Particle Nature of Matter and Separation Techniques

What is boiling?

Vapourisation (liquid to gas) of particles at a specific temperature, it occurs throughout a liquid.

What is the boiling point?

The temperature at which the vapour pressure of a liquid is equal to the atmospheric pressure.

What is evaporation?

Vapourisation (liquid to gas) of particles from the surface of a liquid at any temperature (at which liquid state exists).

On a heating curve when the gradient is 0 what does this mean?

It is melting or boiling. Temperature remains constant because the energy is used to weaken intermolecular forces.

What is an element?

Primary constituents of matter can’t be chemically broken down into simpler substances. Pure substance that contains only one type of atom.

What is an atom?

Smallest part of an element that can still be recognised as that element.

What is a compound?

Pure substance formed when two/more elements combine chemically in a fixed ratio. Chemical/physical properties differ greatly to those of the elements from which it is formed.

What is a mixture?

Contains two/more substances mixed together (not chemically bonded). Retains the individual properties. Can be mixed together in any proportion.

What is a homogenous mixture?

Has the same uniform compositions throughout the mixture. Consists of only one phase. Can’t pick out the individual components. Eg. Air

Heterogenous mixture?

Doesn’t have uniform composition and consists of separate phases (regions of uniform composition)

When is filtration used?

To separate a solid from a liquid or a solid from a gas

Method of filtration?

Filter paper acts as a physical barrier to pieces of solute but allows liquid to pass through the gaps between fibres. The solid left on the filter paper is the residue. The liquid that passes through the filter paper is the filtrate.

What is vacuum filtration?

Vacuum filtration can be used for very finely suspended solids which can clog up pores in filter paper when only using gravity filtration. Vacuum filtration is faster than gravity filtration.

When is evaporation used?

Can be used to remove solvent from a solution to leave solute.

What does solvation involve chemically?

Involves breaking of intermolecular forces within the solute and the formation of new interactions between solute and solvent particles.

What solvent extraction used for?

Separation technique involving two immiscible liquids (organic solvent and water) to separate compounds based on their solubility.

Process of solvent extraction (solvation)?

1. Add two immiscible solvents: Water (aqueous layer, polar) and organic solvent (non-polar).

2. Add the mixture containing the solute (the solute will partition between the two layers based on solubility and polarity).

3. Shake mixture

4. Allow the layers to separate (denser layer at the bottom, water, and less dense layer on top, organic solvent)

5. Drain and collect the layers.

Why does the mixture need to be shaken in solvent extraction?

To increase the contact between layers, allows the solute to come into contact with both solvents and helps the solute move into the solvent which it is more soluble in.

Why do the two solvents in solvent extraction need to be immiscible?

So that they do not mix and form separate layers.

What is recrystallisation used for?

Technique used to purify solids which contain relatively small amounts of impurities. This happened because when the lattice formed during precipitation, the ions get trapped inside and disrupt the pattern of the lattice.

Method for recrystallisation?

1. Solid dissolved in minimum amount of hot solvent.

2. mixture filtered while still hot to remove insoluble impurities, then cooled so that crystals of product form.

3. Filter off crystals with vacuum filtration.

4. Wash crystals using tiny amounts of cold water to remove last traces of surface impurities.

Why is the sold dissolved in hot solvent?

Keeps everything warm and stops premature recrystallisation

What do the properties of the solvent have to be?

Solute must be insoluble in solvent in room temperature, as temperature increases, solubility must also increase. It is important that impurities present are soluble in the solvent at room temperature and insoluble at higher temperatures.

How can crystals be encouraged?

by scraping the inside of the flask with a glass rod.

Why does the mixture need to be cooled in recrystallisation?

Soluble impurities will be fully soluble in cold water so will pass through the filter paper.

What are the properties of a pure substance?

Should have a well-defined melting point. Impure solid will melt at a lower temperature and over a wider range of temperatures.

How does percentage yield indicate purity of the initial sample?

If there is a high percentage yield, then there was a low concentration of impurities. If there is a low percentage yield then there were lots of impurities in the initial sample.

What is simple distillation used for?

Can be used to separate the solute and solvent from a solution or a mixture of two liquids with sufficiently different boiling points.

What are the two types of chromatography?

Paper chromatography and thin layer chromatography (TLC)

What is chromatography used for?

To separate various dyes in coloured inks. To separate mixture of sugars or amino acids. To test the purity of a substance.

Process of chromatography?

1. Line is drawn in pencil across chromatography paper, 1cm from bottom.

2. Sample of mixture placed on pencil line and allowed to dry.

3. Paper suspended in contained with small amount of solvent at bottom so end of paper dips into solvent

4. Close container so that atmosphere becomes saturated with solvent

5. Solvent drawn up the paper by capillary action.

6. Process stopped when solvent front is 1cm from the top of paper.

What is TLC?

It is the same process as paper chromatography but a plate (plastic/glass/metal) is coated in silica gel or alumina is used instead of paper.

What does the separation rely on in TLC?

Separation due to adsorption. Relies on the formation of intermolecular forces between solute molecules and the silica surface, stronger interactions are formed with polar molecules compared to non-polar molecules.

Adsorption in relation to distance travelled?

The greater the tendency of a solute molecules to be adsorbed onto the stationary phase, the more slowly it moves along the plate.

How do components travel in paper chromatography?

Components are dissolved onto the stationary phase.

Column chromatography?

Used for large-scale separation of mixture, each component collected separately and can then be further processed.

Column chromatography process?

1. Column packed with the Stationary phase

2. Mixture to be separated added at the top of the column.

3. Tap opens so that solvent (mobile phase) flows through the column.

4. As solvent flows through column, the components of the mixture move down the column.

5. Different components in the mixture interact differently with the mobile and stationary phase. Higher adsorption to the stationary phase means it will not travel as fast down the column.

6. Different components of mixture are collected in separate beakers depending on the speed they travel down the column.

What is the retention time?

The time at which components emerge from column chromatography

How can retention time be represented?

As a graph where each peak represents a different component of the mixture. Area under peak is proportional to the amount of that component in the mixture.

what is the stationary phase?

Part of the chromatograph that doesn’t move. Silica on TLC. Paper strip in paper chromatography.

What is the mobile phase?

Part of the chromatograph that does move. The solvent in paper chromatography and TLC. The inert gas in gas chromatography

What is partition?

Position of equilibrium between the mobile and stationary phase.

X (mobile phase) ⇌ X (stationary phase)

What is the Rf value?

The distance the solute moves / distance the solvent front moves

What affects the Rf value?

Depends on the mobile and stationary phase used.