IB CHEM 1 - Lesson 1 Part 1: Chemical Elements, Compounds & Mixtures

Q: What is an element (chem definition)?

A: A pure substance made of one kind of atom; cannot be chemically broken down into simpler substances.

Q: What is a compound?

A: A substance formed when two or more elements chemically bond in fixed ratios, creating a substance with properties different from the constituent elements. Compounds can be ionic or molecular.

Q: What does “fixed ratio” mean in compounds?

A: Atoms combine in definite, whole-number proportions (e.g., NaCl is 1:1; H₂O is 2:1), giving constant composition.

Q: Why do atoms form compounds in fixed ratios?

A: To achieve full outer electron shells (stable electron configurations).This occurs due to the drive for stability and lower energy states through chemical bonding.

Q: Typical energy pattern during chemical reactions forming compounds?

A: Energy change occurs (often energy released on forming stable bonds).

Q: Define a mixture.

A: A physical combination of two or more elements and/or compounds in no fixed ratio, not chemically bonded.

Q: Key property difference: mixture vs compound.

A: Mixtures retain individual properties of components, while compounds have distinct properties different from their constituents. Compounds are chemically bonded, while mixtures are not.

Q: At the particle level, what distinguishes a compound from a mixture?

A: Compound: particles are molecules/ions with fixed internal ratios (e.g., H₂O). Mixture: separate particles interspersed (e.g., O₂ + N₂). Particles in a compound are chemically bonded, resulting in a fixed composition, whereas a mixture contains physically combined substances that retain their individual properties.

Q: Homogeneous mixture — definition + example.

A: Uniform composition throughout; components not visibly distinct (e.g., air, bronze).

Q: Heterogeneous mixture — definition + example.

A: Non-uniform composition; components often visible (e.g., concrete, orange juice with pulp).

Q: Is air a substance or a mixture? Why?

A: Homogeneous mixture of gases (mainly N₂, O₂); no fixed ratio and components retain properties (e.g., O₂ supports combustion).

Air is a mixture because it consists of various gases, such as nitrogen and oxygen, that are physically combined and can vary in composition. It does not have a fixed composition and retains the properties of its individual gas components.

Q: Why is bronze homogeneous?

A: Bronze is homogeneous because it is an alloy composed of copper and tin, resulting in a uniform composition where components are not visibly distinct.

Q: Classify: sodium + chlorine → sodium chloride. What changed?

A: Chemical reaction produced a compound (NaCl) with new properties (white crystalline solid vs reactive metal + toxic gas). A: The reactants, sodium and chlorine, underwent a chemical change to form sodium chloride, which is a stable compound with distinct properties compared to its constituent elements.

Can elements be “broken down”?

A: Elements cannot be broken down into simpler substances by chemical means. They are the most basic form of matter and consist of only one type of atom. A: Not by chemical means (only nuclear processes change elements).

Q: Why can mixtures be separated by physical methods?

A: Components are not chemically bonded; they keep distinct physical properties (bp, solubility, magnetism, density).

Q: Separation method by boiling point difference.

A: Distillation (or simple evaporation if one component is a dissolved solid).

Q: Separation method by solubility difference.

A: Dissolve + filtration (insoluble residue remains) → evaporate solvent to recover solute.

Q: Separation method by magnetism.

A: Use a magnet to pull out ferromagnetic components (e.g., iron filings from sand).

Q: Separation method by density/particle size.

A: Decantation (pour off), centrifugation, or sedimentation.

Q: Procedure: separate a mixture of salt and sand.

A:
1) Add water (salt dissolves).
2) Filter (sand = residue; salt solution = filtrate).
3) Evaporate water to recover salt.

Q: Common trap: “Mixtures have fixed ratios.” Correct it.

A: False — only compounds have fixed ratios; mixtures vary in composition.

Q: Common trap: “You can separate compounds by filtration.” Correct it.

A: False — compounds require chemical reactions to separate into elements; filtration only separates mixtures.

Q: Sight test: heterogeneous vs homogeneous — what visual cue helps?

A: Visible phases/particles → heterogeneous; uniform appearance → homogeneous (but note: some homogenous mixtures still require microscopic evidence).

Q: Particle-box reading skill — what do you look for to identify a compound?

A: Repeated identical clusters of different atom types in a fixed pattern/ratio.

Q: Why oxygen in air still supports combustion though diluted by N₂?

A: In a mixture, components keep their chemical behavior; O₂ retains reactivity.