Chemistry

The Scientific Method

An organized approach to solve problems.

Steps in the Scientific Method

Observe

Use your five senses

Define the problem

Hypotheses

"Educated guess"

Experiments

Tests each hypothesis to prove/disprove them

Analysis

Compares the results to the original hypothesis

Conclusion (Theory)

A hypothesis supported by experimental evidence

Law

Used to describe a natural phenomenon which has been tested over a long period of time under different conditions.

UNIT 2

Chemistry

The study of matter and the changes it undergoes.

Matter

Anything that has mass and volume (EVERYTHING!)

Mass

The amount of matter in an object (How much “stuff”)

Energy

Anything that can do work or produce heat.

Weight

The force of gravity acting on an object’s mass.

Review: Solids

Liquids

SOLID

Definite shape

Definite volume

Not compressible

High density

LIQUID

Indefinite shape

Definite volume

Not compressible

Less dense than solids

GAS

Indefinite shape

Indefinite volume

Are compressible

Very low density

There are two major areas that matter are classified into

pure substances and mixtures.

Pure Substances

Have uniform and definite composition

Elements

Found on the Periodic Table (approx. 118)

Described by symbols

H

Compounds

Formed when elements chemically combine: H2O

Mixtures

Two or more pure substances physically mixed together.

In compounds

the elements are bonded to each other. In mixtures

No definite composition – cannot assign a fixed ratio (ex: H2O)

Two Types of Mixtures

Heterogeneous mixtures

Does not have a uniform composition. Parts of the mixture can be physically seen and “picked out” of the mixture. Examples: Cereal

Homogeneous mixtures

Has a uniform composition. Parts of the mixture cannot be “picked out”. Examples: sugar water

Pure Substances

Pure substances have a unique set of chemical and physical properties.

Physical Properties

Properties that can be measured or observed without changing the identity (or composition) of a substance.

The chemical make-up is not changing when these properties are observed!

Examples: color

odor

Chemical Properties

Properties that indicate how a substance reacts with other substances.

These properties are only observed when the substance undergoes a chemical change.

Examples: flammable

combustible

Physical Changes

Changes in appearance without changing composition.

Examples: cutting

breaking

Changes in state such as: melting

freezing

Chemical Changes (Reactions)

One or more substances react to form new substances with different chemical and physical properties.

The beginning substance is different than the ending substance.

Examples: Rusting

burning

Chemical Changes (cont.)

All chemical reactions can be described by a chemical equation.

Reactants 🡪 Products

Iron + Oxygen 🡪 Iron (III) Oxide

Fe + O2 🡪 Fe2O3

(Formula Equation)

(Word Equation)

“yields”

Law of Conservation of Mass

In any chemical or physical change

matter cannot be created or destroyed.

Mass Reactants = Mass Products

Hydrogen + Oxygen 🡪 Water

10 grams + 5 grams

Accuracy

How close a measurement is to the true or accepted value

Precision

How close multiple measurements are to each other

Two new conversion factors you must know

1 inch = 2.54cm and 1 mile = 5

280ft

UNIT

DEMOCRITUS (400 B.C)

A Greek philosopher

Was the first person to think about an atom’s existence.

Believed that matter was composed of tiny indivisible particles called atoms.

He had no experimental evidence to support his thoughts.

John Dalton (1766-1844)

A meteorologist

Unlike Democritus

he had experimental evidence to support his theory.

Dalton had four major points (postulates) to his theory.

DALTON’S THEORY

All elements are composed of indivisible particles called atoms.

Atoms of the same element are identical. The atoms of any one element are different from those of another.

Atoms of different elements mix or combine in whole number ratios.

Chemical reactions occur when atoms separate

join

In a chemical reaction

atoms of one element NEVER change into another.