Elements, Compounds and Mixtures (experimental techniques)

1. Composition: A compound has a strictly fixed ratio by mass ($H_2O$ is always $1:8$ by mass), whereas a mixture can be prepared in any variable proportion.
   2. Properties: A compound exhibits entirely new chemical properties completely distinct from its constituents (water extinguishes fire, though hydrogen is explosive and oxygen supports burning); in a mixture, constituents retain their individual original properties.
   3. Separation: Compounds cannot be separated by physical methods (requires electrolysis or chemical reactions); mixtures are easily separated by physical methods (filtration, magnets, distillation).
   4. Melting/Boiling Points: Compounds possess sharp, fixed melting and boiling points; mixtures boil over a broad temperature range.

• Step 1: Sublimation (to remove $NH_4Cl$):
Place the mixture in a china dish covered with an inverted glass funnel plugged with cotton. Heat gently. Ammonium chloride sublimes into vapor and deposits as a solid white crust on the cool funnel walls. Remove the funnel. The residue in the dish contains salt and sand.
• Step 2: Solvent Extraction (Dissolution in Water):
Add water to the residue and stir well. Salt dissolves completely in water, while sand remains insoluble.
• Step 3: Filtration (to separate Sand):
Filter the mixture through a filter paper cone. Sand is collected as the residue on the filter paper. Wash and dry it.
• Step 4: Evaporation to Dryness (to recover Salt):
Heat the clear filtrate (salt water) in an evaporating dish until all water evaporates completely, leaving pure dry common salt crystals.

• Principle: A separating funnel operates on the physical principle of immiscibility (inability to mix) and difference in densities between two liquids.
• Working:
1. Pour the oil-water mixture into the separating funnel, clamp it vertically on a stand, and allow it to stand undisturbed for 15 minutes.
2. The mixture resolves into two distinct horizontal layers: denser water (density $1.0\text{ g/cm}^3$) forms the bottom layer, while lighter mustard oil (density $~0.9\text{ g/cm}^3$) floats on top.
3. Place a clean beaker under the funnel stem and carefully open the stopcock.
4. Drain out the lower water layer completely and close the stopcock the exact instant the boundary interface reaches the valve.
5. Place a second beaker and drain out the pure mustard oil, achieving complete separation.

• Simple Distillation: A process in which a liquid solution is boiled to vaporize the solvent, and the resulting vapor is cooled and condensed back into liquid form using a Liebig condenser.
• Key Difference from Evaporation:
In Evaporation, the solvent liquid is lost into the atmosphere as vapor, leaving behind only the solid solute residue. In Distillation, the solvent is condensed and fully recovered as pure liquid distillate, allowing both solute and solvent to be preserved.

1. Cut a narrow rectangular strip of Whatman chromatographic filter paper.
   2. Draw a faint pencil line about $2\text{ cm}$ from the bottom edge.
   3. Place a small concentrated drop of black fountain pen ink at the center of the pencil line using a capillary tube and let it dry.
   4. Suspend the paper strip inside a glass boiling tube containing a small amount of water or alcohol solvent such that the lower tip is immersed, but the ink spot remains strictly above the liquid level.
   5. As the solvent rises through the paper fibers by capillary action, it carries the ink dyes upward.
   6. Because the constituent pigments have different solubilities in water and different adsorption affinities for the cellulose paper, they travel at different speeds, separating into distinct bands of cyan, yellow, and magenta on the developed chromatogram.

• Definition: Metalloids (or semimetals) are chemical elements that possess physical and chemical properties intermediate between those of typical metals and non-metals (e.g., they look metallic but are brittle, and act as semiconductors).
• Examples:
1. Silicon ($Si$)
2. Germanium ($Ge$)
3. Arsenic ($As$)
4. Antimony ($Sb$) (also Boron $B$ and Tellurium $Te$).

1. Variable Composition: The percentage of gases in air (e.g., water vapor, carbon dioxide, dust) varies from place to place and season to season; compounds have a strictly fixed mass ratio.
   2. Retained Properties: The gases in air retain their individual chemical properties (oxygen supports combustion, carbon dioxide extinguishes flame).
   3. Physical Separation: The constituents of air can be separated into liquid nitrogen and oxygen by physical fractional distillation of liquefied air without chemical reactions.

• (a) Cream from milk: Centrifugation (high-speed centrifugal spinning).
• (b) Pure copper sulfate crystals from impure solution: Crystallization.
• (c) Kerosene oil and water: Separating Funnel.
• (d) Iron filings from wood shavings: Magnetic Separation (using a magnet).