ACID and ALKALIS

Lemon juice and vinegar taste sour because they contain weak acids.An acid is a substance that dissolves in water to form positively charged particles called hydrogen ions (H+). The opposite of an acid is an alkali, which dissolves in water to form negatively charged ions of hydrogen and oxygen, because they cancel out acidity. Toothpaste, for example, contains an alkali to cancel out acidity in the mouth that would otherwise damage teeth.









pH scale
The concentration of hydrogen ions in a solution is known as its pH. Scientists use the pH scale to measure acidity and alkalinity. On the pH scale, a solution with a pH lower than 7 is acidic, and a solution with a pH greater than 7 is alkaline. Water is neutral, with a pH of 7. A solution's pH can be tested with universal indicator solution or paper, which changes colour in acids alkalis.

Strong acids 
 The more hydrogen ions an acid form in water, the stronger it is, and the lower its pH. Strong acids, such as sulphuric acid and nitric acid, are very dangerous and must be handled carefully.
Sulphuric acid 
Concentrated sulphuric acid will dehydrate (remove water from) any substance with which it comes into contract. For example, the acid dehydrates sugar, a carbonhydrate, to leave a mass of smouldering black carbon. 
Nitric aacid
Organic matter, such as paper, cork, rubber, fabric, and skin, is rapidly decomposed by nitric acid.The acid is so corrosive because it oxidizes (supplies oxygen to) any material with which it comes into contact.