The pH values of some substances are given in the table. Analyse the table and answer the questions that follow.

a. Is blood acidic or alkaline in nature?

b. The pH value of pure milk is 6.4. Does the pH value increase or decrease when milk changes to curd? Justify your answer.

c. Among the substances given in the table,

i) Which one is strongly alkaline?

ii) Which one has weak acidic nature?

a. Blood is slightly alkaline in nature.

A neutral solution has a pH of exactly 7, while an acidic solution has a pH less than 7 and a basic solution has a pH more than 7. Since the blood has a pH value of 7.36, it is alkaline in nature but we can say it only slightly alkaline.

b. The pH value decreases when milk changes to curd.

The process of conversion of milk into curd involves the formation of lactic acid. This lactic acid makes the curd more acidic than milk. On pH scale the acidity increases with the decrease in pH value. Thus pH value decreases from 6.3 to 4.5 when milk changes into curd.

c. i) Lime water is strongly alkaline.

The pH scale varies from 0 to 14. A neutral solution has a pH of exactly 7. Acidic solution has a pH less than 7 and basic solution has a pH greater than 7.

General rule: Lesser the pH more will be acidity.

OR

More the pH more will be alkalinity.

Since lime water has a pH of 10.5 (which is much greater than 7), so it is strongly alkaline.

ii) Milk has the weak acidic nature.

Both milk and vinegar has a pH less than 7 so both are acidic in nature. But milk has a pH slightly less than 7 (6.3) while vinegar has a pH which is much lower than 7 (4.2). Therefore, milk has weak acidic nature as compared to vinegar.