Acids & Alkalis

• The pH scale shows how acidic or alkaline a solution is
• Strong acids have a pH of 1, weak acids have a pH of 6
• Neutral solutions are neither acidic or alkaline. They have a pH of 7
• Weak alkalis have a pH of 8, strong alkalis have a pH of 14
• Indicators contain dyes which turn a different colour in acidic or alkaline solutions
• Litmus paper is an indicator. Blue litmus paper turns red when placed in acid, red litmus paper turns blue when placed in an alkaline solution
• A base is a substance that neutralises an acid
• An alkali is a soluble base
• Adding bases or acids to soil can change its pH making it suitable for different crops
• If an acid reacts with a base, there are two products- a salt and water
• If an acid reacts with a metal, there are two products- a salt and hydrogen
• Hydrochloric acid forms chloride salts, sulfuric acid forms sulfate salts and nitric acid forms nitrate salts

The pH scale measures how acidic or alkaline a solution is.

• Strong acids have a pH of 1
• Weak acids have a pH of 6
• Neutral solutions are neither acidic or alkaline. They have a pH of 7
• Weak alkalis have a pH of 8
• Strong alkalis have a pH of 14

Indicators are used to find out whether a solution is acidic or alkaline. They contain dyes which turn different colours in acids or alkalis. 

Some plants can make good indicators:

• Red cabbage juice turns red in hydrochloric acid and turns yellow/green in sodium hydroxide solution (alkali)
• Beetroot juice turns red/purple in hydrochloric acid and turns dark green in sodium hydroxide solution (alkali)

Red litmus paper turns blue on adding an alkali. Blue litmus paper turns red on adding an acid.

Universal indicator is a mixture of dyes. It changes colour to show how acidic or alkaline a solution is. Universal indicator turns orange in vinegar and red in hydrochloric acid.

Extra stomach acid makes your stomach hurt. An indigestion tablet reacts with this acid in a neutralisation reaction. In a neutralisation reaction, an acid reacts with a substance that cancels it out. The pH gets closer to 7.

A base is a substance that neutralises acids. Bases include sodium hydroxide, calcium oxide and copper oxide. Some bases are soluble in water- a soluble base is an alkali. 

Some soils are more acidic than others. Different plants grow best in different soil pH's. If the soil is too acidic, farmers add a base to the soil, which neutralises some of the soil acid.

A salt is a compound that forms when an acid reacts with a metal. Reacting an acid with a metal makes two products- a salt and hydrogen.

Metals react with hydrochloric acid to form chloride salts:

Sodium + Hydrochloric acid -> Sodium chloride + Hydrogen

2Na     +          2HCl          ->         2NaCl       +     H2

Metals react with sulfuric acid to make sulfate salts:

Zinc + Sulfuric acid -> Zinc sulfate + Hydrogen

Zn   +    H2SO4    ->   ZnSO4   +     H2

Metals react with nitric acid to make nitrate salts:

Magnesium + Nitric acid -> Magnesium nitrate + Hydrogen

Mg          +   2HNO3 ->    Mg(N03)2        +   H2

Reacting an acid with a base also makes a salt. The products are a salt and water.

Sodium hydroxide + Hydrochloric acid -> Sodium chloride + Water

Copper oxide + Nitric acid -> Copper nitrate + Water