Elements, Compounds and Mixtures (1b)

Unit 1, Principles of Chemistry: Part 1, b

Element

• a substance made of atoms that all contain the same number of protons and can’t be split into anything simpler

• there 118 elements

• ex: Copper, Iron, Arsenic

Atom

Smallst part of an element that has the element's properties.

Compound

• a pure substance made of two or more different elements chemically bonded

• ∞ number of compounds

• can’t be separated by physical means

• ex: NaCl, MgO

Mixture

• combination of two or more substances mixed but not chemically bonded together.

• can be separated by physical means like filtration/evaporation

• ex: sand and water, oil and water

Molecule

One or more element chemically combined

Heating Curve

Graph showing how a substance changes state when heated

Cooling Curve

A graph showing how a substance changes state when cooled.

Filtration

Used to separate insoluble solid impurities from a mixture.

Fractional Distillation

• Used to separate miscible substances with different boiling points

• Can separate more than 2 substances

Miscible Liquids

• Liquids that mix

• They form one layer

• Eg: ethanol and water

Immiscible Liquids

• Liquids that don't mix

• They form more than one layer

• Eg: Oil and water

Properties of the compound vs Properties of the element it's made of

Different

Pure salt from Rock Salt

• Filtration and Crystallisation

• Rock salt = Pure salt + insoluble impurities

• Filtration: separates the insoloble impurities from the mixture

Element, Compound, Mixture Diagram

Pure Substance vs Mixture

• Natural language: natural and clean, nothing added

• Chemistry: pure substance = single element/compound with no other substances

• ex: pure water has only H₂O molecules

• drinking water isn’t pure because it has additional substances like dissolved ions and chlorine

Distinguishing Purity

• Pure substances melt and boil at specific temperatures                                              ex, pure water m.p. = 0^°C, b.p = 100°C

• impure substances have a range of melting and boiling points because they consist of different substances

• so, this data can be used to distinguish pure and impure substances

• this also helps us assess the purity of drugs and foodstuffs

Assessing Purity: Process

• using a melting point apparatus to heat a small portion of the sample and observe the exact melting point

• compare to a data table

• the closer the value is to the actual melting point, the purer the substance

Simple Distillation

• separate a liquid and a soluble solid from a solution or a pure liquid from a mixture of liquids

• the solution is heated, and the liquid evaporates

• the vapour rises through the neck of the round-bottomed flask

• the vapour passes through the condenser, where it condenses and cools to a pure liquid that is collected in a beaker

Fractional Distillation

• used to separate two or more miscible liquids

• solution is heated to the temperature of the substance with the lowest m.p

• this substance will evaporate first, and vapours will pass through a condenser

• the liquid will be condensed and collected in a beaker

• once all of the substance is collected, the other liquid will be left behind

Fractional Distillation of Ethanol and Water

• Ethanol b.p = 78 °C, Water b.p = 100 °C

• mixture is heated until it reaches 78 c, and the ethanol boils and distills out of the mixture and condenses into the beaker

• when the temperature starts to increase to 100 °C, heating should be stopped. Water and ethanol are now separated

Filtration

• used to separate undissolved solids from a mixture of solids and a liquid (centrifugation can also be used for this mixture)

• filter paper is placed in a filter funnel above a beaker

• the mixture is poured in

• the filter paper will only allow small liquid particles to pass through as filtrate

• solid particles are too large, so the stay behind as filtrate

Crystalllisation

• used to separate a dissolved solid from a solution, when the solute is more soluble in hot solvent than cold

• solution is heated, allowing the solvent to evaporate and leave a saturated solution behind

• test if the solution is saturated by dipping a glass rod into the solution (if the solution is saturated, crystals will form on the glass rod)

• saturated solution will cool slowly

• crystals will grow as solids come out of the solution due to decreasing solubility

• crystals are collected by filtering, they are washed with cold distilled water to remove impurities and then allowed to dry 

Paper Chromatography

• used to separate substances with different solubilities in a given solvent

• a pencil line drawn on chromatography paper, and spots of the sample are placed on it (pencil is used as ink would run into the chromatogram along with the samples)

• the paper is then lowered into the solvent container (the line must be above solvent line so samples don’t wash into the solvent container)

• solvent travels up the paper by capillary action, taking the coloured substances with it

• different substances have different solubilities, so will travel at different rates    (this causes the substances to separate, higher solubility = travel further)

• this will show the different components of the ink/dye

Interpreting Chromatograms

• pure substances = only one spot

• impure substance = multiple spots

• same substance = identical chromatograms

• mixture = separates to show different components as separate spots

• use a known compound to identify spots

Rf Values

• used to identify components of mixtures

• always the same for a particular compound

• solvent changed = Rf value changes

• allows to identify compounds because it can be compared with known values

• Rf = distance moved by substance / distance moved by solvent