Solutions in test tubes containing H20 and aqueous NaOH can be differentiated with the help of:

1. Fundamentals of Acids, Bases, and Neutral Substances (basic)

To understand the chemistry of the world around us, we categorize substances based on their chemical nature into acids, bases, and neutral substances. At the most fundamental level, these are defined by how they behave when dissolved in water. Acids are substances that release hydrogen ions (H⁺) or hydronium ions (H₃O⁺) in an aqueous solution, whereas bases are substances that release hydroxide ions (OH⁻) in water Science, Acids, Bases and Salts, p.23. Neutral substances, such as pure water (H₂O), do not exhibit either property strongly and maintain a balance.

Since we cannot safely taste or touch every chemical, we use indicators like litmus paper to identify them. Litmus is a natural dye that changes color based on the acidity or basicity of a solution. This is a crucial diagnostic tool in the laboratory:

Substance Type	Ion Released in Water	Red Litmus Paper	Blue Litmus Paper
Acid	H⁺ / H₃O⁺	Stays Red	Turns Red
Base	OH⁻	Turns Blue	Stays Blue
Neutral	None (Balanced)	Stays Red	Stays Blue

When an acid and a base are mixed, they undergo a neutralization reaction. In this process, the H⁺ from the acid and the OH⁻ from the base combine to form water (H₂O), and the remaining parts form a salt Science, Acids, Bases and Salts, p.21. It is a common misconception that all salts are neutral; however, only salts formed from a strong acid and a strong base (like Sodium Chloride, NaCl) result in a neutral solution with a pH of 7 Science, Acids, Bases and Salts, p.29. Understanding these interactions is the bedrock of chemical analysis and industrial applications.

Sources: Science (NCERT 2025 ed.), Acids, Bases and Salts, p.23; Science (NCERT 2025 ed.), Acids, Bases and Salts, p.21; Science (NCERT 2025 ed.), Acids, Bases and Salts, p.29

2. The pH Scale and Strength of Solutions (basic)

To understand the strength of an acid or a base, we use a specialized tool called the pH scale. The 'p' in pH stands for 'potenz', a German word meaning power. Essentially, this scale measures the concentration of hydrogen ions (H⁺) in a solution. It ranges from 0 to 14, providing a numerical value that tells us exactly how acidic or basic a substance is Science, Class X (NCERT 2025 ed.), Chapter 2, p.25. A solution with a pH of 7 is considered neutral (like pure water). Any value less than 7 is acidic, while any value greater than 7 is basic (or alkaline).

The relationship between pH and hydrogen ion concentration is inverse and logarithmic. This means as the concentration of hydronium ions (H₃O⁺) increases, the pH value decreases Science, Class X (NCERT 2025 ed.), Chapter 2, p.25. Because it is a logarithmic scale, each single unit change in pH represents a tenfold (10x) difference in acidity. For example, a solution with a pH of 4 is ten times more acidic than a solution with a pH of 5, and a hundred times (10 × 10) more acidic than one with a pH of 6 Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.102.

In practical chemistry, we differentiate between "strong" and "weak" solutions based on how much they ionize in water. Strong acids (like HCl) and strong bases (like NaOH) dissociate almost completely, releasing a high concentration of ions and pushing the pH toward the extremes of the scale. Conversely, salts formed from a strong acid and a strong base result in a neutral solution with a pH of 7 Science, Class X (NCERT 2025 ed.), Chapter 2, p.29. Understanding these values is vital because most living organisms and chemical processes function only within a very narrow, optimal pH range.

pH Value	Nature of Solution	Ion Concentration
0 to <7	Acidic	High H⁺ / H₃O⁺ concentration
7	Neutral	Balanced H⁺ and OH⁻ ions
>7 to 14	Basic (Alkaline)	High OH⁻ concentration

Sources: Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.25, 29; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.102

3. Salts and their Chemical Nature (intermediate)

In chemistry, a salt is an ionic compound produced when an acid and a base react in a process called neutralization. While the word 'salt' often brings to mind common table salt (NaCl), the chemical nature of salts is diverse. A salt’s personality—whether it is acidic, basic, or neutral—is determined by the relative strength of its 'parents' (the acid and base that formed it) Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.34. To identify the nature of a salt solution, we look at its pH value on a scale from 0 to 14. A neutral solution has a pH of exactly 7, an acidic solution has a pH less than 7, and a basic solution has a pH greater than 7 Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.34. We can verify this using indicators like litmus paper. For instance, neutral substances like a pure salt solution do not change the color of either red or blue litmus paper Science-Class VII, Exploring Substances, p.10. In contrast, basic salts will turn red litmus blue. Understanding this 'parentage' is the key to predicting how a salt will behave in water:

Salt Nature	Parent Acid + Parent Base	Example
Neutral	Strong Acid + Strong Base	Sodium Chloride (NaCl), Potassium Nitrate (KNO₃)
Acidic	Strong Acid + Weak Base	Ammonium Chloride (NH₄Cl), Aluminium Chloride (AlCl₃)
Basic	Weak Acid + Strong Base	Sodium Carbonate (Na₂CO₃), Sodium Acetate (CH₃COONa)

Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.34; Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.10

4. Chemical Indicators: Types and Mechanisms (intermediate)

In chemistry, we often need a way to determine whether a solution is acidic, basic, or neutral without tasting it—which is dangerous. This is where chemical indicators come in. These are substances that undergo a distinct physical change, usually a color change, when they come into contact with an acid or a base. At a molecular level, indicators are often weak acids or bases themselves; they exist in an equilibrium that shifts when H⁺ ions are added or removed, causing their molecular structure to rearrange and reflect different wavelengths of light Science, Class X, Acids, Bases and Salts, p.17. Indicators can be broadly categorized into natural and synthetic types. Litmus is perhaps the most famous natural indicator; it is a purple dye extracted from lichens (plants belonging to the division Thallophyta). Other natural indicators include turmeric, red cabbage leaves, and the petals of flowers like Hydrangea and Petunia. Synthetic indicators, such as phenolphthalein and methyl orange, are lab-synthesized chemicals designed for high precision in titrations Science, Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.19. Understanding the specific color transitions is vital for laboratory work. For instance, in a neutralization reaction, indicators help us identify the 'end-point'—the moment when an acid has completely reacted with a base. If you add dilute HCl to a pink solution of NaOH containing phenolphthalein, the solution eventually turns colorless, signaling that the base's effect has been nullified Science, Class X, Acids, Bases and Salts, p.21.

Common Indicator Color Changes:

Indicator	Acidic Solution	Basic Solution	Neutral Solution
Blue Litmus	Turns Red	Stays Blue	Stays Blue
Red Litmus	Stays Red	Turns Blue	Stays Red
Phenolphthalein	Colorless	Pink	Colorless
Turmeric	No Change (Yellow)	Reddish-Brown	No Change (Yellow)

Sources: Science, Class X, Acids, Bases and Salts, p.17; Science, Class X, Acids, Bases and Salts, p.21; Science, Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.19

5. Reaction Dynamics: Neutralization and Carbonates (exam-level)

At its heart, chemical dynamics involve predictable patterns of how substances transform when they meet. One of the most fundamental interactions is the neutralization reaction. This occurs when an acid and a base interact to cancel out each other's properties, resulting in the formation of salt and water. For instance, when Sodium Hydroxide (NaOH) reacts with Hydrochloric Acid (HCl), they form common table salt (NaCl) and water (H₂O) Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21. Beyond just forming products, this process is generally exothermic, meaning it releases heat into the surroundings Science - Class VII, NCERT (Revised ed 2025), Exploring Substances: Acidic, Basic, and Neutral, p.18. This energy change is a physical sign that a chemical transformation has occurred.

Another critical reaction pattern involves metal carbonates (like Na₂CO₃) and metal hydrogencarbonates (bicarbonates). When these substances encounter an acid, the reaction is more complex than simple neutralization. They produce salt, water, and carbon dioxide gas. You can identify the presence of this gas because it causes effervescence (fizzing) and will turn lime water milky due to the formation of a white precipitate of calcium carbonate Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21. If you continue to pass CO₂ through that milky lime water, it eventually becomes clear again as soluble calcium hydrogencarbonate forms.

To distinguish between these substances in a lab, we rely on indicators like litmus paper. These chemical 'detectives' change color based on the pH level of the solution. For example, bases like NaOH have the characteristic property of turning red litmus paper blue, while neutral substances like pure water leave the color unchanged Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.34.

Reactants	Primary Products	Visual/Physical Evidence
Acid + Base	Salt + Water	Release of heat (Exothermic)
Acid + Metal Carbonate	Salt + Water + CO₂	Effervescence; gas turns lime water milky
Base + Certain Metals (e.g., Zn)	Complex Salt + H₂	Gas bubbles that burn with a 'pop' sound

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21; Science - Class VII, NCERT (Revised ed 2025), Exploring Substances: Acidic, Basic, and Neutral, p.18; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.34; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20

6. Testing for Alkalinity: The Role of Litmus (exam-level)

In our journey to understand chemistry, we often need a way to "see" the invisible properties of a solution. This is where indicators come in. An indicator is a substance that changes its color when added to an acidic or basic solution. The most common natural indicator used in laboratories is litmus, which is extracted from lichens. It is typically used in the form of small strips of paper, available in two varieties: blue litmus and red litmus Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.10.

To identify alkalinity (the property of being a base), we look for a specific color shift. Basic substances, such as sodium hydroxide (NaOH) or soap solutions, have the unique ability to turn red litmus paper blue. If you were to use blue litmus paper on a base, it would remain blue, providing no visible proof of a reaction. This is a critical distinction for a chemist: to confirm a base, you must start with red litmus. Conversely, substances that are neutral, like pure water (H₂O) or a common salt solution, are neither acidic nor basic and will not change the color of either red or blue litmus paper Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.10, 20.

When you are faced with two clear liquids—for example, distilled water and aqueous sodium hydroxide—they look identical to the naked eye. To tell them apart safely, red litmus is your best tool. In the water, the red paper stays red. In the NaOH solution, the red paper turns a distinct blue. If you had used blue litmus instead, it would have stayed blue in both liquids, leaving you unable to distinguish the neutral water from the basic NaOH Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.18. Beyond litmus, nature provides other indicators like turmeric (which turns red in bases) and red rose extract (which can turn green in basic environments) Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.13, 20.

Substance Nature	Red Litmus Paper	Blue Litmus Paper
Acidic (e.g., Lemon Juice)	Stays Red	Turns Red
Basic (e.g., NaOH)	Turns Blue	Stays Blue
Neutral (e.g., Water)	Stays Red	Stays Blue

Sources: Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.10; Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.13; Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.18; Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.20

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental properties of Acids, Bases, and Salts, this question tests your ability to apply those building blocks to a practical laboratory scenario. The core of this problem lies in identifying the chemical nature of the two liquids: H2O (water) is a neutral substance, while aqueous NaOH (sodium hydroxide) is a strong base. To differentiate between them, you need a reagent that produces a visible, distinct change in one but not the other, which is exactly why we use indicators.

The correct answer is (A) red litmus. In your conceptual study, you learned that bases turn red litmus blue. Therefore, when you dip red litmus paper into the test tube containing NaOH, it will undergo a clear color transformation to blue. In contrast, H2O, being neutral, will result in no color change. This provides the immediate visual evidence required to tell them apart. Reasoning through the traps, blue litmus is ineffective because it remains blue in both neutral and basic solutions, offering no distinction. Similarly, while HCl would react with NaOH in a neutralization reaction, this process is generally invisible to the naked eye without a separate indicator or a thermometer to detect heat, making it an unreliable choice for simple differentiation.

UPSC often includes options like Na2CO3 to test if you can distinguish between two basic substances; since both Na2CO3 and NaOH are basic in nature, they will not react in a way that helps you identify the contents of the tubes. As noted in Science, Class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts, the observable change is the key to solving such identification problems. Always look for the reagent that creates a unique visual signature for the specific functional group or pH level you are testing.