The Basics: Chemistry and the Scientific Method

Chemistry : the study of matter and the changes it undergoes
Scientific Method : a systematic approach to research
1. Define problem
2. Experiment, observe, record

• Information is either qualitative or quantitative.
  • qualitative : consisting of general observations about the system
  • quantitative : comprising numbers obtained by various measurements and observations

3. Hypothesize

• hypothesis : a tentative explanation for a set of observations
• law : concise verbal or mathematical statement of a relationship between phenomena that is always the same
  under the same conditions
• theory : a unifying principle that explains a body of facts and/ or those laws that are based on them

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Classifications of Matter

matter : anything that has mass and occupies space
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-Substances and Mixtures

• substance : form of matter that has a definite composition and distinct properties
• mixture : a combination of two or more substances in which the substances retain their distinct identities
  • homogenous mixture : composition of the mixture is the same throughout
  • heterogeneous mixture : composition of the mixture is not uniform

-Elements and Compounds

• element : substance that can not be separated into simpler substances by chemical means
• compound : a substance composed of atoms of two or more elements chemically united in fixed proportions

-The Three States of Matter

solidliquidgas

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• All substances can exist in three states: solid, liquid, and gas. The three states differ in distance between the molecules. Solids have molecules that are held close together, and as it melts to a liquid the molecules spread out more, and then when it is a gas the molecules are spread very wide.
• The composition of the substance doesn't change as the state of matter changes. For example, with heating, a certain substance will melt to form a liquid. This temperature at which it melts is called the melting point. Further heating will change the liquid into a gas. This temperature at which this takes place is called the boiling point.

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-Physical and Chemical Properties

All substances have properties that we can use to identify them. The two basic types of properties associated with matter are called Physical and Chemical properties.

• Physical properties : properties that do not change the chemical nature of matter
  • Some examples are....color, smell, freezing point, melting point, attraction or repulsion to magnets, density and viscosity.
• Chemical properties : properties that do change the chemical nature of matter.
  • Some examples are...heat of combustion, reactivity with water, PH, and electromotive force.

The more properties identified for a substance, the better we know the nature of that substance. These properties help model the substance and understand how the substance will behave under various conditions.

• Extensive property : depends on how much matter is being considered
  • mass : quantity of matter in a given sample of a substance
  • volume : length cubed
• Intensive property : does not depend on quantity of matter being considered
  • density : mass divided by volume
  • temperature

Measurements

-SI Units

International System of Units ( abbreviated SI for the French Systeme Internationale d'Unites)
SI BASE UNITS
BASE QUANTITY ---> UNIT
Length --> meter
Mass --> kilogram
Time --> second
Electrical current --> ampere
Temperature --> kelvin
Amount of substance --> mole
Luminous Intensity --> candela

Example 1: 10-6 kg = 1mg (one milligram), but not 10-6 kg = 1 µkg (one microkilogram)

Example 2: Consider an example of the height of the Washington Monument. You can write it in more than one form. hW = 169 000 mm = 16 900 cm = 169 m = 0.169 km 9 (using the millimeter, centimenter, or kilometer)

-Mass and Weight

• The terms "mass" and "weight" are often used interchangeably even though they are totally different quantities, when strictly speaking.
  • mass : measure of the amount of matter in an object
  • weight : the force that gravity exerts on an object
  • weighing : the process of measuring mass

***all this information was extracted from the Chang Chemistry textbook

Chapter 1 : continued