Class 8 › Science › Chapter 9
The Amazing World of Solutes, Solvents, and Solutions
Uniform and non-uniform mixtures, solutions and solubility, why objects float or sink, and density: how to measure mass and volume and how temperature and pressure affect density.
1. Uniform and Non-uniform Mixtures
Uniform
Evenly distributed
Components are evenly mixed and not visible separately. Every part has the same composition (e.g. salt or sugar in water, ORS, mixture of gases in water). Uniform mixtures are called solutions.
Non-uniform
Not evenly distributed
Components can be seen with the naked eye or a magnifying device (e.g. chalk powder, sand or sawdust in water).
- ORS example: every sip tastes the same because sugar and salt are evenly distributed in water.
- Air is also a uniform mixture of gases.
2. Solute, Solvent and Solution
Solution
Solute + Solvent → Solution
- Solid in liquid: the solid is the solute; the liquid is the solvent.
- Liquid in liquid: the component present in smaller amount is the solute; the one in larger amount is the solvent.
- Chashni (gulab jamun syrup): lots of sugar in a little water, yet water is still the solvent and sugar the solute (a solid dissolved in a liquid).
- Heritage: Ayurveda and Siddha use water (and also oils, ghee, milk, hydro-alcoholic extracts) as solvents for medicines. Asima Chatterjee used solvents to extract compounds from medicinal plants (anti-epileptic and anti-malarial drugs); she was the first woman to receive the Shanti Swarup Bhatnagar Award in chemical science.
3. How Much Solute Can a Solvent Dissolve?
| Term | Meaning |
|---|---|
| Unsaturated solution | More solute can still be dissolved at that temperature |
| Saturated solution | No more solute dissolves at that temperature; extra solute settles at the bottom |
| Concentration | Amount of solute present in a fixed quantity of solution (or solvent) |
| Dilute solution | Less solute (relative term) |
| Concentrated solution | More solute (relative term) |
| Solubility | Maximum amount of solute that dissolves in a fixed quantity of solvent (e.g. 100 mL) at a particular temperature |
- Example: one spoon of salt in a tumbler gives a dilute solution compared with two or more spoons. Comparing 2 spoons in 100 mL (1 spoon per 50 mL) with 4 spoons in 50 mL (4 spoons per 50 mL), the 4 spoons in 50 mL solution is more concentrated.
- Saturation depends on temperature: a solution saturated at one temperature can behave as unsaturated at a higher temperature.
4. Effect of Temperature on Solubility
Solids in liquids
Solubility increases
For most substances, solubility increases with temperature. Baking soda: more dissolves in water at 70 °C than at 50 °C, and least at 20 °C.
Gases in liquids
Solubility decreases
More oxygen dissolves in cold water; warm water holds less. Dissolved oxygen sustains aquatic life (plants, fish, other organisms) even though it is present in small amounts.
- Gases dissolve evenly in water, so a gas-in-water mixture is uniform.
- Heritage (Ningel, Manipur): salt is still made traditionally by boiling salt-well water in pans; as water evaporates, salt crystals remain and are shaped into salt cakes.
5. Why Do Objects Float or Sink?
- Generally, objects that float in a liquid are lighter (less dense) than the liquid, and objects that sink are heavier (denser) than it. Examples: husk floats while rice sinks; oil floats on water; sawdust floats while sand sinks.
- "Heavier" here means density, not total weight. A wooden stick and an iron rod of the same size differ in density.
- Note: density is not the only factor that decides whether something floats or sinks. (Example from the exercises: an unpeeled orange floats but a peeled one sinks, since the peel traps air.)
- Heritage: bamboo and wooden logs were used for rafts and boats because they are light, hollow and float.
6. Density
Definition
$$\text{Density} = \dfrac{\text{Mass}}{\text{Volume}}$$
Relative density
$$\text{Relative density} = \dfrac{\text{Density of the substance}}{\text{Density of water at that temperature}}$$
- Density is the mass present in a unit volume of a substance. High density means particles are packed more closely (crowded bus, dense forest).
- Independent of shape and size (for the same substance), but depends on temperature and pressure.
- SI unit: kg/m³. Other units: g/mL and g/cm³ (for liquids and solids).
- Conversion: 1 kg/m³ = 1 g/L = 1 g/1000 mL = 1 g/1000 cm³ (so 1 g/cm³ = 1000 kg/m³).
- Water: 1 mL of water has a mass of about 1 g at room temperature, so 100 mL water ≈ 100 g and its density is about 1 g/mL.
- Relative density has no unit. Example: aluminium block of 27 g and 10 cm³ has density 2.7 g/cm³, so it is 2.7 times denser than water (relative density 2.7).
- Oil packet: 1 litre of oil with a mass of 910 g has density 0.91 g/mL, which is less than water, so oil floats.
- Rule of thumb: density > 1 g/cm³ → sinks in water; density < 1 g/cm³ → floats in water (as a general guide).
- Earth's layers: crust (lightest), upper mantle, lower mantle, outer core (liquid), inner core (solid). Density increases towards the centre because of rising pressure and temperature.
7. Measuring Mass
- Mass: quantity of matter in an object. SI unit kg (also g). Measured with a balance (digital weighing balance, two-pan balance).
- Using a digital balance: switch on, make sure it reads zero (press tare/reset), place a clean dry watch glass or butter paper, tare again, then place the solid and read the mass (e.g. 16.400 g). For a liquid use a beaker in place of the watch glass.
| Mass | Weight | |
|---|---|---|
| Meaning | Quantity of matter | Force with which Earth attracts an object |
| Unit | g, kg | newton (N) |
| Note | Most balances (except two-pan) measure weight but are marked in mass units. | |
8. Measuring Volume
- Volume: space occupied by an object. SI unit m³; smaller units dm³ and cm³ (1 cm³ = 1 cc).
- Liquids: litre (L) = 1 dm³; millilitre (mL) = 1 cm³.
- Measuring cylinder: narrow, tall, transparent cylinder with markings to measure liquid volume; sizes 5, 10, 25, 50, 100, 250, 500 mL. Narrow shape gives more accurate readings.
- Smallest volume it can read (least count): volume between two big marks ÷ number of small divisions. For a 100 mL cylinder: 10 mL ÷ 10 = 1 mL.
| Cylinder capacity | Smallest volume it can measure |
|---|---|
| 10 mL, 25 mL | 0.1 mL |
| 100 mL | 1 mL |
| 250 mL | 2 mL |
| 500 mL | 5 mL |
- Choosing a cylinder: to measure 70 mL, a 50 mL cylinder needs two steps; a 250 or 500 mL cylinder is less accurate; a 100 mL cylinder is best.
- Meniscus: the curved surface of the liquid. Keep the eye level with the meniscus. For water and colourless liquids read at the bottom of the meniscus; for coloured liquids read at the top.
- Regular solid (cuboid): Volume = length × width × height. Example: 25 cm × 18 cm × 2 cm = 900 cm³.
- Irregular solid (displacement method): tie the object with thread and lower it into water in a measuring cylinder. Volume of object = final level − initial level (e.g. 55 − 50 = 5 mL = 5 cm³).
9. Finding the Density of an Object
Worked example: stone
1. Mass using balance = 16.400 g.
2. Volume by displacement = 55 mL − 50 mL = 5 mL = 5 cm³.
3. Density = 16.400 g ÷ 5 cm³ = 3.28 g/cm³ (denser than water, so it sinks).
- Steps: measure mass, measure volume (formula or displacement), then divide mass by volume.
- If density is known, volume = mass ÷ density, and mass = density × volume.
10. Effect of Temperature and Pressure on Density
Temperature
Heating lowers density
On heating, particles move apart, volume increases, mass stays the same, so density decreases; cooling increases density. Hot air is less dense than cool air, so it rises (hot-air balloons).
Pressure
Mainly affects gases
Gases: more pressure → particles closer → volume decreases, density increases. Liquids (nearly incompressible) and solids: negligible effect.
Why ice floats: water has its highest density at 4 °C. On freezing at 0 °C its particles arrange themselves to take up more space (expansion), so ice is less dense than liquid water and floats. The ice layer on top keeps the water below warm enough for fish and other creatures in cold weather.
11. Quick Revision
| Quantity | Formula / Unit | Measured by |
|---|---|---|
| Mass | g, kg | Balance |
| Volume | mL, cm³, L, m³ | Measuring cylinder / formula / displacement |
| Density | Mass ÷ Volume (kg/m³, g/cm³, g/mL) | Calculated |
| Relative density | Density of substance ÷ density of water (no unit) | Calculated |
Remember
Temperature trends
Solids dissolve more in hot water; gases dissolve less. Density of most substances falls as temperature rises.
Exam tip
Density questions
Write the formula, substitute with units, and compare with 1 g/cm³ to decide float or sink (e.g. 225 g and 90 cm³ gives 2.5 g/cm³, so it sinks).
Common mistakes:
• Calling a sand-and-water mixture a solution (it is non-uniform).
• Thinking an unsaturated solution has more solute than a saturated one.
• Mixing up volume (space occupied) and mass (matter present).
• Reading the meniscus from the top for water (read the bottom; top for coloured liquids).
• Saying oxygen is more soluble in hot water (it is less soluble).
• Giving relative density a unit (it has none).
• Forgetting that density does not change with shape or size (flattening clay does not change its density).
• Mixing mass in g with volume in m³ without converting units.
• Calling a sand-and-water mixture a solution (it is non-uniform).
• Thinking an unsaturated solution has more solute than a saturated one.
• Mixing up volume (space occupied) and mass (matter present).
• Reading the meniscus from the top for water (read the bottom; top for coloured liquids).
• Saying oxygen is more soluble in hot water (it is less soluble).
• Giving relative density a unit (it has none).
• Forgetting that density does not change with shape or size (flattening clay does not change its density).
• Mixing mass in g with volume in m³ without converting units.