Acids, Alkalis and Bases

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Acids - substances with a pH of less than 7 are acids, the more strongly acidic a solution the lower its pH number. Acidic solutions turn blue litmus paper red. They turn universal indicator paper red if they are strongly acidic and orange or yellow if they are weakly acidic.

When acids dissolve in water they produce aqueous hydrogen ions, H+(aq). For example, looking at hydrochloric acid: HCl(aq) → H+(aq) + Cl–(aq)

Alkalis - Bases that dissolve in water are called alkalis. Copper oxide is not an alkali because it does not dissolve in water. Sodium hydroxide is an alkali because it does dissolve in water.

Alkaline solutions have a pH of more than 7. The stronger the alkali, the higher the pH number. Alkalis turn red litmus paper blue. They turn universal indicator paper dark blue or purple if they are strongly alkaline, and blue-green if they are weakly alkaline.

When alkalis dissolve in water they produce aqueous hydroxide ions, OH–(aq). For example, looking at sodium hydroxide:

NaOH(aq) → Na+(aq) + OH–(aq)

Neutralisation reaction - When the H+(aq) ions from an acid react with the OH–(aq) ions from an alkali, a neutralisation reaction occurs to form water. This is the equation for the reaction:H+(aq) + OH–(aq) → H2O(l)

For example, hydrochloric acid and sodium hydroxide solution react together to form water and sodium chloride solution. The acid contains H+ ions and Cl– ions, and the alkali contains Na+ ions and OH– ions. The H+ ions and OH– ions produce the water, and the Na+ ions and Cl– ions produce the sodium chloride, NaCl(aq).

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