A chemical reaction involves the formation of one or more new substances. We can tell that a chemical reaction has taken place by looking for one or more of the following changes:
- A change in appearance: e.g. Colour change, gas given off, solid appears or disappears, etc.
- An energy change: e.g. Heat is given out or taken in (and therefore the temperature changes), light is produced, sound is produced, etc.
Exothermic is a reaction which gives out heat, i.e. the test tube gets hot.
Endothermic is a reaction which takes in heat, i.e. the test tube gets cold.
There are about 100 elements known to scientists at present. About 90 occur naturally in the earth.
Every substance on earth is made from these elements. Each element has a symbol, consisting of one or two letters, to represent its name.
The elements above number 103 have a symbol consisting of three letters to represent their names. For example, number 105 is called unnilipentium, symbol Unp.
Some naturally occurring elements are radioactive, for example, radium.
Plutonium and the other elements above 92 are made in nuclear reactors.
Some elements are named after scientists, e.g. einsteinium, and others after places, e.g. californium.
Element, Compound or Mixture?
Element: A substance made of only one type of atom.
Compound: Made up of two or more atoms from different elements which are chemically bonded together, and is extremely difficult to separate.
Mixture: Made up of two or more elements which are easily separated because they are not chemically bonded together.
Solution: a mixture composed of two or more substances.
Solute: a form that gets dissolved.
Solvent: a form that dissolves the solute.
Concentrated Solution: a little solute is dissolved in the solvent.
Dilute Solution: a fair amount of solute is dissolved in the solvent, but more can be dissolved.
Saturated Solution: is when no more solute can be dissolved in the solvent.
Separation By Filtering
A soluble substance is one which is able to be dissolved in liquid.
An insoluble substance does not dissolve in liquid.
Insoluble substances, such as sand, can be seperated from soluble substances, such as salt, by a process called filtering.
The insoluble substance, called the residue remains in the filter paper, and the soluble substance is collected in the conical flask and is called the filtrate.
The soluble substance may be recovered by a process called evaporation.
Most compounds can be named by applying the following rules:
- The names of the elements are written from left to right as they appear on the periodic table, e.g. sodium chloride and lithium fluoride.
- If there are only two elements forming the compound then the ending of the second name is '-ide'. For example, a compound of copper and sulphur only is called copper sulphide, and a compound of sodium and oxygen only is called sodium oxide.
- If the compound contains oxygen and two other elements, the ending of the second name is usually '-ate'. For example, a compound of copper, sulphur and oxygen is called copper sulphate.