Milk of magnesia, which is used as a mild antacid and laxative, contains which basic compound as its active ingredient?

Introduction / Context:
Many common medicines and household products contain well known chemical compounds. Milk of magnesia is a familiar example used as an antacid to neutralise excess stomach acid and as a mild laxative. General science and chemistry examinations often test whether candidates can identify the key active ingredient in such preparations. This question asks which base is present in milk of magnesia, so you must link the trade name with the underlying chemical compound.

Given Data / Assumptions:
- The product mentioned is milk of magnesia, commonly available in pharmacies.
- The question specifies that the relevant component is a base, that is, an alkaline substance.
- Four possible bases are listed: magnesium hydroxide, calcium hydroxide, sodium hydroxide, and ammonium hydroxide.
- Standard formulations of milk of magnesia as described in textbooks and pharmacopeias are assumed.

Concept / Approach:
Milk of magnesia is essentially a suspension of magnesium hydroxide in water. Magnesium hydroxide is only slightly soluble in water, so it forms a milky white suspension, which is why the product looks like milk. When ingested in small doses, magnesium hydroxide neutralises excess hydrochloric acid in the stomach, providing relief from acidity. It also draws water into the intestines, acting as a mild laxative. Other bases such as sodium hydroxide and calcium hydroxide are far too caustic for direct medicinal use in this way, and ammonium hydroxide is not used for this purpose either. Therefore the only base among the options that correctly matches milk of magnesia is magnesium hydroxide.

Step-by-Step Solution:
Step 1: Recall from basic chemistry and general knowledge that milk of magnesia is a pharmaceutical preparation of magnesium hydroxide. Step 2: Understand that the term milk refers to the appearance of the suspension and not to the presence of dairy products. Step 3: Consider sodium hydroxide and calcium hydroxide, both of which are strong caustic bases. They can cause severe burns and are not used as gentle antacids in this form. Step 4: Consider ammonium hydroxide, which is mainly used as a cleaning agent and in industry, not as a standard over the counter antacid and laxative. Step 5: Conclude that magnesium hydroxide is the only base that fits both the name milk of magnesia and the medicinal uses described.

Verification / Alternative check:
Checking a standard school chemistry book or a reliable pharmacology reference confirms that milk of magnesia is defined as a suspension of magnesium hydroxide in water. Composition labels on commercial bottles also list magnesium hydroxide as the active ingredient. These references emphasise its antacid and laxative properties, matching the description students remember from science lessons. No reputable source connects milk of magnesia to sodium hydroxide, calcium hydroxide, or ammonium hydroxide. This documentary evidence supports the conclusion that magnesium hydroxide is the correct answer.

Why Other Options Are Wrong:
Calcium hydroxide: Known as slaked lime, used in construction and agriculture, and too caustic for direct use in this common form as an antacid suspension.
Sodium hydroxide: A very strong base called caustic soda, used in industry and drain cleaners, not in gentle over the counter antacid products.
Ammonium hydroxide: Primarily used in cleaning agents and industrial applications, and not the active base in milk of magnesia.

Common Pitfalls:
Students sometimes confuse different hydroxides because many share similar naming patterns. Calcium hydroxide, for example, is also white and can form suspensions in water, which may lead to confusion if one relies only on appearance. Another mistake is to assume that any base listed in the options could be used medicinally, forgetting that strong caustic bases are unsafe for ingestion at typical cleaning concentrations. To avoid these pitfalls, it helps to memorise a few key medicinal compounds, such as magnesium hydroxide in milk of magnesia and sodium bicarbonate as baking soda, along with their specific uses.

Final Answer:
Milk of magnesia contains Magnesium hydroxide as its active basic ingredient, which acts as a mild antacid and laxative.