Antacids are commonly used to get rid of acidity in the stomach. A commonly used antacid is

1. The pH Scale and Chemical Nature of Substances (basic)

To understand the chemistry of the world around us, we must first master the pH scale. The term 'pH' comes from the German word potenz, meaning 'power'—specifically, the power of Hydrogen. It is a numerical scale ranging from 0 to 14 that indicates how acidic or basic a solution is Science, Class X (NCERT 2025 ed.), Chapter 2, p.25. A solution with a pH of 7 is neutral (like pure water), while values less than 7 are acidic and values greater than 7 are basic (alkaline) Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.102.

At a molecular level, the nature of a substance depends on its ion concentration. Acids produce hydrogen ions (H⁺) or hydronium ions (H₃O⁺) in water, while bases produce hydroxide ions (OH⁻). Interestingly, the scale is logarithmic. This means that each whole pH value below 7 is ten times more acidic than the next higher value. For example, a solution with a pH of 4 is ten times more acidic than a solution with a pH of 5, and one hundred times more acidic than a solution with a pH of 6 Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.102.

Understanding these relationships is vital because most biological and chemical processes occur within a very narrow pH range. Our bodies, the soil used for farming, and even the aquatic ecosystems in our oceans are sensitive to these changes Science, Class X (NCERT 2025 ed.), Chapter 2, p.34. Below is a quick comparison to help you visualize the scale:

Sources: Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.25; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.102; Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.34

2. Neutralization Reactions: Chemistry of Balance (basic)

At its heart, a neutralization reaction is a chemical "handshake" where the opposing properties of an acid and a base cancel each other out. When you combine an acid and a base, they react to produce two primary products: salt and water. This is not just a laboratory curiosity; it is a fundamental process that maintains balance in our bodies and the environment. As observed in Science, Class X (NCERT 2025 ed.), Chapter 2, p. 21, the general chemical equation is written as:

Base + Acid → Salt + Water

To understand this at a deeper level, we look at the ions involved. Acids release hydrogen ions (H⁺), while bases (specifically alkalis, which are water-soluble bases) release hydroxide ions (OH⁻). In a neutralization reaction, these two ions find each other to form H₂O (water), as noted in Science, Class X (NCERT 2025 ed.), Chapter 2, p. 24. This process is typically exothermic, meaning it releases energy in the form of heat Science-Class VII, NCERT (Revised ed 2025), p. 18. The "salt" produced isn't just common table salt (NaCl); in chemistry, a salt is any ionic compound formed from the cation of a base and the anion of an acid.

In our daily lives, this chemistry is most visible when we treat indigestion. Our stomachs produce hydrochloric acid (HCl) to help digest food, but overproduction can lead to pain and irritation. To find relief, we use antacids, which are mild bases like magnesium hydroxide (Milk of Magnesia) or sodium hydrogencarbonate (baking soda). These bases react with the excess acid in the stomach to neutralize it, effectively "calming" the chemical environment Science, Class X (NCERT 2025 ed.), Chapter 2, p. 27.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.21, 24, 27; Science-Class VII, NCERT (Revised ed 2025), Exploring Substances: Acidic, Basic, and Neutral, p.18

3. Chemistry in the Human Digestive System (intermediate)

The human digestive system is essentially a highly sophisticated chemical reactor. When food enters the stomach, it triggers the gastric glands to release Hydrochloric Acid (HCl). While we often think of acid as corrosive, in the stomach it serves two vital purposes: it creates an acidic environment (low pH) necessary for the protein-digesting enzyme pepsin to function, and it acts as a primary defense by killing harmful bacteria that enter with our food Science, Class X (NCERT 2025 ed.), Chapter 5, p.85. To prevent this potent acid from digesting the stomach itself, the body secretes mucus, which provides a protective chemical barrier for the inner lining Science-Class VII, NCERT(Revised ed 2025), Chapter 9, p.125.

In chemistry, the acidity of a solution is measured on the pH scale, which ranges from 0 to 14. The term pH comes from the German word 'potenz', meaning power, referring to the concentration of hydrogen ions Science, Class X (NCERT 2025 ed.), Chapter 2, p.25. A pH of 7 is neutral; values below 7 are acidic, and those above are basic (alkaline). Under normal conditions, the stomach maintains a very low pH. However, during indigestion, the stomach produces an excess of HCl, leading to pain and irritation. This is a classic example of a chemical imbalance in the body Science, Class X (NCERT 2025 ed.), Chapter 2, p.27.

To treat this acidity, we apply the principle of neutralization: Acid + Base → Salt + Water. We use antacids, which are mild bases, to counteract the excess acid. Common examples include Magnesium Hydroxide [Mg(OH)₂], popularly known as Milk of Magnesia, and Sodium Hydrogencarbonate [NaHCO₃], also known as baking soda Science, Class X (NCERT 2025 ed.), Chapter 2, p.27, 31. These substances are chosen specifically because they are mild enough not to harm the digestive tract while being effective enough to raise the pH back to comfortable levels.

Sources: Science, Class X (NCERT 2025 ed.), Life Processes, p.85; Science-Class VII, NCERT(Revised ed 2025), Life Processes in Animals, p.125; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.25; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.27, 31

4. Common Sodium Compounds and Their Uses (intermediate)

Sodium compounds are foundational to both industrial chemistry and our daily lives. Among these, Sodium hydrogencarbonate (NaHCO₃), popularly known as Baking Soda, stands out due to its unique chemical properties. It is a mild, non-corrosive basic salt. Because of its alkaline nature, it is highly effective at neutralizing acids without being as harsh or caustic as compounds like Sodium Hydroxide. This property makes it a safe ingredient in food and medicine Science, Class X (NCERT 2025 ed.), Chapter 2, p.31.

In the kitchen, the distinction between baking soda and baking powder is crucial. Baking powder is actually a mixture of baking soda and a mild edible acid, such as tartaric acid. When this mixture is heated or added to water, it releases Carbon Dioxide (CO₂) gas. This gas gets trapped in the dough, causing bread or cakes to rise and become soft and spongy Science, Class X (NCERT 2025 ed.), Chapter 2, p.31. Beyond cooking, NaHCO₃ acts as a versatile household remedy for acidity. As an ingredient in antacids, it neutralizes excess hydrochloric acid in the stomach, providing rapid relief from indigestion Science, Class X (NCERT 2025 ed.), Chapter 2, p.31.

Interestingly, the physical properties of sodium hydrogencarbonate also vary with environmental conditions. For instance, its solubility in water is temperature-dependent; it dissolves much more readily at 70 °C than at 20 °C Science, Class VIII (NCERT 2025 ed.), Chapter 9, p.138. This allows for controlled reactions in industrial processes where concentration matters.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.31; Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.36; Science, Class VIII (NCERT 2025 ed.), Chapter 9: The Amazing World of Solutes, Solvents, and Solutions, p.138

5. Calcium Compounds: From Construction to Medicine (intermediate)

Calcium compounds form the backbone of both our physical infrastructure and our biological environment. The journey usually begins with Calcium Oxide (CaO), commonly known as Quicklime. When CaO reacts vigorously with water, it undergoes a combination reaction to produce Calcium Hydroxide (Ca(OH)₂), or Slaked Lime. This process is highly exothermic, meaning it releases a significant amount of heat Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6. This Slaked Lime is a versatile base used extensively in construction and agriculture.

In the construction industry, Slaked Lime is the secret behind the durability of traditional buildings. When applied as whitewash, it slowly absorbs Carbon Dioxide (CO₂) from the atmosphere. Over two to three days, a chemical transformation occurs where the Calcium Hydroxide turns into a thin, hard layer of Calcium Carbonate (CaCO₃). This provides a characteristic shiny finish to walls. Interestingly, this is the same chemical compound that makes up marble, which explains why old whitewashed walls can eventually feel stony and smooth Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7.

Beyond buildings, these compounds are vital for managing the health of our soil and oceans. Farmers often encounter soil that has become too acidic due to excessive rain or chemical use. To restore the pH balance, they treat the fields with Quicklime, Slaked Lime, or Chalk (CaCO₃). These basic compounds neutralize the acid, making the soil habitable for crops again Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28. In nature, Calcium Carbonate appears in two mineral forms: Calcite (found in algae and oyster shells) and Aragonite (found in most corals). While chemically similar, aragonite is more soluble and thus more sensitive to changes like ocean acidification Environment, Shankar IAS Academy (ed 10th), Ocean Acidification, p.263.

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6-7; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28; Environment, Shankar IAS Academy (ed 10th), Ocean Acidification, p.263; Physical Geography by PMF IAS, Major Landforms and Cycle of Erosion, p.227

6. The Chemistry of Antacids: Milk of Magnesia (exam-level)

In our daily lives, we often encounter the discomfort of acidity or indigestion. To understand the remedy, we must first look at the chemistry of the stomach. Our stomach naturally produces Hydrochloric Acid (HCl), which is essential for digesting food without damaging the stomach lining itself Science, Class X, Chapter 5, p. 85. However, during indigestion, the stomach produces an excess of this acid, leading to pain, irritation, and a burning sensation. To counter this, we use antacids—which are essentially mild bases designed to neutralize the excess acid.

The chemical principle at work here is a neutralization reaction: Acid + Base → Salt + Water. One of the most common and effective antacids is Magnesium Hydroxide [Mg(OH)₂], popularly known as Milk of Magnesia. It is categorized as a mild base Science, Class X, Chapter 2, p. 27. Unlike strong bases like Sodium Hydroxide (NaOH), which are highly corrosive and dangerous to ingest, Milk of Magnesia is gentle enough for the digestive tract while being chemically active enough to raise the stomach's pH to a comfortable level.

While Magnesium Hydroxide is a go-to remedy, it isn't the only basic substance used in the kitchen or medicine cabinet. For instance, Sodium Hydrogencarbonate (Baking Soda) is another mild, non-corrosive basic salt often used to neutralize acidity Science, Class X, Chapter 2, p. 31. However, chemists must be selective; while Calcium Carbonate is used in some tablets, Calcium Hydroxide [Ca(OH)₂] is generally considered too caustic for direct ingestion as a standard home remedy for acidity.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.27; Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.85; Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.31

7. Solving the Original PYQ (exam-level)

This question is a perfect application of the neutralization reaction (Acid + Base → Salt + Water) you just studied. In the human body, the stomach maintains a highly acidic environment by secreting hydrochloric acid to aid digestion. However, during indigestion, the stomach produces an excess of this acid, leading to pain and irritation. To solve this, you must apply the concept of a mild base to neutralize the excess acid without causing chemical burns to the digestive tract. As explained in Science, class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts, this is the fundamental principle behind the use of antacids.

To arrive at the correct answer, you must evaluate which substance is a safe, ingestible base. Magnesium hydroxide, popularly known as Milk of Magnesia, is the ideal candidate because it is a mild base that effectively attenuates gastric acidity. A common UPSC trap is to provide options that are technically bases but unsuitable for the human body. For example, calcium hydroxide (slaked lime) is too caustic and harsh for the stomach lining, while sodium hydrogen phthalate is a buffer used primarily in laboratory settings rather than medicine. Manganese acetate is simply a chemical salt with no antacid properties. Therefore, by focusing on the requirement for a mild, non-corrosive base, you can logically conclude that the correct answer is (B) magnesium hydroxide.