P

Particles     The bits of matter from which everything is made. If the particles stick together in fixed places and vibrate, we have a solid; if they can roll and slide about but remain in contact with each other, we have a liquid; if they fly around and are free from each other, we have a gas.

The particle theory (the particulate nature of matter) can explain many properties of the three states of matter. Our understanding of diffusion of gases and liquids, dissolving, solids maintaining their own shape, melting and boiling etc all depend on the particle theory.

Particulate     When fossil fuels are burned, some tiny bits of carbon can be left. These small particles are a source of air pollution and can lead to global dimming.

Particulate theory     Matter is made of particles.

Periodic table     Most people use the form of periodic table formulated by Mendeleev. This shows all the elements that have so far been discovered. They are in order of atomic number. the sets of elements with similar properties are arranged in vertical columns called GROUPS. The horizontal rows are called PERIODS.

When Mendeleev created the table, he was brave enough to leave gaps for elements that had not at that time been discovered. later, scientists looked for elements to fill the gaps hoping that they would have the properties Mendeleev predicted. This worked out well.

Permanent change     The new substance that you have made cannot be changed back into the starting materials. Chemical changes are permanent changes. Some people use the term "irreversible change" too.

pH     NOT the "power of hydrogen"!!! It is a mathematical expression:

pH = -log₁₀[H⁺]                   [H⁺] means "the concentration of hydrogen ions in mol dm^-3 "

At GCSE you need to know that
Acids have pH less than 7.
Alkalis have pH greater than 7.

Weak acids have a pH nearer to 7 than strong ones.
Weak alkalis have pH nearer to 7 than strong ones.

Here is a chance to prove your English teacher wrong! You CAN start a sentence with a small letter:

pH is a measure of the strength of an acid.    This is correct because the p of pH is always lower case.

Phlogiston     An old theory said that when metals burn they lose phlogiston. The theory explained the fact that metals get heavier when they burn by stating that phlogiston had negative mass. Although interesting as a theory to show how science has developed, we no longer believe the phlogiston theory.

Photosynthesis     Plants take in carbon dioxide and water. They use the energy from the Sun to make new substances glucose and oxygen. Glucose and oxygen have a higher energy content than carbon dioxide and water.

Physical change     No new substances are made. It is fairly easy to reverse this kind of change. 
An example would be dissolving salt in water. The change can be reversed simply by eveporating the water.

Physical weathering     Physical processes (such as temperature change, wind-borne grit etc) that can cause a rock to be broken down into smaller pieces.

Porous     Porous rocks have tiny holes in them that can soak up water or other liquids. Crude oil is found soaked into porous rocks.

Potassium     Watch out for the spelling of this word. It is a silvery metal that reacts violently with water. It is one of the alkali metals. By comparing the reactivity of lithium, sodium and potassium you can see tone of the trends in the groups of the periodic table.

Precipitate     An insoluble solid which is formed when two solutions react together.

Pressure     The force caused by particles hitting the walls of a vessel.

Product     The new substance or substances that are formed during a chemical reaction.

Property     A feature of a substance that can be described or measured. Chemical proerties refer to how the substance behaves in a chemical reaction (eg it is an acid, it burns in air) while a physical property is to do with the substance itself (eg it is a hard, shiny solid with a melting point of 465^oC).

Pure     A pure substance contains only one substance. Pure substances have a single melting point and a single boiling point.

Many people wrongly believe that "natural" and "organic" things are pure.
"Natural" spring water contains many salts.
 "Organic" in this context means that it has been grown without artificial fertiliser or weedkillers etc.

MORE WORDS AND DEFINITIONS WILL BE ADDED.

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