Mixtures

• A molecule is a group of two or more atoms held together by chemical bonds.
• Molecules can consist of atoms of the same element, such as oxygen (O₂), or different elements, such as water (H₂O).
• A single atom, like helium, does not count as a molecule because it requires at least two atoms.

• A compound is a substance that contains two or more different elements chemically bonded together.
• Water (H₂O) and carbon dioxide (CO₂) are compounds because they each contain more than one type of element.
• Molecules such as oxygen (O₂), chlorine (Cl₂), and nitrogen (N₂) are not compounds because they only contain one type of element.
• In a compound, the elements are always present in fixed proportions, for example water always has two hydrogen atoms for every one oxygen atom.

• Chemical formulae use element symbols and subscript numbers to show how many atoms of each element are in a molecule, e.g.
• H₂O means two hydrogen atoms and one oxygen atom.
• If only one atom of an element is present, no subscript number is written, as seen with the single carbon in CO₂.
• A more complex example is H₂SO₄ (sulfuric acid), which contains two hydrogen atoms, one sulfur atom, and four oxygen atoms.
• Brackets in formulae group atoms together, and the number outside the bracket shows how many of those groups are present, e.g. Ca(OH)₂ means one calcium and two groups of OH.

• Some compounds, such as sodium chloride (NaCl), exist as giant structures containing millions or billions of atoms rather than small individual molecules.
• Compounds that form large structures rather than molecules typically contain ionic bonds between their atoms.
• The formula of a non-molecular compound acts as a ratio; for example, NaCl shows that sodium and chloride ions are present in a 1:1 ratio.

• A mixture consists of two or more substances that are not chemically combined together.
• In a mixture, the individual substances are physically combined but no chemical bonds form between them.
• Because the substances in a mixture are not chemically bonded, they retain their individual properties and can be separated by physical methods.

• Mixtures can be separated using physical methods such as filtration, crystallisation, or distillation.
• These separation techniques work because the components of a mixture are not chemically bonded and have different physical properties.

Key Steps in the Process

• Heat the solution: Gently heat the solution in an evaporating basin to evaporate some of the solvent, making the mixture more concentrated.
• Check for saturation: Stop heating when crystals start to form at the edge of the basin (the point of crystallisation) or when a saturated solution is reached.
• Cool and crystallise: Leave the solution to cool so that more solid crystals form, because solutes are less soluble at cooler temperatures.
• Filter and dry: Separate the pure crystals from the remaining liquid using filter paper and a funnel, then dry them.

How Distillation Works

• Heating (Evaporation): The mixture is heated so the liquid with the lowest boiling point turns into a gas (vapor) first.
• Cooling (Condensation): The hot vapor travels into a condenser (surrounded by cold running water), which cools the gas back into a liquid.
• Collection: The purified liquid collected at the end is called the distillate.

Chromatography is an analytical separation technique used to separate mixtures of soluble substances—such as inks, dyes, or plant pigments—based on their different solubilities and their attraction to two specific phases.

The Two Phases
• Mobile phase: The liquid solvent (such as water or ethanol) that moves through or along the stationary phase, carrying dissolved substances.
• Stationary phase: The solid or paper support (such as chromatography paper) that stays fixed in place.

A chromatogram is the visual pattern or result of spots left on the stationary phase (like chromatography paper) after a chromatography experiment is complete.

• Stationary Phase: The solid or material that does not move, such as the filter paper.
• Mobile Phase: The liquid solvent that moves through the stationary phase, carrying dissolved substances.
• Pure vs. Impure: A pure substance produces a single spot on a chromatogram. An impure substance or mixture produces two or more spots.
• Rf Value (Retention Factor): A ratio used to identify components by dividing the distance traveled by the substance by the distance traveled by the solvent.

Paper chromatography separates mixtures of soluble substances, such as food colourings, into their individual dye components.

• A molecule contains two or more atoms bonded together, whereas a compound specifically requires two or more different elements.
• All compounds are chemically bonded substances with fixed ratios of elements, while mixtures have no chemical bonds and can vary in composition.
• Compounds cannot be separated by physical methods, whereas the components of a mixture can be separated physically.