Solutions

Solvents, solutes and solutions

Solvent: A substance which can dissolve other substances. E.g water

Solute: A substance which can dissolve E.g sugar

Solution: Is the mixture of a dissolved solute and a solvent uniformly distributed.

What is dissolution?

This is where the solute particles are separated by the forces inside the solvent causing it to disperse and become evenly distributed in the mixture.

Example: ionic solute (salt - NaCl)

Oxygen atoms in water are slightly more electronegative than hydrogen so they pull the shared electrons closer to it, making the molecule polar. When it comes in contact with the ionically-bonded salt molecules opposite charges attract and the salt atoms are pulled away from the main crystalline structure.



Source: www.apsubiology.org

 

Example: Covalent solute (sugar - C12H22O11)

Sugar bonds covalently so the atoms cannot disassociate, however sugar as a molecule is slightly polar like water. So when water comes in contact with it, the water molecules rotate until oppositely charged faces are facing each other. Opposite charges attract and the water pulls  the outer layer of molecules away from the main structure.

Source: www.ck12.org

 

Solubility

Soluble: A solute is able to dissolve in a solvent

Insoluble: A solute is unable to dissolve in a solvent

Solubility: The ability for a solute to dissolve in a solvent

What factors affect solubility?

Factor

How it affects solubility

Explanation

Temperature

Higher temperature generally gives higher solubility in solids.

The higher the temperature the more energy the solute and solvent particles have, so the solvent can break the solutes bonds more easily. Also if the dissolution is endothermic (which most are), using Le châtlier’s principle Increased temperature will shift the reaction towards the products causing more of the solute to dissolve

Higher temperature generally gives lower solubility in gases

As the temperature increases the gas molecules find it easier to escape from the solvent and  so the solubility decreases.

Pressure

As pressure increases the solubility of gases increases. Pressure has no effect on solids.

Extra pressure causes the gas to try and fill spaces inside the solvent, it also makes it harder for the gas to leave the solvent. This is why carbonated soda is under so much pressure; it allows more CO2 to be dissolved in the drink.

Solvent polarity

Solvents must be polar to dissolve.

If a solvent is not polar it can not pull apart the solute molecules.

 

Precipitation reactions

Definition: Mixing two solutions containing soluble salts forms an insoluble salt.

 

When two solutions of soluble salts mix together, oppositely charged ions are pulled together and react. Like how charged sides of water are pulled towards the charged ions of salt. Sometimes an insoluble salt is formed and this will fall out of solution. This is called a Precipitate

 

For example:

Lead nitrate + potassium iodide → lead + iodide + potassium nitrate.

 

In the case of the equation above, potassium nitrate is formed and as this is insoluble it is no longer dissolved and becomes the precipitate

Predicting the products of a precipitate reaction with solubility rules

 

You may be wondering how to predict if mixing two soluble salts will produce a precipitate. To do this we need to know how to apply the solubility rules.

 

Solubility rule table

Rule for solubility

Exceptions to rule

All nitrates

None

Most sulfates

Lead sulfate, barium sulfate and calcium sulfate

Most chlorides, bromides and iodides

Silver chloride, silver bromide, silver iodide, lead chloride, lead bromide, lead iodide

Sodium carbonate, potassium carbonate, ammonium carbonate

Most other carbonates

Sodium hydroxide, potassium hydroxide, ammonium hydroxide

Most other hydroxides

Source: BBC bitesize



Step in process

Compounds in solution

Firstly, when when soluble salts dissolve their ions dissociate so when two salt solutions are mixed the product will be a solution of cations and anions

lead + iodide + potassium + nitrate.

These ions are pulled together as they have opposite charge causing nitrate to go with lead and potassium to go with iodide.

lead iodide + potassium nitrate.

According to the  solubility rule nitrates are soluble and lead iodides are insoluble. So lead iodide is a precipitate and potassium nitrate will remain dissolved in the solution.

Lead Iodide + potassium + nitrate.

 

This reaction is actually called a double displacement reaction but you don’t need to know this till DP!

 

Physical changes vs Chemical changes

 

A reaction can either be physical or Chemical. The main rule is that chemical reactions create a new substance while physical does not.

 

Chemical change

Physical reaction

Changes chemical composition

No change in composition

Change colour

Stays the same colour

Often irreversible

Reversible

 

Chromatography

Definition: A process used to separate a mixture of coloured solutions.

 

Using it to separate dyes in ink

 

  1. A line is drawn at the bottom of a strip of paper and a spot of the mixture is placed on the line.
  2. The paper is then dunked into a beaker of water up till the line. The water is absorbed into the paper and causes the ink to run up.
  3. This separates out the ink as different dyes move at different rates due to their varying weights.
  4. The Rf value of the inks can be calculated by dividing the distance moved by the dye over the distance moved by the solvent. By comparing these to industry standards the specific dye can be identified.

Source: http://www.bbc.co.uk

 

Colloids and Emulsions

Colloids

Definition: A substance consisting of large molecules of one substance dispersed equally in another substance

 

There are 4 types:

  1. Sol - Solid particles in a liquid
  2. Emulsion - Liquid in a Liquid
  3. Foam - Gas in a solid or liquid
  4. Aerosol - Liquid or solid in a gas

 

Continuous phase: The solvent

Dispersed Phase: The solute

Emulsion

There are many liquids which don’t mix together called immiscible Liquids. An example of two such liquids are oil and water. However by adding a molecule called an Emulsifier, a colloid can be formed making it look like a dissolution has taken place even though it hasn’t.

An emulsifier is a molecule  that is soluble in water on one side and soluble in oil on the other side. By putting this molecule in the solution it will connect the two solutions together allowing them form a stable not separated solution called an emulsion.

 

Source: www.nonmajorsbiology.files.wordpress.com

Examples of colloids from everyday life

Milk/butter: Milk and butter are good examples of emulsions. In milk the fat is the dispersed phase while in butter the fat is the continuous phase.

 

Deodorant: This a good example of an aerosol, as the liquid is dispersed in a gas.








Editors

View count: 3407