Mixtures & Compounds: Chemistry Notes for NTSE

Mixtures & Compounds – Study Notes

Program: NTSE – Chemistry
Topic: Mixtures & Compounds

Concept	What it is	How it appears in everyday life
Mixture	A physical combination of two or more substances that are not chemically bonded. Each component keeps its own identity.	Salt‑water, air, coffee with milk, soil.
Compound	A substance formed by a chemical combination of elements in a fixed ratio. The elements lose their individual identities.	Water (H₂O), table salt (NaCl), glucose (C₆H₁₂O₆).

Type	Definition	Examples	Key Properties
Homogeneous (Solution)	Uniform composition throughout.	Salt in water, air.	Same appearance, no phase boundaries.
Heterogeneous	Different phases or physically separable components.	Sand in water, oil and water.	Visible phase boundaries, components can be separated mechanically.
Physical Mixtures	No chemical reaction between components.	Sugar in tea, soil.	Components can be recovered intact.
Chemical Mixtures	Components react chemically but are not fully combined into a single compound.	Bleach (NaOCl + NaCl + H₂O).	Reaction may be reversible; components often cannot be recovered unchanged.

Separation Techniques

Technique	Principle	When to use	Example
Filtration	Solid‑liquid separation using a porous medium.	Solid suspended in liquid.	Gravel from muddy water.
Magnetic Separation	Uses magnetic attraction to separate ferromagnetic solids.	Iron filings in sand.	Removing iron filings from a mixture of sand and iron.
Distillation	Uses differences in boiling points.	Liquid‑liquid mixtures or volatile liquids.	Distilling ethanol from a fermented mash.
Extraction	Solutes move between two immiscible liquids.	Organic compounds in aqueous solution.	Extracting caffeine from coffee with an organic solvent.
Chromatography	Separation based on differential migration on a stationary phase.	Complex mixtures of dyes, pigments.	Paper chromatography of plant pigments.
Precipitation	Solid forms from a soluble salt by ion exchange.	Removing lead from water.	Adding sulfate to lead‑containing water to precipitate PbSO₄.

Feature	Detail
Chemical Formula	Empirical (simplest ratio) or molecular (actual number of atoms).
Laws	Definite Proportions – a compound always contains the same mass ratio of elements. Multiple Proportions – if two compounds contain the same element, the mass ratios of the other element(s) are in a small whole‑number ratio.
Types	<ul><li>Ionic – electron transfer, e.g. NaCl.</li> <li>Covalent – electron sharing, e.g. H₂O, CO₂.</li> <li>Coordinate (Dative) – one atom donates both bonding electrons, e.g. NH₃·H₂O (ammonium hydroxide). </li></ul>
Bond Character	Ionic bonds are electrostatic; covalent bonds involve shared electron pairs.
Molecular Geometry	Determined by VSEPR theory; affects physical properties.
Formation	Typically via chemical reactions that obey stoichiometry.

Problem:
A mixture contains 5 g of iron filings (Fe) and 20 g of sand (SiO₂). Separate the two components.

Solution Steps

Identify a distinguishing property
- Iron is magnetic; sand is not.
Choose a separation technique
- Magnetic separation using a bar magnet.
Procedure
- Pass the mixture through a magnetic field.
- Magnetic filings will stick to the magnet and can be collected.
- The sand will pass through and can be collected separately.
Yield
- Iron filings recovered: ≈5 g (assuming negligible loss).
- Sand recovered: ≈20 g.

Key Takeaway: Use a physical property (magnetism) to separate components without any chemical change.

Problem:
Show that hydrogen (H₂) and oxygen (O₂) produce water (H₂O) and obey the law of multiple proportions.

Data

Element	Mass per mole	Moles in 1 g of H₂O	Mass in mixture