Acids and Alkalis

• Substances can be classified into acidic, alkaline and neutral solutions
• The pH scale, from 0 to 14, is a measure of the acidity or alkalinity of a solution
• The pH scale can be measured using litmus, universal indicator or a pH probe
• A solution with pH 7 is neutral
• Aqueous solutions of acids have pH values of less than 7
• Aqueous solutions of alkalis have pH values greater than 7
• An aqueous solution is any solution in which the solvent is water
• Strong acids have a pH from 0 to 3
• Weak acids have a pH of 4 to 6
• Strong alkalis have a pH from 11 to 14
• Weak alkalis have a pH from 8 to 10
• Strong acids and strong alkalis are both corrosive
• Weak acids and alkalis are less corrosive
• Many substances we use every day are acidic or alkaline.
• Lemon juice is acidic
• Bleach (and many other cleaning agents) are alkaline
• Acids will turn universal indicator red or orange
• Neutral solutions will turn universal indicator green
• Alkaline solutions will turn universal indicator blue or purple
• In neutralisation reactions an acid reacts with an alkali to form a salt and water
• Neutralisation forms a neutral (pH7) solution
• A salt is a metal compound made from acid
• A salt is formed when the hydrogen in an acid is replaced by a metal.
• Indicators will show the pH of the substance by a colour change
• Litmus indicator can show if a solution is acidic or alkaline
• Litmus indicator is red in an acidic solution
• Litmus indicator is blue in an alkaline solution
• Litmus indicator remains the same colour in a neutral solution
• If using litmus paper, blue litmus paper turns red in an acidic solution
• Red litmus paper turns blue in an alkaline solution
• Universal indicator is sometimes called UI
• Universal indicator can be used as a liquid solution or as paper strips to dip into a solution
• Metal carbonates react with acids in neutralisation reactions to form a salt, water and carbon dioxide
• In an open system these products can escape, and the system is neutral
• In a closed system carbon dioxide reacts with water to form carbonic acid, which makes the system acidic