The Amazing World of Solutes, Solvents, and Solutions Class 8: Solubility and Density

What The Amazing World of Solutes, Solvents, and Solutions is about

Key concepts

Solute, solvent and solution

NCERT section 9.1 What Are Solute, Solvent, and Solution?

A solution is a uniform mixture: its components cannot be seen separately. Sugar or salt in water forms a solution; chalk, sand or sawdust in water forms a non-uniform mixture.

• When a solid dissolves in a liquid, the solid is the solute and the liquid is the solvent: solute + solvent → solution.
• When two liquids mix, the one in smaller amount is the solute and the one in larger amount is the solvent.
• In chashni (sugar syrup for gulab jamun), there is more sugar than water, yet water is still the solvent and sugar the solute, because the solid dissolves in the liquid.
• Gases can form solutions too: air (a uniform mixture of gases) and oxygen dissolved in water.

ORS is a solution of sugar and salt in water, which is why every sip tastes the same.

Worked example

Identify the solute and solvent in (a) ORS, (b) a mixture of 10 mL lemon juice and 200 mL water, (c) chashni. Which mixtures among ORS, muddy water and sawdust in water are solutions?

Try it first, then show the solutionSolution

1. (a) ORS: solutes are sugar and salt; solvent is water.
2. (b) Two liquids: lemon juice (smaller amount) is the solute, water the solvent.
3. (c) Chashni: sugar (solid) is the solute, water the solvent, even though there is more sugar.
4. Only ORS is a solution (uniform); muddy water and sawdust in water are non-uniform.

Answer: ORS: sugar and salt in water; lemon juice in water; sugar in water; only ORS is a solution.

Saturated and unsaturated solutions, concentration and solubility

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

Activity 9.1: add salt spoon by spoon to half a tumbler of water, stirring each time. At first all of it dissolves; after a few spoons some salt stays at the bottom.

• An unsaturated solution can dissolve more solute at that temperature.
• A saturated solution has dissolved the maximum solute it can at that temperature; extra solute settles.

Concentration is the amount of solute in a fixed quantity of solution (or solvent). A dilute solution has less solute, a concentrated one more; these are relative terms. To compare, work out solute per mL: 2 spoons in 100 mL is 0.02 spoon per mL, while 4 spoons in 50 mL is 0.08 spoon per mL, four times as concentrated.

Solubility is the maximum amount of solute that dissolves in a fixed quantity (for example 100 mL) of solvent at a particular temperature.

Our scientific heritage: Ayurveda and Siddha used water, and also hydro-alcoholic extracts, oils, ghee and milk, as solvents for medicines. Asima Chatterjee used solvents to extract compounds from medicinal plants for anti-epileptic and anti-malarial drugs.

Worked example

Glass X holds 100 mL of water with 2 spoons of salt; glass Y holds 50 mL with 4 spoons. Compare their concentrations.

Try it first, then show the solutionSolution

1. First: $\frac{2}{100}=0.02$ spoon per mL.
2. Second: $\frac{4}{50}=0.08$ spoon per mL.
3. $\frac{0.08}{0.02}=4$, so the second is four times as concentrated.

Answer: Glass Y is four times as concentrated as glass X.

Temperature and solubility of solids

NCERT section 9.2.1 How does temperature affect the solubility of a solute?

Activity 9.2 (demonstration): baking soda (sodium hydrogen carbonate) is added to 50 mL of water at 20 °C until some stays undissolved. Heating to 50 °C dissolves it, and more can be added until it again stays undissolved; heating to 70 °C dissolves that too.

So for most solids, solubility increases with temperature. A solution that is saturated at one temperature becomes unsaturated when heated, because it can now dissolve more.

This is why sugar dissolves faster and in larger amounts in hot tea than in cold lemonade. On cooling a hot saturated solution, some solute may come out again.

Worked example

Some baking soda lies undissolved at the bottom of a saturated solution at 20 °C. What happens when the beaker is warmed to 50 °C, and what does this show?

Try it first, then show the solutionSolution

1. At 50 °C, the undissolved baking soda dissolves.
2. The solution, saturated at 20 °C, is now unsaturated.
3. So the solubility of baking soda increases with temperature.

Answer: It dissolves; solubility of the solid rises with temperature.

Solubility of gases

NCERT section 9.3 Solubility of Gases

Many gases dissolve in water, including oxygen, but only to a small extent. This tiny amount of dissolved oxygen keeps aquatic plants, fish and other organisms alive. A gas dissolved in water forms a uniform mixture (a solution).

Temperature works the other way for gases: the solubility of gases generally decreases as temperature increases. Cold water holds more oxygen; when water warms, less oxygen stays dissolved, which can stress aquatic life.

So: in liquids, the solubility of solids usually increases with temperature, while that of gases decreases.

Worked example

Why might fish in a shallow pond struggle on a very hot summer afternoon?

Try it first, then show the solutionSolution

1. Fish depend on oxygen dissolved in the water.
2. Warm water can hold less dissolved oxygen, because gas solubility decreases as temperature rises.
3. So the water may not have enough oxygen for the fish.

Answer: Hot water holds less dissolved oxygen.

Density

NCERT section 9.5 What Is Density?

An iron rod feels heavier than a wooden stick of the same size: iron has a greater density. A crowded bus or a dense forest is a picture of high density.

Density is the mass present in a unit volume of a substance:
$\text{density}=\frac{\text{mass}}{\text{volume}}$

• It does not depend on the shape or size of the sample, but it does depend on temperature (and, for gases, pressure).
• SI unit: kg/m³; for convenience g/cm³ or g/mL (1 mL $=$ 1 cm³).
• 1 mL of water has a mass of about 1 g, so water's density is about 1 g/mL.
• An aluminium block of mass 27 g and volume 10 cm³ has density 2.7 g/cm³.

Relative density (with respect to water) $=\frac{\text{density of substance}}{\text{density of water}}$; it has no unit. Aluminium's is 2.7.

A 1-litre oil pack marked 910 g has oil of density $\frac{910\text{ g}}{1000\text{ mL}}=0.91$ g/mL, less than water, so oil floats. Density is one important factor (not the only one) in floating and sinking.

Worked example

A metal block measures 5 cm by 4 cm by 2 cm and has a mass of 108 g. Find its volume, density and relative density. Will it float in water?

Try it first, then show the solutionSolution

1. Volume $=5\times 4\times 2=40$ cm³.
2. Density $=\frac{108}{40}=2.7$ g/cm³.
3. Relative density $=\frac{2.7}{1}=2.7$; it is denser than water, so it sinks.

Answer: 40 cm³; 2.7 g/cm³; relative density 2.7; it sinks.

Measuring mass and volume

NCERT section 9.5.1 Determination of density

Mass (Activity 9.3): switch on a digital balance; if it does not read zero, press tare (reset). Place a clean watch glass or butter paper, press tare again so its mass is ignored, then put the object on it and read the mass. (Most balances actually sense weight but show mass units.)

Volume of liquids (Activities 9.4, 9.5): use a measuring cylinder.

• Find its capacity and least count: volume between big marks $÷$ number of small divisions (for a 100 mL cylinder with 10 divisions between 10 mL marks, 1 mL).
• Choose a suitable size: for 70 mL a 100 mL cylinder is best (a 50 mL one needs two steps; a 250 mL or 500 mL one is less precise).
• Keep the cylinder on a flat surface, eyes level with the liquid; read the bottom of the meniscus for water and colourless liquids, the top for coloured liquids. Use a dropper to adjust.
• Cylinders are tall and narrow so that small volume changes give big height changes, making readings more precise.

Volume of regular solids (Activity 9.6): $V=l\times w\times h$; a notebook 25 cm by 18 cm by 2 cm is 900 cm³.

Volume of irregular solids (Activity 9.7), displacement: note the water level, lower the object (on a thread) into the water, note the new level; the rise is the object's volume (mL = cm³).

Worked example

Water in a measuring cylinder stands at 50 mL. A metal key tied to a thread is lowered in and the level rises to 53 mL. The key's mass is 24.3 g. Find its volume and density.

Try it first, then show the solutionSolution

1. Volume $=53-50=3$ mL $=3$ cm³.
2. Density $=\frac{24.3}{3}=8.1$ g/cm³.
3. Check: $8.1\times 3=24.3$ g.

Answer: Volume 3 cm³; density 8.1 g/cm³.

Temperature, pressure and density; why ice floats

NCERT section 9.5.2 Effect of temperature on density

Temperature (9.5.2): on heating, particles of a solid, liquid or gas move apart, so the volume increases while the mass stays the same: density decreases. Cooling does the opposite. That is why hot air rises above cooler, denser air, and how hot-air balloons fly.

Pressure (9.5.3): squeezing a gas pushes particles closer: volume falls, density rises. Liquids are nearly incompressible and solids even less so, so pressure hardly changes their density.

Earth's layers: density increases from the light crust towards the core, where pressure and temperature are very high.

Ice floats: water is densest at 4 °C. When it freezes at 0 °C, its particles arrange to take up more space (expansion), so ice is less dense than water and floats. A floating ice layer keeps the water below warm enough for fish in very cold weather.

Floating: objects less dense than a liquid tend to float in it (husk, oil, wood, bamboo rafts); denser ones sink (rice, sand, iron). A raw egg sinks in tap water but can be made to float by dissolving enough salt to make the water denser.

Worked example

A raw egg sinks in a glass of tap water. Suggest how to make it float and explain using density.

Try it first, then show the solutionSolution

1. Dissolve salt in the water, stirring, a little at a time.
2. Salt increases the mass in the same volume, so the solution becomes denser.
3. When the salt water becomes denser than the egg, the egg floats.

Answer: Add salt to make the water denser than the egg.

Science in real life: The Amazing World of Solutes, Solvents, and Solutions

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Density, Measuring Mass and Volume, and Floating
• Solutes, Solvents, Solubility and Saturated Solutions

The Amazing World of Solutes, Solvents, and Solutions: formulas and key facts

Solution

$\text{solute}+\text{solvent}\to \text{solution}$

Concentration (comparison)

$\text{concentration}=\frac{\text{amount of solute}}{\text{volume of solution (or solvent)}}$

Density

$\rho =\frac{m}{V}$

Relative density

$\text{RD}=\frac{\text{density of substance}}{\text{density of water}}$

No unit.

Units

$1\text{ mL}=1{\text{ cm}}^3;1\text{ L}=1{\text{ dm}}^3;1{\text{ g/cm}}^3=1000{\text{ kg/m}}^3$

Volume of a cuboid

$V=l\times w\times h$

Volume by displacement

$V_{\text{object}}=V_{\text{final}}-V_{\text{initial}}$

Least count of a measuring cylinder

$\frac{\text{volume between big marks}}{\text{number of small divisions}}$

Temperature effects

$T\uparrow :\text{solubility of solids}\uparrow ,\text{solubility of gases}\downarrow ,\text{density}\downarrow$

The Amazing World of Solutes, Solvents, and Solutions: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks The Amazing World of Solutes, Solvents, and Solutions

The question patterns to expect, and the method that works for each.

Tricks to remember The Amazing World of Solutes, Solvents, and Solutions

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Solid solute, liquid solvent

  The thing that dissolves is the solute; the thing that does the dissolving is the solvent.

  Use it when: Naming parts of a solution.

• Saturated = full

  Like a full sponge, a saturated solution cannot take any more at that temperature.

  Use it when: Saturated versus unsaturated.

• Hot helps solids, hurts gases

  Warming dissolves more sugar but drives out oxygen.

  Use it when: Temperature effects on solubility.

• D = M over V

  Density is 'mass per volume': draw the triangle M on top, D and V below.

  Use it when: Density calculations.

• Bottom for clear, top for coloured

  Read the meniscus bottom for water, top for coloured liquids.

  Use it when: Measuring cylinders.

The Amazing World of Solutes, Solvents, and Solutions: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in The Amazing World of Solutes, Solvents, and Solutions

The slips examiners see most often, and what to write instead.

• ✗Mistake: Calling water the solute in sugar syrup

  There is more sugar than water.

  Wrong working

  In chashni, sugar is the solvent because there is more of it.

  Right working

  Sugar (the solid) dissolves in water (the liquid), so water is the solvent.

  ✓Do this: For a solid in a liquid, the liquid is always the solvent.

  Usually costs: 1 mark.

• ✗Mistake: Saying gases dissolve more in hot water

  The solid rule is applied to gases.

  Wrong working

  Oxygen is more soluble in hot water.

  Right working

  Gas solubility decreases with temperature; cold water holds more oxygen.

  ✓Do this: Solids up, gases down with temperature.

  Usually costs: 1 mark.

• ✗Mistake: Forgetting units or using the wrong ones for density

  Mass and volume units are mixed.

  Wrong working

  Density of a 24.3 g key with volume 3 mL is 8.1.

  Right working

  8.1 g/cm³ (or g/mL); in SI, 8100 kg/m³.

  ✓Do this: Always write the unit; 1 g/cm³ = 1000 kg/m³.

  Usually costs: 1 mark.

• ✗Mistake: Reading the top of the meniscus for water

  The edge looks like the level.

  Wrong working

  Reading the curved edge where water climbs the glass.

  Right working

  For water, read the bottom of the meniscus with the eye level with it.

  ✓Do this: Bottom for colourless liquids, top for coloured ones.

  Usually costs: 1 mark.

• ✗Mistake: Thinking density changes with size

  Bigger pieces feel heavier.

  Wrong working

  A big iron block has a higher density than a small iron nail.

  Right working

  Both are iron; density is the same, only mass and volume differ.

  ✓Do this: Density depends on the material (and temperature), not size or shape.

  Usually costs: 1 mark.

NCERT exercise guide: The Amazing World of Solutes, Solvents, and Solutions

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on The Amazing World of Solutes, Solvents, and Solutions

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The SI unit of density is
   (a) g/cm³
   (b) kg/m³
   (c) g/mL
   (d) kg

   Hint

   SI units of mass and volume.

   Answer

   (b) kg/m³

2. Q2 · 1 mark · MCQExam level

   Chalk powder stirred into water is
   (a) a solution
   (b) a saturated solution
   (c) a non-uniform mixture
   (d) a uniform mixture

   Hint

   The chalk settles.

   Answer

   (c) a non-uniform mixture

3. Q3 · 1 mark · Very short answerExam level

   What is the volume of a notebook 25 cm long, 18 cm wide and 2 cm thick?

   Hint

   $l\times w\times h$.

   Answer

   900 cm³

4. Q4 · 2 marks · Very short answerExam level

   Name the curved surface of water in a measuring cylinder and state where to read it.

   Hint

   Colourless liquid.

   Answer

   Meniscus; read at the bottom of the meniscus with the eye level with it.

5. Q5 · 2 marks · Very short answerHarder exam level

   A block of mass 72.9 g is a cube of side 3 cm. Find its density.

   Hint

   Volume $3^3$.

   Answer

   Volume 27 cm³; density $\frac{72.9}{27}=2.7$ g/cm³.

6. Q6 · 2 marks · Very short answerHarder exam level

   How much mass of a liquid of density 0.92 g/mL is there in 500 mL? And of a liquid of density 1.2 g/mL in 250 mL?

   Hint

   $m=\rho V$.

   Answer

   $0.92\times 500=460$ g; $1.2\times 250=300$ g.

7. Q7 · 2 marks · Very short answerHarder exam level

   Why does a saturated solution at room temperature become unsaturated on heating?

   Hint

   Solubility of solids.

   Answer

   For most solids, solubility increases with temperature, so the hot solution can dissolve more solute than it holds.

8. Q8 · 3 marks · Short answerHarder exam level

   Explain why gas density changes a lot with pressure but solid density hardly changes.

   Hint

   Spaces between particles.

   Answer

   Gas particles have large spaces between them, so pressure pushes them closer, reducing volume and raising density. Solid particles are already very close, so pressure barely changes their volume or density (liquids are nearly incompressible too).

9. Q9 · 3 marks · Short answerHarder exam level

   Correct the false statements: (i) Sugar dissolves more in cold water than in hot water. (ii) Density is the mass of an object. (iii) A dilute solution has more solute in the same volume than a concentrated one.

   Hint

   Definitions.

   Answer

   (i) For most solids, including sugar, solubility increases with temperature, so more dissolves in hot water. (ii) Density is mass per unit volume, not the mass itself. (iii) A concentrated solution has more solute in the same volume; a dilute one has less.

10. Q10 · 5 marks · Long answerHarder exam level

    Describe how to determine the density of an irregular stone and explain each measurement, then calculate the density for mass 39.5 g and a level change from 50 mL to 55 mL.

    Hint

    Balance and displacement.

    Answer

    Mass: tare a digital balance with a watch glass, then weigh the stone: 39.5 g. Volume: note the water level (50 mL), lower the stone on a thread, note the new level (55 mL); volume = 5 cm³. Read at the bottom of the meniscus. Density $=\frac{39.5}{5}=7.9$ g/cm³ (relative density 7.9).

11. Q11 · 3 marks · Short answerHOTS / competency

    A 1 L bottle of water is completely filled and frozen in a freezer. Predict what may happen to the bottle and explain.

    Hint

    Expansion on freezing.

    Answer

    The bottle may bulge or crack. Water expands when it freezes (ice is less dense, so the same mass takes up more volume), and a full bottle has no space for the extra volume.

12. Q12 · 3 marks · Short answerHOTS / competency

    100 mL of water (100 g) dissolves 25 g of sugar, giving 115 mL of solution. Find the density of the solution and compare it with water. Would an object of density 1.05 g/mL float in it?

    Hint

    Total mass over total volume.

    Answer

    Density $=\frac{125}{115}\approx 1.09$ g/mL, more than water's 1 g/mL. An object of 1.05 g/mL is less dense than the solution, so it floats (though it would sink in plain water).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is a solution?Show answer

   A uniform mixture of two or more substances.

2. 2.In a solution of two liquids, which is the solute?Show answer

   The one present in the smaller amount.

3. 3.What is a saturated solution?Show answer

   One in which no more solute can dissolve at that temperature.

4. 4.What does concentration mean?Show answer

   The amount of solute present in a fixed quantity of solution or solvent.

5. 5.How does temperature affect the solubility of gases in water?Show answer

   It generally decreases as temperature increases.

6. 6.Define density.Show answer

   Mass per unit volume of a substance.

7. 7.What is the SI unit of density?Show answer

   kg/m³.

8. 8.What is relative density?Show answer

   The density of a substance divided by the density of water; it has no unit.

9. 9.Where do you read the volume of water in a measuring cylinder?Show answer

   At the bottom of the meniscus, with the eye level with it.

10. 10.At what temperature is water densest?Show answer

    4 °C.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you name the solute and solvent in ORS, chashni and lemonade?
• ☐Can you tell a saturated from an unsaturated solution?
• ☐Can you compare concentrations given in different volumes?
• ☐Can you state how temperature affects solubility of solids and gases?
• ☐Can you calculate density with correct units?
• ☐Can you convert g/cm³ to kg/m³?
• ☐Can you find the least count of a measuring cylinder?
• ☐Can you find the volume of a stone by displacement?
• ☐Can you explain why hot air rises?
• ☐Can you explain why ice floats on water?

The Amazing World of Solutes, Solvents, and Solutions: frequently asked questions