Elements, Compounds, and Mixtures

Chemistry Unit 4: Elements, Compounds, Mixtures

• Refer to pages 2.34-2.45 of notebook, Obj.4.a Slides.
• Refer to pages 2.30-2.32 of notebook for phenomenon.

Pure Substances

• Pure substances comprise of one type of matter (element or compound).
• A pure substance will have unique physical and chemical properties like melting/boiling points, density, and hardness.

Impure Substances

• Impure substances consist of more than one type of matter physically combined.
• Physical properties of pure substances in the mixture can separate them.

Elements

• Elements consist of only one kind of atom.
• Some elements exist as diatomic molecules.
• Examples of elements from group 7 (halogens) include H₂, O₂, N₂.
• Elements identified by a chemical symbol.

Compounds

• Compounds consist of two or more different elements.
• Elements are chemically bonded.
• The elements are in fixed amounts.
• Formulas are used, with numbers as subscripts, to write elements.

Mixtures - Homogenous (Solutions)

• Refer to page 2.35 for more examples
• Homogenous mixtures look uniform throughout.
• Solutions are made of solvents and solutes.
• Solvents are usually present in large amounts, typically liquid (like water).
• Solutions in water are aqueous, denoted by the symbol (aq).
• Solutes are present in smaller amounts and dissolve.
• Alloys are solid metal solutions.
• Air is a solution of gasses.
• N₂ is the solvent; O₂, CO₂, H₂O, etc., are solutes.

Dissolving Ionic Substances vs. Covalent Substances (like Sugar) in Water

Ionic Substances:

• Water molecules interact with the substance:
  • If soluble, water molecules overcome ionic attractions within the lattice.
  • Solvated ions move off the surface and separate in the solution.
• Water molecules cluster around anions:
  • Positive hydrogens bond to negative anions.
  • Negative oxygen bonds to positive cations.

Covalent Substances:

• Water molecules arrange around the substance according to opposite polar areas.
• The attraction of water molecules/motion overcomes the attraction between the molecules.
• Molecules dissolve because they are separated from each other and mixed in with water.

Mixtures - Heterogeneous (Suspensions)

• Adding a solid to water turns it "cloudy".
• If left alone, solid particles precipitate out and settle out to the bottom.
• Heterogeneous mixtures do not look uniform throughout; particles can usually be seen.
• Example: chocolate chip cookies.
• Suspensions: examples of heterogeneous mixtures in a liquid.
• If left to stand, suspended particles may separate, settle, or precipitate out.
• Particles do not dissolve (insoluble), like medicines or drinks that say "shake well before use".
• Immiscible: when liquids (like oil and water) do not mix.

Examples of Homogenous vs. Heterogeneous Mixtures

Homogenous:

• lemonade
• tea w/ sugar
• salt water
• milk
• ketchup
• paint
• these are the same throughout

Heterogenous:

• gravel
• chocolate chip cookies
• soda (w/ carbonation)
• fruit yogurt
• buttered popcorn
• crayons (wrapped in paper)
• salad
• chicken noodle soup
• these are not the same throughout
• Refer to page 2.44 for particle/substance info.
• Refer to pages 2.38-2.40 for more phenomenon examples.

Separations - Filtration

• Separations (pages 2.46-2.47 of notebook, Obj.4.b Slides)
• Used to separate a suspension (heterogeneous mixture).
• Method depends on properties of the substance to be collected.

Common properties include:

• Particle size (big or small): soluble vs. insoluble.
• Boiling point: temperature to change from liquid to gas.
• Solubility: whether it dissolves in a specific solvent or not.
• Density: whether it is likely to float or sink in a solvent.
• One can collect material in the residue and/or filtrate.
• Insoluble substances (that don't dissolve) won't pass through pores (holes) in the filter paper.
• The solid trapped on filter paper is the residue.
• Soluble substances (that dissolve) pass through the paper with the solvent.
• The liquid collected is the filtrate (collected by another method if needed).

Separations - Evaporation

• Evaporation is used to separate a solution (homogenous mixture).
• One can only collect the solute (dissolved solid) in evaporation.
• Solvent evaporates (changes from liquid to gas), leaving behind the solid (can be scraped off).

Separations - Distillation

• Distillation is used to separate a solution (homogenous mixture).
• One can only collect solvent (liquid) in distillation.
• The mixture is heated.
• Lower boiling point liquid gains enough energy to evaporate.
• Gas is cooled and condenses into pure liquid.
• The condenser is always cold (gas condenses back to liquid).
• The solid is left in the original beaker, and the liquid is in a new beaker.