Question

what is the nature of substance of the solutions with ph
lemon juice-2.2
distilled water-7
coffee-5.0
saliva-5.5-6.9

Answer 1

lemon juice -2.2-   0-7- acid
distilled water-7-    7-neutral
coffee-5.0-             0-7 -acid
saliva 5.5-              0-7 -acid
saliva-6.9                0-7 -acid

Answer 2

In general the pH values indicate the concentration of H+ ions in the solution. If pH is less than 7, then the solution is an acid.  If pH is more than 7 then it is a base because the concentration of [OH-]  is more than that of [H+].  If the pH of the solution is 7, then it is neutral.

According to the given pH values:
The given sample of lemon juice = pH = 2.2   =>  strongly Acidic
given sample of distilled water : pH = 7   => Neutral
given sample of coffee = pH = 5.0   =>  moderately acidic
Given sample of saliva = pH = 5.5 to 6.9   =>  weakly acidic

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In general in real life and nature :

Lemon juice is acidic because of citric acid.

Distilled or pure water is neutral, there are equal concentrations of H+ or OH-.

Coffee is slightly acidic in nature, but its acidity depends on its variety and on whether it is roasted or not.  There are special organic coffees which are neutral. These are perfectly healthy and do not cause acidity problems.

Saliva contains many components and ions Its pH varies between 6.4 to 7.4 and helps in digestion by breaking down the food.  Different Amylases, which are part of saliva, are slightly acidic to strongly acidic.