The pH value of Milk of Magnesia is approximately
- (a)Zero
- (b)7
- (c)10
- (d)14
Answer
Why
Correct — C, about 10. Milk of Magnesia is a suspension of magnesium hydroxide, Mg(OH)2, a mild alkali, so a fresh sample reads a pH of roughly 10 to 10.5. That is why it neutralises excess hydrochloric acid in the stomach and is sold as an antacid and mild laxative. A pH near 10 sits clearly on the basic side of the neutral value of 7, yet stays well below the strongly caustic end of the scale.
Why the others are wrong
- (a)Zero — A pH of zero marks a very strong acid such as concentrated hydrochloric acid; Milk of Magnesia is basic, not acidic, so it lies at the opposite end of the scale.
- (b)7 — A pH of 7 is neutral, like pure water. Milk of Magnesia is distinctly alkaline, which is exactly the property that lets it neutralise stomach acid, so it cannot be neutral.
- (d)14 — A pH of 14 belongs to a strong, caustic alkali such as concentrated sodium hydroxide. Magnesium hydroxide is only sparingly soluble, so its solution is a mild base near 10, not a caustic 14.
Concept
The pH scale runs from 0, strongly acidic, through 7, neutral, to 14, strongly basic. Milk of Magnesia is a milky suspension of magnesium hydroxide, which is only slightly soluble in water. That limited solubility keeps the hydroxide ion concentration low, giving a mild alkalinity around pH 10 rather than a caustic value.
Because it is a medicine, students are tempted to call it neutral at 7 or to over-rate it as a strong base at 14. The reasoning shortcut is that an antacid must be basic enough to swallow the acid but gentle enough to drink, which places it in the pH 10 band.
Key facts
- Milk of Magnesia is an aqueous suspension of magnesium hydroxide, Mg(OH)2.
- Its pH is approximately 10 to 10.5, on the basic side of neutral.
- It is used as an antacid to relieve acidity and as a mild laxative.
- Its low solubility keeps it mildly basic rather than strongly caustic like sodium hydroxide.
Study next
Common traps
- Assuming a medicine or edible substance must be neutral at pH 7.
- Confusing a mild base near 10 with a strong caustic alkali at 14.
Asked as the approximate pH of a familiar household base; place it on the basic side near 10, not at neutral 7 or caustic 14.
Related PYQs
Which one among the following is known as Milk of Magnesia?
- (a) Magnesium bicarbonate
- (b) Magnesium carbonate
- (c) Magnesium sulphate
- (d) Magnesium hydroxide
Answer(d) Magnesium hydroxide
Same substance from the other side: that NDA item asks what Milk of Magnesia is (magnesium hydroxide), while this one asks its pH. Knowing it is a hydroxide base immediately places its pH on the alkaline side near 10.
Practice
- practice — not a real PYQ
Which one of the following is the chemical present in Milk of Magnesia?
- (a)Calcium carbonate
- (b)Magnesium hydroxide
- (c)Sodium bicarbonate
- (d)Aluminium hydroxide
Answer(b) Magnesium hydroxide — a mildly basic suspension used as an antacid. - practice — not a real PYQ
A solution has a pH of 10. It is best described as
- (a)strongly acidic
- (b)neutral
- (c)mildly basic
- (d)strongly caustic
Answer(c) mildly basic — pH 10 lies above neutral 7 but well below the caustic 14 end.