IB Diploma Programme Chemistry: Structure 1.1.1 - Elements, Compounds, Mixtures & Separation Techniques

Foundations of Matter: Elements, Compounds, and Mixtures

• Matter is classified into two broad categories: pure substances and mixtures.

• Elements are the primary constituents of matter and represent the building blocks of all substances.

• Elements are defined as the simplest form of matter, consisting of only ONE type of atom.

• Because they consist of only one type of atom, elements cannot be broken down chemically into simpler substances.

• There are currently 92 naturally-occurring elements discovered.

• Any elements beyond these 92 have been synthesised by scientists since the mid-1940s.

• An example of a synthesised element is nihonium ($Nh$), which was created in the early 2000s.

• Elements can exist in different physical arrangements:

  • Solid iron ($Fe$) is made up of only iron atoms arranged in fixed positions.

  • Elements can exist as individual atoms or as molecules where atoms of the SAME element are bonded together.

• Diatomic elements are composed of two atoms of the same element bonded together:

  • Hydrogen ($H_2$)

  • Nitrogen ($N_2$)

  • Oxygen ($O_2$)

  • Fluorine ($F_2$)

  • Chlorine ($Cl_2$)

  • Bromine ($Br_2$)

  • Iodine ($I_2$)

• Polyatomic elements consist of more than two atoms of the same element bonded together:

  • Phosphorus ($P_4$)

  • Sulfur ($S_8$)

• Elements such as carbon can exist as allotropes, which are different structural forms of the same element in the same physical state.For example, carbon can be found as graphite and diamond, each exhibiting distinct physical and chemical properties despite being composed solely of carbon atoms.

Compounds and Chemical Bonding

• Compounds are pure substances composed of two or more DIFFERENT elements chemically combined in a FIXED ratio.

• Atoms in a compound are chemically bonded, meaning they cannot be separated using physical methods.

• Examples of compounds with fixed ratios include:

  • Methane ($CH_4$): 1 carbon atom to 4 hydrogen atoms.

  • Water ($H_2O$): 2 hydrogen atoms to 1 oxygen atom.

• Carbon alone forms millions of compounds, leading to the dedicated branch of organic chemistry.

• Chemical bonding completely changes the properties of the constituent elements; a compound is NOT a blend of its elements' properties.

• Case Study: Sodium Chloride ($NaCl$)

  • Sodium ($Na$) is a very reactive metal that reacts vigorously with water.

  • Chlorine ($Cl_2$) is a toxic gas used to kill bacteria in water treatment.

  • Sodium chloride ($NaCl$) is a stable ionic compound forming a lattice structure; it is safe and used as food flavouring.

Mixtures and Solvation

• Mixtures combine two or more elements or compounds in NO fixed ratio.

• Mixtures are not classified as pure substances because their components are NOT chemically bonded.

• Each component of a mixture retains its own individual properties (e.g., iron filings in a sulfur mixture remain magnetic).

• Mixtures can be separated by physical methods.

• Solvation is the specific process where solvent particles surround and interact with solute particles as they dissolve.

• Mixtures are categorised into two types:

  • Homogeneous Mixtures: Uniform composition throughout with no visible boundaries. The solute (e.g., salt) dissolves fully into the solvent (water). Examples include air (nitrogen, oxygen, and argon evenly mixed) and salt water.

  • Heterogeneous Mixtures: Non-uniform composition with visible phases or boundaries. Examples include oil and water, carbonated water (containing visible $CO_2$ bubbles), and a mixture of iron and sulfur.

Summary of Matter Classification and Exam Pitfalls

• Matter is divided into pure substances (Elements and Compounds) and Mixtures (Homogeneous and Heterogeneous).

• Pure substances have a fixed composition; mixtures have a variable composition.

• Common Exam Pitfalls to avoid:

  • Diatomic elements ($Cl_2$, $O_2$) are NOT compounds; they contain only one element.

  • Alloys are mixtures, even though metallic bonds hold the atoms together.

  • Homogeneous does not mean pure; salt water is homogeneous but is a mixture.

  • A fixed subscript ratio in a formula signals a compound; variable composition signals a mixture.

Global Water Crisis and Separation Techniques

• Separation techniques are used to isolate components of a mixture based on their physical properties.

• According to the CDC, approximately 2 billion people lack access to clean drinking water at home.

• Two-thirds of Earth is covered in water, but seawater contains roughly $35\,g$ of dissolved salt per litre ($35\,g\,dm^{-3}$), which is more than the human body can tolerate.

• UN Sustainable Development Goal 6 calls for clean water and sanitation for all, making separation techniques like desalination vital.

• Factors determining the choice of separation technique include the type of mixture and the specific physical properties of the components.

Detailed Physical Separation Methods

• Filtration:

  • Separates an INSOLUBLE solid from a liquid or solution.

  • The mixture is poured through filter paper.

  • The liquid that passes through is the filtrate; the solid remaining in the paper is the residue.

  • Example: Separating sand and water.

• Evaporation:

  • Separates a solute dissolved in a solvent (usually water).

  • The solution is heated so the solvent evaporates, leaving the solid solute behind.

  • Example: Obtaining solid salt from salt water.

• Solvation:

  • Separates a heterogeneous mixture of TWO SOLIDS based on differing solubility.

  • One solid dissolves into a solvent; the remaining insoluble solid is removed via filtration.

• Distillation:

  • Separates a liquid mixture based on differences in volatility (boiling point).

  • Example: Ethanol (boiling point $78\,^{\circ}C$) evaporates before water (boiling point $100\,^{\circ}C$). It rises through a condenser and is collected as the distillate.

  • Fractional distillation is used on an industrial scale to separate crude oil into gasoline, kerosene, and lubricating oil.

• Chromatography:

  • Separates dissolved components by their differing affinity for a stationary phase (e.g., paper) versus a mobile phase (solvent).

  • Components with a greater affinity for the mobile phase travel further up the stationary phase.

  • Common methods include paper chromatography and thin-layer chromatography (TLC).

• Recrystallisation:

  • Purifies a solid by exploiting how its solubility changes at different temperatures.

  • Used in pharmaceutical industries to remove contaminants from medication and to purify sugar crystals from cane juice.

Method Selection Guide and Worked Examples

• Selection based on properties:

  • Insoluble solid + liquid: Use Filtration (property: insolubility).

  • Solid dissolved in liquid: Use Evaporation (property: volatility of solvent).

  • Two solids (one soluble): Use Solvation + Filtration + Evaporation (property: solubility).

  • Two miscible liquids: Use Distillation (property: different boiling points).

  • Dissolved coloured components: Use Chromatography (property: affinity for solvent).

  • Solid with soluble impurities: Use Recrystallisation (property: solubility changes with temperature).

• Worked Example: Separating Sand, Salt, and Water:

  • Step 1: Filter the mixture. The sand is retained as residue; salt water passes through as filtrate.

  • Step 2: Evaporate the filtrate. The water evaporates, leaving the solid salt behind.

• Challenge Example: Separating Sand and Iron Filings:

  • Iron is magnetic while sand is not; a magnet can be used to pull the iron filings away from the sand.

Advanced Separation Pitfalls

• Filtration Limitations: If a solute is fully dissolved, filtration will not separate it; there must be an insoluble solid. Filtration cannot desalinate water.

• Chromatography Limitations: Components must be DISSOLVED in the mobile phase; it cannot separate insoluble solids.

• Recrystallisation Limitations: This process only works if the solids have significantly different solubilities at different temperatures.