Is Matter Around Us Pure? — NCERT Solutions, AI Tutor & Practice

Understand: In everyday language 'pure' means clean or unadulterated, but in science it has a stricter meaning.

Definition: A pure substance is a substance that is made up of only one type of particle (atoms or molecules) throughout. It has a fixed composition and fixed properties (like a definite melting point and boiling point).

Examples:

1. Gold (an element made of only gold atoms)
2. Water (a compound made of only $H_2O$ molecules)

Conclude: Any material containing only one kind of particle is a pure substance.

Understand: A mixture contains two or more pure substances that are not chemically combined.

Homogeneous mixture:

• Has a uniform composition throughout.
• Components are not visible separately; no boundary between them.
• Example: salt dissolved in water (a solution).

Heterogeneous mixture:

• Has a non-uniform composition.
• Components are visible separately and have visible boundaries.
• Example: a mixture of sand and iron filings.

Conclude: The key difference is uniformity — homogeneous mixtures are uniform, heterogeneous mixtures are not.

Given: Mass of solute (salt) $= 40\ \text{g}$, mass of solvent (water) $= 320\ \text{g}$.

Formula: $\text{Mass \% of solute} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100$

Step 1 — Mass of solution: $\text{Mass of solution} = 40 + 320 = 360\ \text{g}$

Step 2 — Substitute: $\text{Mass \%} = \frac{40}{360} \times 100 = 11.11\%$

Conclude: The mass percentage of salt in the solution is about $11.1\%$.

Understand: A colloid is a type of mixture that lies between a true solution and a suspension.

Definition: A colloid is a heterogeneous mixture in which the particle size is between $1\ \text{nm}$ and $1000\ \text{nm}$ — too small to be seen individually but large enough to scatter light.

Two components:

1. Dispersed phase — the particles that are spread throughout (like the solute).
2. Dispersing medium — the medium in which the particles are spread (like the solvent).

Tyndall effect: Colloidal particles are large enough to scatter a beam of light, making the path of light visible. This scattering is called the Tyndall effect.

Conclude: Colloids appear homogeneous but are actually heterogeneous and show the Tyndall effect.

Understand: Oil and water do not mix and form two separate layers, with the denser liquid (water) at the bottom.

Method — Using a separating funnel:

1. Pour the mixture into a separating funnel and let it stand.
2. The mixture separates into two distinct layers based on density — water below, oil above.
3. Open the stopcock to let the lower layer (water) run out into a beaker.
4. Close the stopcock as soon as the oil reaches the opening.
5. Collect the oil separately in another beaker.

Principle: Immiscible liquids of different densities settle into separate layers, which can be drained one at a time.

Conclude: A separating funnel efficiently separates immiscible liquids like oil and water.

Understand: The technique used is crystallisation, which separates a pure solid from its solution.

Procedure:

1. Dissolve the impure copper sulphate in the minimum amount of hot water in a china dish.
2. Filter the solution to remove insoluble impurities.
3. Heat the filtrate gently on a water bath to concentrate it until it becomes a saturated solution.
4. Allow the solution to cool slowly and undisturbed.
5. Pure copper sulphate separates out as crystals, while soluble impurities remain in the solution (mother liquor).
6. Filter and dry the crystals.

Why crystallisation is preferred over evaporation: Evaporation may decompose some solids and cannot remove soluble impurities; crystallisation gives purer crystals.

Conclude: Crystallisation gives pure copper sulphate crystals.

Understand:

• Element = one type of atom.
• Compound = two or more elements chemically combined in a fixed ratio.
• Mixture = two or more substances physically mixed.

Classification:

| Substance | Type | Reason | |-----------|------|--------| | (a) Air | Mixture | Contains nitrogen, oxygen, etc., not chemically combined | | (b) Sodium | Element | Made of only sodium atoms | | (c) Carbon dioxide | Compound | Carbon and oxygen chemically combined ($CO_2$) | | (d) Soda water | Mixture | $CO_2$ dissolved in water, not chemically bonded |

Conclude: Correct classification requires checking whether the components are chemically combined or just mixed.

Understand: The three components have different properties: ammonium chloride sublimes, salt is soluble in water, and sand is insoluble.

Step 1 — Sublimation: Heat the mixture. Ammonium chloride sublimes (turns to vapour) and can be collected on a cool surface. This removes ammonium chloride.

Step 2 — Dissolving: Add water to the remaining mixture of sand and salt. Salt dissolves; sand does not.

Step 3 — Filtration: Filter the mixture. Sand remains on the filter paper (residue); salt solution passes through (filtrate).

Step 4 — Evaporation/Crystallisation: Evaporate the salt solution to obtain common salt.

Conclude: Sublimation, filtration and evaporation together separate all three components.

Understand: Solubility depends on the amount of solute a solvent can dissolve at a given temperature.

Saturated solution: A solution in which no more solute can be dissolved at a given temperature is called a saturated solution.

Unsaturated solution: A solution that can still dissolve more solute at that temperature.

Making unsaturated → saturated without adding solute: Since solubility usually increases with temperature, we can:

1. Cool the solution — at a lower temperature the solvent can hold less solute, so the same amount of dissolved solute may now be enough to saturate it.

Alternatively, evaporating some solvent reduces the amount of solvent, so the existing solute saturates it.

Conclude: Cooling or evaporating the solvent can make an unsaturated solution saturated.

Understand: Liquids A ($78^\circ C$) and B ($100^\circ C$) are miscible and their boiling points differ by more than $25^\circ C$.

Technique: Simple distillation.

How it works:

1. Heat the mixture in a distillation flask fitted with a thermometer and condenser.
2. Liquid A (lower boiling point, $78^\circ C$) vaporises first.
3. The vapour passes into the condenser, where it cools and condenses back into liquid.
4. Pure liquid A is collected in a receiving flask.
5. Liquid B, having a higher boiling point, remains behind in the flask.

Note: If the difference in boiling points were less than $25^\circ C$, fractional distillation would be required instead.

Conclude: Simple distillation separates two miscible liquids with a sufficient difference in boiling points.

Understand: These three differ mainly in particle size, which affects their other properties.

| Property | True Solution | Colloid | Suspension | |---------|--------------|---------|-----------| | Nature | Homogeneous | Heterogeneous | Heterogeneous | | Particle size | $< 1\ \text{nm}$ | $1$–$1000\ \text{nm}$ | $> 1000\ \text{nm}$ | | Visible to eye | No | No (but scatter light) | Yes | | Tyndall effect | No | Yes | Yes | | Settling on standing | Does not settle | Does not settle | Settles down | | Filtration | Cannot be filtered | Cannot be separated by ordinary filter | Can be filtered | | Example | Salt in water | Milk | Chalk in water |

Conclude: Particle size is the fundamental difference that determines all other properties.

Understand: A physical change does not form a new substance and is usually reversible; a chemical change forms a new substance and is usually not easily reversible.

Analysis:

| Change | Type | Reason | |--------|------|--------| | (a) Melting of ice | Physical | Only state changes (solid → liquid); still water | | (b) Burning of candle | Chemical | New substances ($CO_2$, water vapour) are formed | | (c) Dissolving sugar in water | Physical | Sugar can be recovered by evaporation; no new substance | | (d) Rusting of iron | Chemical | New substance (iron oxide, rust) is formed |

Conclude: Changes (a) and (c) are physical; (b) and (d) are chemical because new substances form.