In the neutralisation reaction between an acid and a base, Acid + Base → _______ + Water, what product correctly fills the blank?

Introduction / Context:
This question addresses the basic idea of neutralisation in acid base chemistry. When an acid reacts with a base, they cancel each other effect to form more neutral products. Students learn a general word equation for this process, and correctly identifying the missing product is important for understanding many real world applications, such as antacid use, wastewater treatment, and laboratory titrations.

Given Data / Assumptions:

• General reaction pattern: acid plus base produces something plus water.
• The options for the missing product are carbon dioxide, metal oxide, hydrogen gas, and salt.
• We consider typical reactions like hydrochloric acid with sodium hydroxide.

Concept / Approach:
The classic neutralisation reaction between an acid and a base can be represented in ionic form as H+ plus OH− giving H2O. In molecular examples, the metal part of the base and the non metal part of the acid combine to form a salt. For instance, HCl plus NaOH gives NaCl plus H2O. Therefore the general pattern is acid plus base gives a salt and water. Carbon dioxide is produced in reactions of acids with carbonates, not simple bases. Hydrogen gas is released when acids react with some metals, not when they react with bases.

Step-by-Step Solution:
Step 1: Write a typical neutralisation example. HCl + NaOH → NaCl + H2O Step 2: Identify the products. The first product is NaCl, a salt, and the second product is water. Step 3: Compare with the word equation given. Acid + Base → _______ + Water. Here the blank must represent the salt that forms from cation of the base and anion of the acid. Step 4: Choose the matching option. Among carbon dioxide, metal oxide, hydrogen gas, and salt, only salt correctly fits the word equation.

Verification / Alternative Check:
Consider another example: sulphuric acid reacting with sodium hydroxide. The balanced equation is H2SO4 + 2NaOH → Na2SO4 + 2H2O. Again, the products are a salt (sodium sulphate) and water, confirming that salt is the general product. Other combinations, such as hydrochloric acid with magnesium hydroxide or calcium hydroxide, follow the same pattern, reinforcing that the blank should always be filled with salt.

Why Other Options Are Wrong:
Option A: Carbon dioxide is produced when acids react with carbonates or bicarbonates, for example HCl with CaCO3, not in simple acid base neutralisation. Option B: Metal oxides can behave as bases or acidic oxides but they are reactants or intermediates, not the typical product of mixing acid and base in solution. Option C: Hydrogen gas is evolved when active metals such as zinc or magnesium react with acids, not when a base is present.

Common Pitfalls:
Learners sometimes mix up different general reaction patterns, such as acid plus carbonate giving salt, carbon dioxide, and water. Confusing those with simple neutralisation leads to wrong choices. A helpful tip is to remember that the word neutralisation emphasises the formation of water from H+ and OH−, with the ions left behind combining into a salt.

Final Answer:
In the reaction Acid + Base → _______ + Water, the blank is filled by Salt.