Elements, compounds and mixtures 1.8-1.13

Element

A substance made of atoms that all contain the same number of protons and cannot be split into anything simpler.

Compound

A pure substance made up of two or more elements chemically combined.

Mixture

A combination of two or more substances that are not chemically combined.

Pure Substance and How To Ditsiguish?

A substance that consists of only one type of particle; can be an element or compound. They have fixed boiling and melting points, compared to impure substances that have a range of boiling + melting points.

Filtration

A physical method used to separate an undissolved solid from a mixture of solid and liquid. The filter paper only allows liquid to pass through hence leaving the solid behind.

Crystallisation

Used to seperate a dissolved solid from a solution, when the solid is much more soluble in hot solvent than in cold. When heated → solvent evaporates → leaving crystals behind

Simple Distillation? and how is it done?

A technique used to separate a liquid and soluble solid from a solution. Involves heating the solution so that the liquid turns into vapor leaving the solid solute behind. The vapor is then cooled and condensed back into liquid.

Fractional Distillation? and when + how is it done?

A method used to separate two or more liquids that are mixed together with different boiling and melting points. Heated to the temperature of the substance with lowest boiling point. The liquid with the lower boiling point evaporates and is cooled through the condesor to collect it as a separate liquid. The liquid with the higher boiling point stays in the orignal beaker.

Chromatography and how is it done?

Used to separate substances that have different solubilities in a given solvent.

1) Penicl line is drawn on chromatography paper → spots of the sample are placed on it

2) Paper is lowered into the solvent container and solvent moves up the paper

3) An impure substance will show more than one spot, while a pure should show only one.

Rf Value

distance travelled by substance - distance travelled by solvent

Solvent Front Line

The line drawn on chromatography paper that indicates how far the solvent has traveled.

Atom

The smallest part of an element that can still be recognised as that element

What does a substance's Rf value depend on?

How soluble it is in the solvent

In chromatography why must the solvent height be lower than the pencil line?

So that the substances do not dissolve into the solvent off the paper