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The pH of the human small intestine is around 7.5 and the pH of the large intestine can be 5.5. As substances travel from the small intestine to large intestine, what would happen to the H+ ions concentration?
(a) Increases 20 fold
(b) Increases 2 fold
(c) Increases 10 fold
(d) Increases 100 fold

Answer
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Hint: pH scale is a logarithmic scale and pH 7.5 and 5.5 values differ by 2.

Complete answer:pH expresses logarithmic function of H+ ion concentration of an aqueous solution. It determines the acidity or basic nature of a solution. pH 7 is neutral. A pH value more than 7 indicates basicity while value less than 7 indicates acidic nature. The large intestine is much more acidic than the small intestine; hence, making the large intestine more appropriate for the digestion process.

pH scale is not a linear scale (e.g. centimetre scale). In linear scale two adjacent values have the same difference. But on a logarithmic scale it is different. In a logarithmic scale two adjacent values can increase or decrease by a factor of 10. From the question we can see pH of small intestine- 7.5 and pH of large intestine- 5.5 differ by 2 (from general point of view). But the difference in H+ ion concentration between them is actually (10)2= 100 times. As substances travel from the small intestine to large intestine, the H+ ion concentration increases 100 times.

So, the correct answer is ‘Increases 100 fold’.

Note: pH scale values range from 1 to 14 and 7 is considered neutral pH because it is the pH value of pure water which is neither acidic nor basic.