The pH of human small intestine is around $7.5$ and the pH of large intestine can be $5.5$. As substances travel from the small intestine to larger intestine, what would happen to the $H^{+}$ ions concentration?

pH scale is not a linear scale like a centimetre or inch scale (in which two adjacent values have the same difference). It is a logarithmic scale in which two adjacent values increase or decrease by a factor of 10. As pH 7.5 and 5.5 differ by 2, the difference in H${}^{+}$ ion concentration between them would be 10${}^2$ = 100.