General Science Review: Physical Sciences

Chemistry is the study of elements and the compounds  they form. Matter can take the form of an element, a  compound, or a mixture. 

An element is the basic form of matter, incapable  of being decomposed by chemical means into simpler  substances. Each element has distinct chemical and  physical characteristics. Hydrogen (H), oxygen (O),  and carbon (C) are elements. 

A compound is a combination of two or more  elements chemically combined in a specific proportion. 

Compounds can be separated by chemical means,  and are represented by chemical formulas that include  the symbols of all the elements present. Examples of  familiar compounds are water (H2O) and table salt  (NaCl). In order to be considered organic, a compound  must contain carbon. 

A mixture is a combination of two or more substances  that are not chemically combined. Dissolving  salt in water results in a mixture. The two compounds  don’t react with each other and can be separated by  physical means—in this case, heating the water so it  evaporates, leaving the salt behind. 

All matter is made up of atoms. The following  terms are used to define atomic structure: 
- Atom: the smallest unit of an element that retains  all of the element’s chemical properties. An atom  is composed of three primary particles: electrons,  protons, and neutrons. 
- Electron: found outside the nucleus (the center of  an atom), it has a negligible mass and a charge  of –1. 
- Proton: found in the nucleus, it has a mass of 1  amu (atomic mass unit) and a charge of +1. 
- Neutron: found in the nucleus, it has a mass of 1  amu and no charge. 

An atom contains an identical number of protons  and electrons, making it electrically neutral. 

Atoms of the same element generally have the  same properties, unless they are isotopes, which can  behave differently. Atoms of different elements have different  properties and different masses. Atoms of elements  combine in simple whole number ratios. 

Periodic Table 
The periodic table lists all of the known elements  according to their atomic numbers. 

Atomic number is the number of protons in the  atom. The atomic number determines the element.  atomic number 11Na 

Mass number is the total number of protons and  neutrons in one atom of an element. Mass number  can vary because the number of neutrons in an atom  can change.  mass number 23Na 

The horizontal rows of elements in the periodic  table are called periods. There are seven periods in all. 

Moving from left to right across a period, the atomic  number increases by one from one element to the next. 

Each successive element has one more electron in its  outer shell. All elements in the same period have the  same number of shells. 
The vertical columns of elements in the periodic  table are groups. Elements in the same group have the  same number of electrons in their outer shell. They  therefore have similar chemical properties. 

Electronic Structure of Atoms 
According to the periodic table, the atomic number of  hydrogen (H) is 1, because it has one proton in its  nucleus. The number of protons in an atom determines  its atomic nucleus. The atomic number of nitrogen  (N) is 7; nitrogen has seven protons in its nucleus. 
Niels Bohr, a Danish physicist, proposed a model  of the atom that had a nucleus surrounded by concentric  orbits. 

Electrons are found in the orbits. When electrons  become excited, they absorb energy and move to an  orbit farther from the nucleus. When they release  energy, they fall to an orbit with lower energy closer to  the nucleus. The energy level (also called electron  shell) is a region of space in which electrons move  around the nucleus. 

Chemical Equations 
Chemical equations are used to show the result of a  physical or chemical change in matter. A physical  change is when the shape or size of matter is changed,  but the molecules remain unchanged. Steam from a  boiling pot of water is an example of a physical change. 
A chemical change is one in which two or more molecules  interact to form new molecules. Paper burning is  an example of a chemical change—the paper changes  to carbon. 
The sum of the atomic weights of all the atoms in  a formula is called the formula weight. A mole is the  amount of substance containing Avogadro’s number of  particles, 6.02  1023 atoms of an element. The abbreviation  is mol. 

Use the following rules in balancing equations: 
1. All reactants (starting materials) and products  must be known, and their formulas must be correctly  written. 
2. The formulas are never changed in order to balance  an equation. 
3. The number of atoms of each element in the  reactants must be equal to the number of atoms  of the element in the products. 
4. The numbers to the left of each formula (coefficients)  must be reduced to the smallest possible  whole numbers. 

Types of chemical reactions include: 
- Combination or synthesis reactions: two chemicals  combine to form a new substance 
- Decomposition reactions: one substance breaks  down to create two or more substances 

Concentration, Acids, Bases, and pH 
Concentration is a measure of how much solute is in  a solution. A solute is a substance that is dissolved in a  medium, and a solvent is a medium in which a solute  is dissolved. For instance: salt water is a solution—  a homogenous mixture consisting of the solute (salt)  and the solvent (water). 

Acids are proton donors; they release hydrogen  ions (protons). Acids have a sour taste. Bases are proton  acceptors; they take up hydrogen ions. Bases have  a bitter taste and feel slippery. Strong bases take up  more hydrogen ions than weak bases. 

The relative concentration of hydrogen ions is  measured in concentration units called pH units. The  pH scale runs from 0 to 14. A substance with a pH of 7 is neutral. Substances such as vinegar and orange  juice, with a pH of less than 7, are considered acidic. 

Substances such as soaps and ammonia, with a pH of  more than 7, are considered alkaline. 

Measurement 
Although you may be more familiar with the English  system of measurement (inches, pounds, and so on),  the metric system is the standard system of measurement  in science. The metric system is a decimal system  based on multiples and fractions of ten. The  meter (m) is the standard unit of length in the decimal  system: 

1 meter = 100 centimeters (cm) 
1 kilometer (km) = 1,000 meters 
The gram (g) is the metric system unit of mass: 
1 gram = 1,000 milligrams (mg) 
1 kilogram (kg) = 1,000 grams 

Volume, the amount of space occupied by a fluid or  body, is usually measured using the liter. The cubic  meter (cm3) is actually the standard metric unit of volume,  but it is infrequently used. 
1 liter (L) = 1,000 milliliters (ml) 
1 cm3 = 1 ml 

In science, temperature is most often measured  using degrees Celsius (°C). On the Celsius scale, the  freezing point for water is 0° and the boiling point for  water is 100°. This makes it much easier to use than the Fahrenheit scale, which has a freezing point of 32° and  a boiling point of 212°. The two equations below show  how to convert a temperature measurement from one  scale to the other. 

Fahrenheit to Celcius: (F - 32) * 5/9
Celcius to Fahreheit: (C * 9/5) + 32

Tip: If you want to convert Celsius to Fahrenheit, just multiply the temperature by 1.8 and then add 32 to the product. 
If you want to convert temperature from Fahrenheit to Celsius, subtract 32 from the number and then divide the difference by 1.8.

Other important equations are as follows: 
- Speed is the distance covered or traveled by an  object per a certain unit or amount of time: 
Speed = distance/time 

- Momentum is the tendency of an object to continue  moving in the same direction: 
Momentum = mass  speed 

- Work is a force applied to an object which, in  turn, results in movement: 
Work = force  distance 

- Power is the rate at which work is done. It is  measured in joules (J): 
Power = work/time 

Note:Mass should not be confused with weight. 
Mass is the measurement of the amount of matter in an  object. Weight is the force by which gravity attracts a  body to Earth. 

Energy 
Energy, the capacity to do work, is never created or  destroyed—it may only be changed in form. There are  many forms of energy, including light, sound, heat,  chemical, mechanical, and electrical. Energy, like  power, is measured in joules. 

The two forms of energy are potential and kinetic  energy. Potential energy is the energy stored in a body  or system as a consequence of its position, shape, or  state. Kinetic energy is the energy of motion, and is  usually defined as the work that will be done by the  body possessing the energy when it is brought to rest. 

Sound and light are measured by their wavelength. 

A wave is a periodic disturbance in a medium  or space. Sound is a vibration at a frequency and intensity  audible to the normal human ear (20–20,000 Hertz). Vibrations that have a lower frequency are  called infrasounds, and those with a higher frequency  are called ultrasounds. Light is a form of electromagnetic  radiation. Light can pass through a vacuum, but  sound cannot. Search online for 'the electromagnetic spectrum'.

Refraction is the bending of light, especially when  it moves from one material to another. The refraction  of sunlight results in a spectrum of colors, such as a  rainbow. The colors in a spectrum are red, orange, yellow,  green, blue, indigo, and violet. 

Another form of energy—heat—may be transferred  via conduction, convection, or radiation. Conduction  is the transmission of heat from a region of  high temperature to a region of lower temperature. 

Putting a cool kettle of water on a stove to boil is a good  example. Convection is the process by which parts of  a fluid or gas change density due to the uneven application  of heat. For instance, as a stove heats a cool  room, the warmer (less dense) air rises and the cold air  remains lower. Radiation is heat transmitted via electromagnetic  waves. The sun’s heat warming the petals  of a flower is an example of radiation.  high frequency  short wavelength  low frequency  long wavelength  radio  waves  micro  waves  infrared  light  ultraviolet  light  x-rays gamma  rays  visible  light