits are significant
Example 40.7 has 3 sig. figs
Rules for Sig. Figs
3. Leading zeros do not count as significant digits.
4. Zeros at the end of a number and to the right of a decimal point are
significant.
5. Trailing zeros are significant only if the number contains a decimal point
How many sig. figs?
1. 28.6 g
2. 3440. cm
3. 910 m
4. 0.04604 L
Rules for Sig. Figs
Mathematical Operations
Mathematical Operations Addition Addition and Subtraction
and Subtraction: The number of : The number of
decimal places in the result equals
decimal places in the result equals
the number of decimal places in the
the number of decimal places in the
least precise measurement.
least precise measurement.
6.
6.88 + 11.934 = 18.734 + 11.934 = 18.734 18. 18.77 ( (3 3
Rules for Sig. Figs.
Multiplication and Division
Multiplication and Division:: # sig # sig
figs in the result equals the number
figs in the result equals the number
in the least precise measurement
in the least precise measurement
used in the calculation.
used in the calculation.
6.38 x 2.0 = 12.76
6.38 x 2.0 = 12.76 13 (2 sig 13 (2 sig
Sig Fig Practice
Sig Fig Practice
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
Sig Fig Practice
Sig Fig Practice
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
Direct Proportions
The quotient (division) of two variables is a constant.
As the value of one variable increases, the other must also increase
As the value of one variable decreases, the other must also decrease
Direct Proportion
Inverse Proportions
The product (multiply) of two variables is a constant
As the value of one variable
increases, the other must decrease
As the value of one variable
decreases, the other must increase
