A solution contains 20 g of solute in 180 g of solvent. If the solvent is water, what is the concentration of the solution in terms of mass by mass percentage ?

1. Matter around us: Pure Substances and Mixtures (basic)

Welcome to your first step in mastering chemical principles! To understand the world around us, we must first look at what things are made of. In science, everything you touch, breathe, or eat is matter—composed of tiny, discrete particles. While we might think of "pure" as meaning "healthy" or "natural" (like pure honey), a scientist uses the term much more strictly. Understanding the distinction between pure substances and mixtures is the foundation of chemistry. Science, Class VIII NCERT, Nature of Matter, p.117

A pure substance is a form of matter that consists of only one type of particle. These particles can be atoms or molecules, but they are identical throughout the substance. Because they are uniform at the particle level, pure substances cannot be separated into other kinds of matter using physical methods like filtration or hand-picking. For example, distilled water (H₂O) is pure because every single particle is a water molecule. Science, Class VIII NCERT, Nature of Matter, p.121

On the other hand, a mixture contains two or more different substances mixed together without a chemical bond. Mixtures are classified based on how well their components blend:

• Uniform (Homogeneous) Mixtures: The components are so well mixed that you cannot see them separately, even with a microscope. A sugar-water solution is a classic example—once dissolved, the sugar particles are evenly distributed throughout the water. Science, Class VIII NCERT, Nature of Matter, p.117
• Non-uniform (Heterogeneous) Mixtures: The components remain distinct and are often visible to the naked eye. Think of a sprout salad or a mixture of iron filings and sulfur powder where you can clearly see the different parts. Science, Class VIII NCERT, Nature of Matter, p.117, 128

Sources: Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.117; Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.121; Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.128

2. Components of a Solution: Solute and Solvent (basic)

When we look at a glass of sugar syrup or even the air we breathe, we are looking at solutions. A solution is a uniform (homogeneous) mixture of two or more substances. To understand how they work, we must distinguish between the two essential components: the solute and the solvent. Think of the solvent as the "host" and the solute as the "guest."

The simplest way to identify them is by their physical state and quantity. In a solution where a solid is dissolved in a liquid (like salt in water), the solid is the solute and the liquid is the solvent Science, Class VIII, p.135. However, when we mix two substances of the same state—such as two liquids or two gases—we use the quantity rule: the component present in the larger amount is the solvent, while the one in the smaller amount is the solute Science, Class VIII, p.149.

It is also important to recognize that there is a limit to how much "guest" a "host" can hold. At a specific temperature, the maximum amount of solute that can dissolve in a fixed quantity of solvent is known as its solubility Science, Class VIII, p.137. If a solution has reached this limit and can no longer dissolve more solute, we call it a saturated solution. If it can still take in more, it is unsaturated.

Sources: Science, Class VIII (NCERT), The Amazing World of Solutes, Solvents, and Solutions, p.135; Science, Class VIII (NCERT), The Amazing World of Solutes, Solvents, and Solutions, p.137; Science, Class VIII (NCERT), The Amazing World of Solutes, Solvents, and Solutions, p.149

3. Solubility and Saturation Levels (intermediate)

When we mix a substance like salt into water, we aren't just making a mixture; we are navigating the limits of chemistry. At first, the salt disappears easily, forming an unsaturated solution, which simply means the solvent (water) has the capacity to hold more solute (salt) Science, Class VIII, NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.137. However, if you keep adding salt, you will eventually notice crystals settling at the bottom no matter how much you stir. This state is called a saturated solution—the point where the solvent has reached its maximum capacity to dissolve that specific solute at a given temperature.

The term solubility refers to this specific limit: the maximum amount of a solute that can dissolve in a fixed quantity (usually 100 g or 100 mL) of solvent at a specific temperature Science, Class VIII, NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.149. It is important to note that temperature acts as a "volume expander" for solubility. Generally, as you increase the temperature of a liquid, its particles move more vigorously, creating more space to accommodate solute particles. Thus, a solution that is saturated at room temperature often becomes unsaturated again if you heat it up Science, Class VIII, NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.138.

To mathematically express how "strong" or concentrated a solution is, we often use the Mass by Mass Percentage (w/w%). This is a critical calculation for any chemistry student. The formula is:

Concentration (%) = (Mass of Solute / Total Mass of Solution) × 100

A common pitfall is dividing the solute by the weight of the solvent alone. Remember: the Mass of Solution = Mass of Solute + Mass of Solvent. For example, if you dissolve 20g of salt in 180g of water, the total mass of your solution is 200g, making the concentration 10% (20/200 × 100).

Sources: Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.137; Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.138; Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.149

4. Separating Components of a Mixture (intermediate)

In our journey through chemistry, we often encounter substances in their mixed form. However, to study a substance's properties or use it effectively in industry, we need it in its pure state. The process of separating components from a mixture is essentially a search for purity Science, Class VIII NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.120. Whether we are filtering tea leaves at home or removing impurities like gangue from metal ores in a factory, the principle remains the same: we exploit the differences in physical or chemical properties between the components Science, class X NCERT (2025 ed.), Metals and Non-metals, p.51.

To master this, we first need to understand the relationship between the parts and the whole. In a solution, the solute is the substance being dissolved (often in a smaller quantity), while the solvent is the medium doing the dissolving Science, Class VIII NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.136. For example, in the thick chashni (sugar syrup) used for Gulab Jamun, sugar is the solute and water is the solvent. When we want to quantify exactly how much solute is present in a mixture, we calculate its concentration.

One of the most common ways to express this is the Mass by Mass Percentage (w/w%). This measures the mass of the solute relative to the total mass of the entire solution. It is vital to remember that the solution's mass is the sum of both the solute and the solvent. The formula is expressed as:

Mass % = (Mass of Solute / Total Mass of Solution) × 100

In industrial settings, such as dairy farming or ore refining, these principles of separation and concentration are critical. For instance, processes like pasteurization and preservation allow for the safe transport and storage of milk products over long distances by managing the components of the milk mixture Fundamentals of Human Geography, Class XII NCERT (2025 ed.), Primary Activities, p.30.

Sources: Science, Class VIII NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.120; Science, class X NCERT (2025 ed.), Metals and Non-metals, p.51; Science, Class VIII NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.136; Fundamentals of Human Geography, Class XII NCERT (2025 ed.), Primary Activities, p.30

5. Methods of Expressing Concentration (Mass %) (exam-level)

When we talk about the concentration of a solution, we are essentially asking: "How much 'stuff' (solute) is packed into this mixture?" In chemistry and environmental science, one of the most fundamental ways to express this is the Mass Percentage (w/w%). This method is highly reliable because, unlike volume, mass does not change with temperature or pressure. As we understand from Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.149, a solution is a uniform mixture where a solute is dissolved in a solvent. To find the mass percentage, we calculate the ratio of the solute's mass to the total mass of the entire solution.

The most critical step—and where many students stumble—is defining the Total Mass of the Solution. A solution isn't just the liquid you started with; it is the sum of both the solute and the solvent. For instance, if you dissolve 20g of salt in 180g of water, the total mass of your solution is 200g. If you only used the mass of the water in your denominator, your concentration would be mathematically incorrect. This principle of "parts per total" is seen across disciplines, from calculating atmospheric gas proportions Physical Geography by PMF IAS, Earths Atmosphere, p.271 to determining the salinity of soil in degraded lands Geography of India, Majid Husain, Contemporary Issues, p.116.

To calculate the Mass Percentage, use this standard formula:

For a practical example, consider a scenario where you have 25g of sugar dissolved in 225g of water. The total mass is 250g. The calculation would be (25 / 250) × 100, resulting in a 10% (w/w) solution. Understanding this ratio is vital for grasping concepts like solubility—the maximum amount of solute that can be dissolved in a fixed quantity of solvent at a specific temperature Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.149.

Sources: Science, Class VIII NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.141, 149; Physical Geography by PMF IAS, Earths Atmosphere, p.271; Geography of India by Majid Husain, Contemporary Issues, p.116

6. Solving the Original PYQ (exam-level)

Now that you have mastered the definitions of matter and mixtures, this question tests your ability to apply the Mass by Mass Percentage formula in a practical scenario. The core concept here is recognizing the relationship between the solute (the substance being dissolved) and the solvent (the medium), which together form the total solution. As covered in NCERT Class 9 Science, the concentration of a solution is not just about the solute alone, but how that solute compares to the entire weight of the mixture.

To solve this, think like a strategist: first, identify your components. You have 20 g of solute and 180 g of solvent (water). The most critical step is calculating the Total Mass of the Solution by adding them together (20 g + 180 g = 200 g). Applying the formula (20 / 200) × 100, the zeroes cancel out, leaving you with 20 / 2, which equals the correct answer (C) 10%. This systematic approach ensures you don't skip the foundational step of defining the total mixture before calculating the ratio.

UPSC often includes "trap" answers to catch students who rush their reasoning. Option (A) 11.1% is the most common pitfall; it occurs if you mistakenly divide the solute by the solvent (20 / 180) rather than the total solution. Option (D) 20% is a distractor designed for those who might confuse the mass of the solute with the final percentage. Always remember: in concentration problems, the denominator must represent the total mass of the final solution to reflect the true weight-by-weight percentage.