,
Recall that density was defined as (we will use the symbol, d, for density):
where m is the mass of a sample of material and V is the volume of that same sample.
Sometimes we see a quantity called "specific gravity." The specific gravity is usually used to describe the density of liquids. It is defined as the density of the sample divided by the density of water at the same temperature.
That is,
Notice that specific gravity has no units..
Example problem:
A medical lab technician has a liquid sample with a specific
gravity of 1.0443 at 25oC. What is the density of the sample?
(The density of water at 25oC is 0.997 g/cm3.)
An important feature of density is that it does not depend on the amount of material that you have. The density of a teaspoon full of sugar is the same as the density of a freight car full of sugar.
That is because the mass of a sample is proportional to the size of the sample and the volume of the sample is also proportional to its size. Thus when you divide the mass by the volume the proportionality constant cancels out.
Density is an example of what we call an intensive property. We have seen some other intensive properties. Temperature,T, and pressure, p, are intensive properties.
The opposite of an intensive property is an extensive property. An extensive property IS proportional to the amount of material that you have. Some of the properties we have been talking about are extensive properties.
Consider the properties:
TIf you were to double the size of your sample the temperature and pressure and density would not change, but the volume and mass would double.
p
m
V
color
hardness
specific gravity
There is an easy way to remember the difference between extensive and intensive properties.
Extensive properties depend on the size, or extent of the sample.
Intensive properties are like an intensity (think of temperature or pressure).
There are lots more intensive and extensive than the ones
we have talked about here. We will see some more later.
Physical and Chemical Changes
A physical change does not change the nature of the material.
A chemical change produces new materials from old materials.
Elements can be combined with each other to give new substances called compounds.
Compounds can be reacted with each other (or with elements) to give new compounds.
These are called chemical changes.Examples:Chemical changes make new substances.
Physical changes do not make new substances.
melt ice
gasoline evaporates
burn cotton cloth
cut paper up
grind salt
strike match
digest food
dissolve penny in acid
corrosion of iron
etc.
Atomic Theory
We have learned so far about the three classes of substances:
elementsWe have learned about some properties of substances.
compounds
mixtures.
Extensive propertiesAnd we have learned about physical and chemical changes.
Intensive properties
All of the facts about elements, compounds, and mixtures, and all of the facts about chemical changes can be explained (or understood) by the
Atomic Theory.
Atomic theory begins with the following set of postulates. (The atomic theory "grew up" slowly over a long period of time.)
1. Matter is made up of atoms.For example, it has been known for many years that the pure substance carbon (an element) will react with the pure substance oxygen (an element) to give a new pure substance (a compound) which we call carbon dioxide.2. Atoms of the same element are the same.
3. Atoms of different elements are different.
4. Atoms can combine to form molecules(tightly bound groups of atoms).
In the language of atomic theory we would say that one atom of carbon joined with two atoms of oxygen to form one molecule of carbon dioxide.
Pretty cumbersome in words, use symbols.
We can say,
1 C atom reacts with 2 O atoms to give one molecule of CO2.Or, better yet,
C + 2 O ® CO2(We will find a more accurate way to write this later.)
The use of these symbols, which we call chemical symbols makes statements regarding chemical changes much easier.
Each element has its own unique symbol.
We can't think about chemistry or work most chemical problems
without knowing the symbols.
Symbols
We can represent an element or the atom of an element by its symbol.
For example:
C = carbon
O = oxygen
B = boron
Pb = lead, etc
C CarbonThe following elements are essential in the human diet at more than 100 mg per day:
H Hydrogen
O Oxygen
N Nitrogen
Ca CalciumThe following elements are essential to health, but in small quantities:
Cl Chlorine
Mg Magnesium
P Phosphorus
K Potassium (Kalium)
Na Sodium (Natrium)
S Sulfur
B BoronThe following elements, although not known to be essential for life, are common enough that we should know them:
Cr Chromium
Co Cobalt
Cu Copper (Cuprum)
F Fluorine
I Iodine
Fe Iron (Ferrum)
Mn Manganese
Mo Molybdenum
Ni Nickel
Se Selinum
Si Silicon
Zn Zinc
Al Aluminum (known as Aluminium in Great Britain)(Some of the elements were known in ancient times and their symbol is derived from their Latin name, as shown in parentheses.)
Ar Argon
As Arsenic
Ba Barium
Bi Bismuth
Br Bromine
Au Gold (Aurum)
He Helium
Pb Lead (Plumbum)
Li Lithium
Hg Mercury (Hydrargyrum)
Ne Neon
Pt Platinum
Pu Plutonium
Rn Radon
Ag Silver (Argentum)
Sr Strontium
Sn Tin (Stannum)
U Uranium
V Vanadium
We also use these symbols to show how atoms (in molecules) change partners in a chemical reaction.
Examples:
CO + PbO ® CO2 + PbHow do the elements themselves exist?
NH3 + HCl ® NH4Cl
Monatomic elements
He, Ne, Ar, Kr, Xe, RnDiatomic elements
(called Noble Gases, they used to be called the inert gases)
H2, O2, N2, F2, Cl2, Br2, I2(Oxygen also exists as O3 = ozone. This is called an allotropic form, many elements exist in several allotropic forms.)
Other forms of elements:
P4, S8, C60, etcDiamond, graphite, and buckminsterfullerene ("buckyball," C60) are all forms of the same element, carbon.
We always write reactions involving elements using their actual formulas.
Recall that a few minutes ago we wrote
C + 2 O ® CO2.This is not a completely correct chemical equation because oxygen does not exist in nature as O atoms. We should write,
2 C + 2 O2 ® 2 CO2.Likewise, we wouldn't write
O + 2 H ® H2Oinstead, write
O2 + 2 H2 ® 2 H2O
The structure of atoms
What's inside an atom? Or what are the components of an atom?
Atoms are composed of elementary particles:
protons - "heavy," positive electric charge (mass = 1.6726 ´ 0-24g)(Electron and proton charges are equal and opposite - they "balance" each other.)neutrons - "heavy," electrically neutral (mass = 1.6749 ´ 0-24g)
electrons - "light," negative electrical charge (mass = 9.1094 ´ 10-28g)
In a neutral atom:
# electrons = # protons = atomic number(Most of the mass of an atom is in the protons and neutrons.)# protons + # neutrons = mass number (or atomic mass number)
mass of proton » mass of neutron » 1836 ´ mass of electron
The protons and neutrons are bound tightly together to form the nucleus. Most of the mass of the atom is concentrated in the nucleus.
The electrons surround the nucleus in a "charge cloud." Since the electrons are 1836 times lighter than protons and neutrons, only about 0.03% of the mass is in the electrons.
On the other hand, the nucleus is very small, most of the size (volume) of the atom is provided by the electrons.
The atomic number
= # protonsIons= # electrons (in a neutral atom)
determines what an element is.
O is atomic number 8H is atomic number 1
He is atomic number 2
C is atomic number 6
Cl is atomic number 17
Ar is atomic number 18
etc.
Sometimes the number of electrons does not equal the number of protons. In this case the atom is no longer electrically neutral. The atom has a charge, it is called an ion.
If it has an excess of electrons it will be a negatively charged ion (an anion). If has a deficiency of electrons it will be a positively charged ion (a cation).
Examples
H+We will find out how and why atoms can form stable ions in Chapter 8.H-
O2-
Na+
Ca2+