Acids produce hydrogen ions, H⁺, in water; alkalis produce hydroxide ions, OH⁻. The pH scale runs from 0 to 14: below 7 is acidic, 7 is neutral and above 7 is alkaline. Universal indicator or a pH probe measures pH. In neutralisation, H⁺ + OH⁻ → H₂O. Strong acids, such as hydrochloric acid, are completely ionised in water; weak acids, such as ethanoic acid, are only partly ionised.
Acids react to make salts. Acid + metal → salt + hydrogen. Acid + metal oxide or hydroxide → salt + water. Acid + metal carbonate → salt + water + carbon dioxide. Hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates. A soluble salt can be made by adding an insoluble base to acid until no more reacts, filtering off the excess, and crystallising the solution.
Worked example
Name the salt made when zinc oxide reacts with sulfuric acid.
- The metal part of the salt comes from the base: zinc.
- Sulfuric acid makes sulfates.
Answer:The salt is zinc sulfate (and water is also made).
Key words
- acid
- a substance that releases H⁺ ions in water
- alkali
- a soluble base that releases OH⁻ ions
- neutralisation
- acid + base → salt + water
- salt
- compound formed when the H of an acid is replaced by a metal
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NGSS-referenced: HS-PS1-2