Chemistry OCR Gateway GCSE Acids, bases & pH

Reacting metals with acids

metal + acid → salt + hydrogen

Reacting metal carbonates with acids

metal carbonate + acid → salt + water + carbon dioxide

reacting acids and bases

acid + metal oxide/hydroxide → salt and water (pH 7)

What is an acid

An acid is a substance that produces hydrogen ions, H+(aq), when dissolved in water. pH less than 7

what is an alkali

An alkali is a base that produces hydroxide ions, OH-(aq), when dissolved in water, pH more than 7

strong acids vs weak acids

strong - completely ionise in water (gives all H+ ions)

weak- partially ionise in water (gives only some H+ ions)

Universal indicator pH range

1- red

7 - green

14 - purple

Methyl orange pH range

acid - red

alkali - yellow

Phenolphtalein ph range

acid - clear

alkali - pink

litmus pH range

acid - red

alkali - blue

Advantages of indicator

• cheap

• easy to use

disadvantage of indicator

• dyes solution

• has to be compared with chart

• not accurate

advantages of pH probes

• very accurate

disadvantages of pH probes

• expensive

-has to be calibrated