An acid in common usage is a substance that tastes sour, reacts with metals and carbonates, turns blue litmus paper red, and has a pH less than 7.0 in its standard state. Examples include acetic acid (in vinegar) and sulfuric acid (used in car batteries). Acid/base systems are different from redox reactions in that there is no change in oxidation state. Acids can occur in solid, liquid or gaseous form, depending on the temperature. They can exist as pure substances or in solution. Chemicals or substances having the property of an acid are said to be acidic.
Acids are generally dangerous. Never drink them or put them near your eyes. Never pour water into a concentrated acid as it heats up and can spit hot acid at you.
An indicator is a chemical that changes colour. Indicators are liquids, but they can be soaked into a type of paper similar to blotting-paper to form strips of indicator paper.
Just dip the indicator paper into the unknown liquid and it will change colour to show the pH.
The simplest indicator is Litmus (from the Litmus plant). It goes red in acids, blue in alkalis.
pH = less than 7 pH = more than 7
Universal indicator is a better indicator than litmus because it can show a greater range of colours. The chart below shows the approximate colour that universal indicator goes when put into liquids of different pH values.
pH = 3 or less pH = 4 or 5 pH = 6 pH = 7 (neutral) pH = 8 or 9 pH = 10 or more
Universal indicator is available as a liquid or soaked into absorbing paper. It is often used to work out the pH of soil samples to see which plants can be grown in a certain patch of ground.
There are other indicators, for example, methyl orange (which changes colour from colourless to orange at pH of about 3). They all have different pH values where they change colour, and each has its own uses.
references:http://richardbowles.tripod.com/chemistry/acids/acids.htm
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