The Amazing World of Solutes, Solvents, and Solutions

9.1 Solute, Solvent and Solution

A solution is a homogeneous mixture. In saltwater, salt is the solute and water the solvent. In liquid–liquid solutions, the smaller-quantity component is often described as the solute and the larger as the solvent. Sugar syrup remains a solution even when its sugar mass exceeds water mass. Air is a homogeneous gaseous mixture. Sand and water or sawdust and water are non-uniform mixtures, not true solutions.

9.2 How Much Solute Can a Fixed Amount of Solvent Dissolve?

An unsaturated solution can dissolve more of a particular solute at a given temperature. A saturated solution has reached its solubility limit at that temperature; excess solute may remain undissolved. Concentration is amount of solute per specified amount of solvent or solution. Dilute and concentrated are relative, whereas saturated and unsaturated refer to the solubility limit. Four spoons in 50 mL water is more concentrated than two spoons in 100 mL, assuming equal spoon size and complete dissolution.

9.2.1 Effect of Temperature on Solubility

The chapter's baking-soda demonstration compares water near 20 °C, 50 °C and 70 °C. More baking soda can dissolve as temperature rises in this example. Many solids become more soluble in warmer water, but not all do. A saturated solution at one temperature may become unsaturated after heating. Heating must be performed by a teacher; never taste laboratory solutions.

9.3 Solubility of Gases

Oxygen dissolves in water and supports aquatic life. Dissolved gas solubility generally decreases as water temperature increases, which can stress aquatic organisms. This differs from the behaviour of many solid solutes. Cold water can generally hold more dissolved oxygen than warm water under comparable conditions.

9.4 Why Objects Float or Sink

A substance or object's average density compared with the surrounding fluid helps predict floating. Oil often floats on water because it is less dense and immiscible. A hollow boat can float even when made of metal because the overall boat-and-air volume displaces sufficient water. Density is not the only consideration: buoyancy, shape and trapped air also matter.

9.5 Density and Units

Density ρ=m/V, where m is mass and V is volume. SI unit kg/m³; convenient units g/mL and g/cm³. 1 mL=1 cm³ and 1 g/cm³=1000 kg/m³. The textbook's aluminium example: 27 g / 10 cm³=2.7 g/cm³. Relative density with respect to water is density of sample divided by density of water at the same temperature; it has no unit. Oil marked 910 g per litre has density 0.91 g/mL.

9.5.1 Measuring Mass

Zero or tare a digital balance with the empty watch glass or container in place; then add the sample and record the displayed mass. The textbook's stone example is 16.400 g. Scientific mass is measured in g or kg; weight is a gravitational force in newtons. Many electronic balances sense force but display calibrated mass.

Measuring Volume and Meniscus

Use a graduated cylinder of appropriate capacity and scale. The textbook's 100 mL cylinder has 10 subdivisions between 10 mL and 20 mL: least count 1 mL. To measure 70 mL in one step, a 100 mL cylinder is preferable to a less finely graduated 250 or 500 mL cylinder. Read the bottom of the water meniscus at eye level; the chapter instructs the top for the illustrated coloured-liquid case. Follow the actual liquid and instrument's measurement convention.

Regular and Irregular Object Volumes

A cuboid's volume is length × width × height: 25 cm × 18 cm × 2 cm=900 cm³. For an irregular non-porous object fully immersed without trapped bubbles, displaced volume equals object volume. The textbook gives initial water 50 mL and final 55 mL, so volume=5 mL=5 cm³. With mass 16.400 g, density=16.400/5=3.28 g/cm³.

9.5.2 Effect of Temperature on Density

For many substances, heating increases volume at nearly unchanged mass, decreasing density. Warm air is generally less dense than nearby cooler air at comparable pressure, explaining buoyant hot-air balloons. Water is a special case: liquid water has maximum density near 4 °C, and ice is less dense than liquid water, so ice floats.

9.5.3 Effect of Pressure on Density

Increasing pressure can compress gases substantially, increasing their density. Liquids and solids are far less compressible, so their density changes much less under ordinary pressure changes. Density depends on conditions, not just a material name.

Summary Comparison Table

Measurement and Formula Table

Activities 9.1–9.7: Observation and Reasoning

Activity 9.1: add successive teaspoons of salt to fixed water and note the first undissolved residue; do not invent the number of spoons. 9.2: teacher heats saturated baking-soda mixtures and observes temperature dependence. 9.3: tare a balance and measure an object. 9.4: calculate the cylinder's least count (1 mL in the pictured example). 9.5: measure 50 mL using the correct meniscus and eye level. 9.6: calculate cuboid volume from measured dimensions. 9.7: immerse an irregular solid and subtract initial from final water level. For each activity, write aim, apparatus, safe method, actual observation, explanation and conclusion.

Scientific Heritage and Applications

The chapter discusses Asima Chatterjee's research using solvent extraction of medicinal plants and Ningel village in Manipur, where traditional salt production involves evaporation of saline water. Research the Dead Sea's high salinity and limited aquatic life; it is not completely devoid of microbial life. Traditional medicinal claims should be distinguished from clinically established efficacy.

Common Mistakes and Diagram Guidance

Do not confuse density with mass, saturation with concentration, or a uniform solution with a non-uniform suspension. Solubility of gases usually decreases with warming, unlike many solids. Read meniscus at eye level to avoid parallax. For Fig. 9.25, orange peel contains air pockets that lower average density; removing it can cause sinking. Fig. 9.26 shows heated water expanding in a tube, indicating decreased density if mass remains constant.

Keep the Curiosity Alive — all 12 textbook questions with solutions

Additional original practice questions and solutions

MCQs, HOTS and revision

MCQ 1: Which is a homogeneous mixture: sand and water or saltwater? Saltwater. MCQ 2: Which is more soluble in cold water: oxygen gas or the same gas in hot water? Usually cold water. HOTS: Why does a hollow metal ship float? It displaces enough water to balance its weight because its overall average density including enclosed air is low enough. Exam tip: Always include units with density, distinguish mass from weight, and show displacement subtraction.

Discover, Design and Debate

Research why the Dead Sea is unusually saline, compare salt dissolution in water, vinegar and oil with teacher-approved materials, and debate whether water is the most versatile solvent. Never taste experimental mixtures or heat chemicals without trained supervision. For research, distinguish high salinity from the incorrect claim that the Dead Sea contains no microbial life.

Source and figures

Official NCERT Chapter 9 PDF. Consult Figures 9.1–9.26 for source illustrations, especially meniscus reading, water displacement, floating oranges and heated water in a tube. Illustrations are described here rather than reproduced. All worked numbers in textbook solutions follow the source exercise values.