CHM Ch6 - Separation of mixtures and identifications of components

Homogenous solution

The solute is evenly spread throughout the solvent

How does a substance dissolve? (4)

1. Solute particles are attracted to solvent particles

2. Particles of solute are separated from one another

3. Particles of solvent are separated from one another

4. Solute particles become surrounded by solvent particles and are carried through the solution

Rule for the strength of bonds to allow a substance to dissolve

For a substance to dissolve, new bonds formed between solute and solvent must be the same strength or stronger than the bonds between particles of solute, and between particles of solvent

Three ways that substances dissolve in water

1. Molecular compounds forming hydrogen bonds in water

2. Molecular compounds that ionise in water

3. Ionic compounds that dissociate

How do substances dissolve in water by forming hydrogen bonds? (provide examples)

1. Hydrogen bonds break between small, polar molecules

2. Hydrogen bonds break between water molecules

3. New hydrogen bonds form between solute and water molecules

4. Water is not a reactant in the equation

(E.g. ethanol, sucrose, glucose)

How do molecular compounds dissolve in water by ionising? (Use HCl as an example)

1. HCl, a small, polar molecule breaks into H+ and Cl- ions

2. H+ and H2O bond to make H3O+

3. The H3O+ and Cl- ions become surrounded by water molecules

4. Strong bonds called ion-dipole bonds form between the ions and water. The ions are now hydrated and their state is aqueous

5. Water is a reactant in the equation

How do ionic compounds dissolve in water by dissociating? (Use NaCl as example)

1. The positive poles of water are attracted to Cl-

2. The negative poles of water are attracted to Na+

3. Ionic bonds between ions in the lattice break

4. New ion-dipole bonds form between ions and water

5. Water is not a reactant

Chromatography

A laboratory technique used to separate different components of a mixture. It is based on the different solubilities of substances

Stationary phase

The phase that does not move (e.g. piece of paper or the inside surface of a tube)

Mobile phase

A solvent or mixture of solvents that move over the stationary phase

Explain how the mixture moves across the stationary phase

• The mobile phase moves over the stationary phase

• Components repeatedly adsorb (stick) onto the stationary phase and then desorb (dissolve) into the mobile phase

Why are components separated using chromatography?

Components are separated because they move at different rates

• Components attracted to the mobile phase desorb more and move faster

• Components attracted to the stationary phase adsorb more and move slower

Retardation factor

The distance a component travels divided by the distance travelled by the mobile phase (solvent front)

Low retardation factor

A low retardation factor indicates that the component is more attracted to the stationary phase and is less soluble in the mobile phase

High retardation factor

A high retardation factor indicates that the component is more attracted to the mobile phase and is more soluble in the mobile phase