Is Matter Around Us Pure?

A pure substance is one that consists of only one type of particles (atoms or molecules). All particles of a pure substance have the same chemical nature.

Key characteristics of a pure substance:

1. It has a fixed (definite) composition throughout.

2. It has a definite melting point and boiling point at a given pressure.

3. It is a homogeneous material.

4. Its properties are uniform throughout.

Examples: Gold (Au), pure water (H₂O), sugar (C₁₂H₂₂O₁₁), common salt (NaCl), pure oxygen (O₂).

Homogeneous Mixture:

1. Has uniform composition throughout.

2. Components cannot be distinguished or seen separately (appear as a single phase).

3. No visible boundary between components.

4. Also called a solution.

5. Examples: Salt in water, sugar in water, air (mixture of gases), alloys like brass.

Heterogeneous Mixture:

1. Has non-uniform composition; composition varies from place to place.

2. Components can often be distinguished and seen separately (appear as two or more phases).

3. A visible boundary exists between components.

4. Examples: Sand in water, oil and water, soil, granite.

Solution (True Solution):

Particle size: Less than 1 nm. Transparent and clear. Passes through both filter paper and semipermeable membrane. Does not scatter light (no Tyndall effect). Particles do not settle on standing. Example: Salt in water, sugar in water.

Colloid (Sol):

Particle size: 1 nm to 100 nm. Appears homogeneous but is actually heterogeneous. Scatters light (shows Tyndall effect). Cannot be separated by filtration. Particles do not settle on standing. Examples: Milk, fog, smoke, starch solution.

Suspension:

Particle size: Greater than 100 nm (visible to naked eye or under microscope). Heterogeneous mixture. Particles are visible. Particles settle on standing (unstable). Can be separated by filtration. Scatters light. Examples: Chalk powder in water, muddy water.

Given:

Mass of solute (NaCl) = 36 g

Mass of solvent (water) = 100 g

Mass of solution = Mass of solute + Mass of solvent = 36 + 100 = 136 g

The concentration of the saturated solution of NaCl at 293 K is 26.47% (by mass).

The mixture can be separated in three steps:

Step 1 — Magnetic separation: Pass a magnet over the mixture. Iron filings are magnetic and will be attracted to and separated by the magnet. Remaining mixture: salt + sulphur.

Step 2 — Addition of water and filtration: Add water to the remaining salt-sulphur mixture and stir. Salt (NaCl) is soluble in water and dissolves to form a solution. Sulphur is insoluble in water. Filter the mixture through filter paper. Sulphur remains on the filter paper and the filtrate (salt solution) passes through.

Step 3 — Evaporation: Heat the filtrate (salt solution) to evaporate the water. Pure salt crystals remain behind.

Result: Iron filings, sulphur, and common salt are all separated.

The technique used to separate butter from curd is Centrifugation.

Principle: When a mixture is rotated at very high speed in a centrifuge machine, the denser (heavier) particles experience greater centrifugal force and are flung outward, settling at the periphery (outer edge or bottom of the tube). The lighter particles remain closer to the centre (or top).

In the case of curd, the fat particles (butter) are lighter than the watery liquid (buttermilk). When curd is spun at high speed, the heavier liquid (buttermilk) moves outward and the lighter butter collects near the centre/top, allowing them to be separated.

Other applications of centrifugation: Separating cream from milk, separating blood cells from plasma, in washing machines (spin cycle removes water from clothes).

The Tyndall effect is the scattering of a beam of light by colloidal particles. When a beam of light passes through a colloid, the path of the light beam becomes visible because the relatively large colloidal particles scatter the light in all directions.

This effect was first described by John Tyndall and is used to distinguish colloids from true solutions (which do not scatter light as the particles are too small).

Examples:

1. When sunlight enters a dark room through a small hole, the beam becomes visible due to dust particles (a colloid) scattering the light.

2. The beam from a projector or car headlights becomes visible in a dusty or foggy atmosphere.

3. The blue colour of the sky is also attributed to scattering of light by colloidal-size particles in the atmosphere.

A mixture of sand and water is a suspension (heterogeneous mixture with large particles). It can be separated by the following methods:

Method 1 — Sedimentation followed by Decantation: Allow the mixture to stand undisturbed. The heavier sand particles settle at the bottom (sedimentation). Carefully pour off the clear water above (decantation) without disturbing the settled sand.

Method 2 — Filtration: Pour the mixture through a filter paper in a funnel. The sand particles (being large) are retained on the filter paper as the residue, while the water passes through as the filtrate.

For completely dry sand, the filtrate (water) can be evaporated and the sand residue dried.

(i) Cutting of a log of wood — Physical change. Only the shape and size change; no new substance is formed.

(ii) Melting of wax — Physical change. Change of state from solid to liquid; the chemical composition of wax remains the same.

(iii) Dissolving sugar in water — Physical change. Sugar can be recovered by evaporating the water; no new substance is formed.

(iv) Cooking of food — Chemical change. New substances with different chemical compositions and properties are formed; the change is irreversible.

(v) Rusting of iron — Chemical change. Iron reacts with oxygen and moisture to form iron oxide (rust), which is a new substance. The change is irreversible.

(vi) Burning of magnesium ribbon — Chemical change. Magnesium reacts with oxygen to form magnesium oxide, a new substance (white ash).

(vii) Electrolysis of water — Chemical change. Water decomposes into hydrogen and oxygen gas, which are entirely different substances.

(viii) Burning of paper — Chemical change. Paper burns to produce ash, CO₂, and water vapour — completely new substances. The change is irreversible.

Cream is separated from milk by the process of centrifugation using a device called a cream separator or centrifuge.

Method:

1. Fresh milk is poured into the centrifuge bowl (cream separator).

2. The bowl is spun at high speed (several thousand rpm).

3. Due to centrifugal force, the denser skim milk (with higher water content) is flung outward to the periphery/bottom of the bowl.

4. The lighter cream (fat globules, density less than water) remains near the centre/top and rises.

5. Separate outlets allow the cream and skim milk to be collected separately.

This method is far more efficient than the traditional method of allowing milk to stand and skimming the cream that rises to the top by gravity.

Chromatography is a technique used to separate the components of a mixture based on the difference in the speed at which they travel through a stationary phase when carried by a mobile phase.

Principle: Different components of a mixture have different degrees of solubility in the mobile phase (solvent) and different tendencies to adsorb onto or interact with the stationary phase (paper, silica, etc.). Components that are more soluble in the mobile phase travel farther and faster, while those that interact more strongly with the stationary phase travel more slowly. This difference in speed causes the components to separate into distinct bands or spots.

Applications of chromatography:

(i) Separating the different coloured dyes present in a mixture of inks (paper chromatography).

(ii) Detecting and identifying drugs, medicines, or metabolites in blood or urine samples in forensic science and medicine.

Other uses: separating amino acids in biochemistry, testing food colours and additives, environmental analysis.

(i) Tearing of paper — Physical change. Only size and shape change; chemical composition of paper remains the same.

(ii) Burning of wood — Chemical change. Wood reacts with oxygen and produces ash, CO₂, CO, and water vapour. New substances are formed; the change is irreversible.

(iii) Melting of ice — Physical change. Only change of state (solid to liquid); chemical formula of water (H₂O) remains unchanged. It is reversible.

(iv) Making glass from sand — Chemical change. Sand (SiO₂) is chemically fused with other substances (soda ash, limestone) at very high temperatures to form glass, a new substance with different properties.

(v) Grinding of wheat into flour — Physical change. Only the particle size changes; the chemical composition of wheat remains the same.

To confirm that a colourless liquid is pure water, we can perform the following tests:

(i) Boiling point test: Pure water boils at exactly 100°C (373 K) at 1 atmosphere pressure. If the liquid boils at a different temperature, it is not pure water.

(ii) Freezing point test: Pure water freezes at exactly 0°C (273 K). A deviation indicates the presence of dissolved substances.

(iii) Anhydrous copper sulphate test: Add anhydrous copper sulphate (white powder). If water is present, it will turn blue (forms blue copper sulphate pentahydrate, CuSO₄·5H₂O). This confirms the presence of water.

(iv) Density test: Pure water has a density of exactly 1.00 g/cm³ at 4°C.

(a) Sodium chloride from its solution in water:

Evaporation or Crystallisation. The salt solution is heated, causing water to evaporate. Pure salt crystals are left behind. (Crystallisation gives purer crystals than simple evaporation.)

(b) Ammonium chloride from a mixture of ammonium chloride and salt:

Sublimation. Ammonium chloride (NH₄Cl) sublimes on heating — it directly converts to vapour and can be collected as pure solid on a cold surface (like a cold funnel placed above), leaving behind the common salt (NaCl) which does not sublime.

(c) Metal filings from paint:

If the metal filings are magnetic (e.g., iron), use a magnet to separate them. If the filings are non-magnetic, use filtration — the metal filings are insoluble in the paint solvent and can be filtered out, or allow them to settle by sedimentation and then decant the paint.

(a) Ice: Yes, ice is a compound. Ice is solid water (H₂O). Water is a pure compound formed by the chemical combination of hydrogen and oxygen in a fixed ratio of 2:1 by atoms (1:8 by mass). It has definite properties and a fixed composition regardless of its state (solid, liquid, or gas).

(b) Wood: No, wood is not a compound — it is a heterogeneous mixture. Wood is made of cellulose, lignin, hemicelluloses, water, minerals, and various other organic substances. Its composition varies from one type of wood to another and even within the same tree.

(c) Air: No, air is not a compound — it is a homogeneous mixture. Air is a mixture of nitrogen (~78%), oxygen (~21%), argon, carbon dioxide, and other trace gases. The composition of air can vary (e.g., more CO₂ near cities). The gases in air can be separated by physical methods (fractional distillation), which would not work on a compound.

Mixture:

1. Formed by physical combination of two or more substances; no chemical reaction involved.

2. Components retain their individual properties.

3. No fixed ratio of components; composition can vary.

4. Can be separated by physical methods (filtration, evaporation, magnetism, etc.).

5. No energy change (heat/light) during formation.

6. Examples: air, salt water, soil, alloys.

Compound:

1. Formed by chemical combination of two or more elements.

2. Has entirely different properties from its constituent elements.

3. Elements combine in a fixed ratio by mass.

4. Can be separated only by chemical methods (electrolysis, etc.).

5. Energy is either absorbed or released during formation.

6. Examples: water (H₂O), common salt (NaCl), carbon dioxide (CO₂).

(a) Saturated solution: A solution in which no more solute can be dissolved at a given temperature is called a saturated solution. At saturation, the dissolved and undissolved solute are in dynamic equilibrium. Example: When 36 g of NaCl is dissolved in 100 g of water at room temperature, the solution becomes saturated — no more NaCl will dissolve.

(b) Pure substance: A substance consisting of only one type of particles with uniform composition and definite properties. Example: Pure gold, distilled water, pure oxygen.

(c) Colloid: A heterogeneous mixture with particle size between 1–100 nm that appears homogeneous to the naked eye. It shows the Tyndall effect. Colloidal particles do not settle on standing. Example: Milk (fat globules in water), fog (water droplets in air).

(d) Suspension: A heterogeneous mixture in which insoluble solid particles (size > 100 nm) are dispersed in a liquid. The particles are visible, settle on standing, and can be filtered out. Example: Chalk powder in water, muddy river water.

A true solution is a homogeneous mixture formed when a solute completely dissolves in a solvent to form a single phase at the molecular or ionic level.

Properties of a true solution:

1. Particle size is less than 1 nm (molecular level).

2. It is transparent and clear.

3. Particles pass through both filter paper and semi-permeable membranes.

4. Does not show the Tyndall effect (no light scattering).

5. Solute does not settle on standing — it is stable.

6. Uniform composition throughout.

Example: Salt in water, sugar in water, copper sulphate solution.

Difference from colloid: In a true solution, particle size is < 1 nm and the path of light is not visible (no Tyndall effect). In a colloid, particle size is 1–100 nm and the path of a light beam through it is visible (Tyndall effect). A colloid is heterogeneous at the microscopic level but appears homogeneous to the naked eye.