Commercial soft drinks and sodas mainly contain which weak acid formed when carbon dioxide dissolves in water?

Introduction / Context:
Soft drinks and carbonated beverages have a characteristic fizz and slightly sharp taste. This is due to the presence of dissolved carbon dioxide gas, which reacts with water under pressure to form a weak acid. Understanding which acid is produced helps connect simple equilibrium chemistry with everyday life. The question asks you to identify the main weak acid present in soft drinks because of dissolved carbon dioxide, distinguishing it from other organic acids found in foods and fruits.

Given Data / Assumptions:

• Soft drinks are carbonated beverages containing dissolved CO2 under pressure.
• When CO2 dissolves in water, it forms a weak acid in equilibrium with the dissolved gas.
• Other acids like citric, tartaric and lactic acid occur naturally in fruits and fermented foods.
• The focus here is specifically on the acid produced from dissolved CO2, not on flavouring acids that may also be added.

Concept / Approach:
When carbon dioxide is dissolved in water, a reversible reaction occurs forming carbonic acid, H2CO3. This weak acid partially dissociates to release hydrogen ions, giving the drink a mildly acidic pH. The overall reaction can be written as CO2 plus H2O giving H2CO3, with an equilibrium between these species. Although some soft drinks may also contain citric or phosphoric acid for flavour and preservation, the acid directly formed from dissolved carbon dioxide is carbonic acid. The approach is to match the name of this acid with the correct option and not confuse it with other common food acids.

Step-by-Step Solution:
Step 1: Recall that soft drinks are often called carbonated beverages because they contain dissolved carbon dioxide gas. Step 2: When CO2 dissolves in water, a weak acid called carbonic acid, H2CO3, is formed in equilibrium with the dissolved gas. Step 3: Carbonic acid is responsible for part of the sharp taste and for the slightly acidic pH of most soft drinks. Step 4: Oxalic acid is found in certain vegetables such as spinach and is not the main acid from carbonation. Step 5: Tartaric acid is present in tamarind and some fruits, and citric acid is found in lemons and oranges. Step 6: Lactic acid is produced in sour milk and curd by bacterial fermentation, not by dissolving CO2 in water. Step 7: Therefore, the correct answer is carbonic acid, the weak acid formed when carbon dioxide dissolves in water.

Verification / Alternative check:
If you measure the pH of a freshly opened soft drink, it is typically acidic, often in the range of pH 3 to 4. This is due to both carbonic acid and any additional acids added for flavour. Laboratory studies of the CO2 water system describe the equilibrium CO2 plus H2O giving H2CO3 giving H plus plus HCO3 minus, which matches the chemistry of carbonic acid in solution. Labels on soft drink bottles may list other acids such as phosphoric or citric acid, but these are separate additives. The fundamental acid generated by carbonation is always carbonic acid, confirming option C as correct.

Why Other Options Are Wrong:
Option A, oxalic acid, is more associated with some green vegetables and certain cleaning uses, not with carbonation in soft drinks. Option B, tartaric acid, is naturally present in grapes and tamarind and is used in some baking powders, but not as the primary acid from dissolved CO2. Option D, citric acid, is widely used in lemon flavoured drinks and fruit juices but is not produced by dissolving carbon dioxide in water. Option E, lactic acid, is a product of fermentation in dairy products and muscles. Only option C correctly identifies carbonic acid as the weak acid formed from dissolved carbon dioxide in soft drinks.

Common Pitfalls:
Students sometimes focus on the brand labels of drinks and see citric or phosphoric acid listed, then assume these are the only acids involved. While those acids do contribute to flavour and preservation, the question here is specifically about the acid formed from dissolved CO2. Another confusion is between carbonic acid and carboxylic acids like acetic or oxalic acid due to similarity of names. To avoid mistakes, remember that carbonic acid is uniquely associated with carbon dioxide and water, whereas other common food acids arise from plant metabolism or fermentation.

Final Answer:
The weak acid mainly present in carbonated soft drinks due to dissolved CO2 is Carbonic acid, formed from dissolved carbon dioxide.