It is alkaline to neutralise hydrochloric acid from the stomach.

The idea

Bile is alkaline, so when it enters the small intestine it neutralises the acidic chyme arriving from the stomach. Stomach cells secrete hydrochloric acid, producing a low pH that helps activate stomach enzymes and kill microbes. That acidity would reduce the activity of pancreatic and intestinal enzymes, which work best at around neutral to slightly alkaline pH. Bile contains sodium hydrogencarbonate, which reacts with hydrochloric acid to raise the pH. This neutralisation protects the lining of the small intestine and provides suitable conditions for enzymes such as lipase, amylase and protease. The alkaline conditions are therefore not a digestive action by bile itself but a necessary preparation for efficient chemical digestion in the duodenum.

What earns the marks

• Bile is alkaline, so it raises the pH of the material entering the small intestine.
• Hydrochloric acid from the stomach makes the stomach contents acidic.
• Neutralisation of the acid creates conditions suitable for enzymes in the small intestine.
• Bile contains sodium hydrogencarbonate, which neutralises the acid.
• The alkaline conditions help protect the small intestine lining from acid damage.

Common mistakes and how to put them right

• Saying bile is acidic because it comes from the digestive system.

  Put it right by stating that bile is alkaline and neutralises stomach acid.

• Saying bile digests food chemically.

  Put it right by stating that bile neutralises acid and emulsifies fat, while enzymes carry out chemical digestion.

• Saying stomach acid is neutralised in the stomach.

  Put it right by stating that neutralisation occurs when bile enters the small intestine.

Exam tips

• Name hydrochloric acid as the acid from the stomach and state that bile neutralises it.
• Link alkaline conditions to the optimum pH of small intestine enzymes.
• Use the term neutralise rather than vague words such as cancel out.