Elements, Mixtures, and Compounds: Comprehensive Notes

Define an atom.
Define an element.
Define a compound.
Differentiate between elements and compounds.
Define melting and boiling points.
Explain why pure substances have fixed melting and boiling points.
Explain the effect of pressure and impurity on melting and boiling points.
Describe the processes of separation: filtration, crystallization, and distillation.

Elements are substances that cannot be split into anything simpler by chemical means.
An element contains only one type of atom.
Represented in models as atoms of a single color or size.
Examples:
- Pure metal (e.g., magnesium)
- Oxygen gas
- Diamond (a form of carbon)
There are 118 elements, shown in the Periodic Table.
Most elements occur naturally (e.g., hydrogen, helium, sulfur).
Some are artificially made (e.g., einsteinium).

Compounds are formed when two or more elements chemically combine.
Elements combine in fixed proportions.
Examples:
- Hydrogen and fluorine form hydrogen fluoride (HF).
- Magnesium and fluorine form magnesium fluoride (MgF₂).
- Carbon dioxide (CO₂)
- Methane (CH₄).
Diagrams of compounds show more than one type of atom bonded together.
Examples of compounds:
- Water
- Silicon dioxide
- Sodium chloride

In a mixture, substances are mixed together, but no chemical reaction occurs.
Mixtures can be made from elements and/or compounds.
Components can be in any proportion.
- Example: sugar in tea or coffee until saturated.
Types of mixtures:
- Mixture of elements: nitrogen and oxygen
- Mixture of compounds: carbon dioxide and water (vapor)
- Mixture of an element with a compound: carbon dioxide and nitrogen

Pure substances (elements and pure compounds) melt and boil at fixed temperatures.
- Melting point of water: $0°C$
- Boiling point of water: $100°C$
Mixtures usually melt or boil over a range of temperatures.
Impurities lower the melting point and raise the boiling point of a substance.
- Example: Dissolving 10g of sodium chloride in 1 liter of water lowers the melting point to about $-0.6°C$ and raises the boiling point to about $100.2°C$ .
Melting point helps determine the purity of a substance.
- Example: Aspirin melts at $138°C$ . If a sample melts between $128-134°C$ , it is impure.

Melting point: solid -> liquid
Boiling point: liquid -> gas

The choice of separation method depends on the nature of the substances being separated.
Methods rely on differences in physical properties (e.g., boiling point).

Differences in solubility can be used to separate solids.
Choose a suitable solvent where only the desired substance dissolves.
- Example: Water dissolves salt but not sand.

Used to separate an undissolved solid from a mixture of solid and liquid (e.g., sand from water).
Centrifugation can also be used.
Process:
- Filter paper is placed in a filter funnel above a beaker.
- The mixture is poured into the filter funnel.
- Liquid passes through the filter paper as filtrate.
- Solid particles remain on the filter paper as residue.

Used to separate a dissolved solid from a solution when the solid is more soluble in hot solvent than cold (e.g., copper sulfate from a copper(II) sulfate solution).
Process:
- Heat the solution to evaporate the solvent and create a saturated solution.
- Test for saturation by dipping a cold glass rod into the solution; crystals will form on the rod if saturated.
- Allow the saturated solution to cool slowly so crystals form.
- Collect crystals by filtering the solution.
- Wash with distilled water to remove impurities and dry.

Used to separate a liquid and soluble solid from a solution (e.g., water from saltwater) or a pure liquid from a mixture of liquids.
Process:
- Heat the solution, evaporating the pure liquid (e.g., water), which rises as vapor.
- The vapor passes through a condenser, where it cools and condenses into a pure liquid, collected in a beaker.
- The solid solute is left behind after all the liquid evaporates.
Can be used to separate alcohol and water from fermentation.

Used to separate two or more miscible liquids (e.g., ethanol and water).
For liquids with closer boiling points and when a higher degree of purity is required.
Process:
- Heat the solution to the temperature of the substance with the lowest boiling point.
- That substance evaporates first; the vapors pass through a condenser, cool, and condense into a liquid collected in a beaker.
- After the first substance is collected, the remaining component(s) are left behind.
  - Example: Ethanol has a boiling point of $78°C$ and water of $100°C$ . Heat the mixture to $78°C$ to distill out the ethanol. Stop heating when the temperature increases to $100°C$ to separate the water.