Elements are made up of only ONE type of atom.
Elements are pure substances.
Atoms can be BONDED together.
- Ex. O2
Compounds are made up of TWO or more types of atoms that
are BONDED together.
- Atoms bound together are called molecules.
- The properties of compounds are very different than the
properties of the elements that make them up.
Compounds (like elements) are considered pure substances.
Mixtures are made up of two or more types of atoms that are
combined together.
Mixtures have the properties of its components.
Mixtures can be…
- Heterogenous - not uniform (like trail mix)
- Homogenous - Evenly distributed (like salt water or milk)
Physical and
Chemical Changes
What is a physical
change?
What is a chemical
change?
Physical Change
- Is a change in physical properties like size, shape, or state
- The original substance can be returned to their original
state.
Chemical Changes
- alters both the physical and chemical properties of a
substance.
- Something new is formed and the original substance
could not easily be returned.
Some indicators that a chemical change has occurred:
-
A temperature change without adding or taking away heat
A color change (be careful here)
New odor
Gas forming
Precipitate forming (solid chunks)
History of the Atom
Democritus (ancient Greece) was the first to hypothesize that
all matter is made of tiny particles.
He named the particles “atomos” meaning indivisible and
indestructible.
His ideas were not tested, they were just based on thought.
John Dalton (1766-1844) did experiments to explore
Democritus’s idea of the atom.
He studied chemical reactions over many years and from that
data proposed Dalton’s atomic theory.
1. All elements are made of tiny, indestructible particles
called atoms that are different from all other element’s
atoms.
2. Atoms of elements can physically mix OR chemically
combine in chemical reactions to create compounds.
3. In chemical reactions, atoms are rearranged, not created or
destroyed.
In present day these are still ALL true!
Conservation of
Mass
The total mass of the products must equal the total mass of
the reactants.
Products -----> Reactants
This is easy to observe in a closed system, where the
environment is controlled.
In an open system, the reaction can interact with the
environment.
It may appear as though the Law is being violated, but if you
think about it, you can always justify the change in mass!