In basic chemistry of bases, which base is present in lime water that is often used in laboratory tests and in construction related chemistry?

Introduction / Context:
This question deals with common bases in chemistry and their familiar names. Lime water is a simple laboratory reagent and is also related to materials used in construction, such as slaked lime. Knowing which base is present in lime water helps you connect theoretical chemical formulas with practical applications, such as tests for carbon dioxide gas and the setting of mortar. The question checks whether you can correctly identify calcium hydroxide as the dissolved base in lime water rather than confusing it with other hydroxides.

Given Data / Assumptions:
- The substance in question is lime water, a common term in school level chemistry.
- You must identify which base is present in this solution.
- Options include sodium hydroxide, magnesium hydroxide, calcium hydroxide, and ammonium hydroxide.
- It is assumed that you know that lime water is a saturated aqueous solution of a specific slightly soluble base.

Concept / Approach:
Lime water is prepared by dissolving a small amount of calcium hydroxide, Ca(OH)2, in water to form a saturated solution. This mixture is distinct from sodium hydroxide solution, which is called caustic soda, and from ammonium hydroxide solution, which is aqueous ammonia. Magnesium hydroxide is very sparingly soluble and is usually encountered as a suspension called milk of magnesia, not as clear lime water. Therefore, the correct base present in lime water is calcium hydroxide. The approach is to associate each common name with its chemical formula and physical form in standard experiments.

Step-by-Step Solution:
Step 1: Recall that lime water is made by dissolving calcium hydroxide in water until a saturated solution is formed. Step 2: Remember that the chemical formula for calcium hydroxide is Ca(OH)2. Step 3: Recognize that sodium hydroxide solution is known as caustic soda and is much more soluble and strongly alkaline than lime water. Step 4: Note that magnesium hydroxide is typically a suspension called milk of magnesia, not the clear solution referred to as lime water. Step 5: Understand that ammonium hydroxide refers to aqueous ammonia, which has a distinctive smell and is not called lime water. Step 6: Conclude that lime water contains calcium hydroxide, so Calcium hydroxide is the correct answer.

Verification / Alternative check:
You can verify this by reviewing basic laboratory procedures for testing carbon dioxide gas. The standard test involves passing carbon dioxide through lime water, causing it to turn milky due to the formation of calcium carbonate. Instructions for this experiment clearly state that lime water is a solution of calcium hydroxide. Chemistry textbooks and practical manuals also define lime water as saturated calcium hydroxide solution. No authoritative source describes lime water as containing sodium hydroxide, magnesium hydroxide, or ammonium hydroxide, confirming that calcium hydroxide is correct.

Why Other Options Are Wrong:
Sodium hydroxide is a strong, highly soluble base known as caustic soda. It is not called lime water and has different uses. Magnesium hydroxide occurs mainly as a suspension and is related to antacids and other medicinal uses, not to lime water. Ammonium hydroxide is a solution of ammonia in water and has a strong pungent smell, quite different from lime water. None of these bases are accurately described by the term lime water in school level chemistry.

Common Pitfalls:
Students often mix up common names of bases, especially lime water, caustic soda, and milk of magnesia. Some may choose sodium hydroxide because it is a familiar strong base, ignoring the specific name lime. Others may not distinguish between suspensions and clear solutions. To avoid such errors, memorize the associations clearly: lime water equals calcium hydroxide solution, caustic soda equals sodium hydroxide, and milk of magnesia equals magnesium hydroxide suspension.

Final Answer:
The base present in lime water is Calcium hydroxide.