General Practical Techniques (SQA National 5 Chemistry): Revision Note

Simple filtration

• Filtration is a technique used to separate an insoluble solid from a liquid

• It is commonly used to:

  • Remove insoluble impurities

  • Collect insoluble products

• Examples of when filtration can be used include:

  • Removing sand from a dissolved mixture of rock salt

  • Collecting a solid product from a precipitation reaction

How to filter

1. Fold a piece of filter paper and place it inside a funnel

2. Place the funnel over a conical flask or beaker

3. Pour the mixture through the filter paper

   • The liquid that passes through the filter paper is called the filtrate

     • The filtrate is a solution

     • The solution is made of the liquid / solvent and any dissolved substances

   • The solid that is trapped in the filter paper is called the residue

Using a balance

• A digital balance is used to accurately measure mass, usually in grams (g)

How to use a balance

1. Place an empty beaker or weighing boat on the balance

2. Press the tare (or zero) button to reset the display to zero

   1. This removes the mass of the container

3. Carefully add the substance to be weighed until you reach the desired mass

4. Wait for the reading to settle before recording it

Collecting gases

• The method used to collect a gas depends on:

  • Its solubility in water

  • Its density compared to air

• The three main methods for collecting gases are:

  • Collecting over water

  • Upward displacement of air

  • Downward displacement of air

1. Collection over water

• This method is used when gases are insoluble or have low solubility in water (e.g., hydrogen)

How it works

• As the reaction proceeds, a gas is produced

• The gas passes through a delivery tube into an inverted measuring cylinder

  • The measuring cylinder is filled with water and placed inside a water-filled trough or container

• The gas displaces (pushes) the water out of the inverted measuring cylinder

  • The volume of gas being produced can be measured using this method

2. Upward displacement of air

• This method is used when gases are soluble in water AND denser (heavier) than air (e.g., carbon dioxide)

How it works:

• As the reaction proceeds, a gas is produced

• The gaseous product passes through a delivery tube into the bottom of an upright gas jar, filled with air

• The gaseous product fills the gas jar and pushes the less dense air out of the top of the gas jar

3. Downward displacement of air

• This method is used when gases are soluble in water AND less dense (lighter) than air (e.g., ammonia)

How it works:

• As the reaction proceeds, a gas is produced

• The gaseous product passes through a delivery tube into the top of an inverted gas jar, filled with air

• The gaseous product fills the gas jar and pushes the less dense air out of the bottom of the gas jar

Heating

• Heating is a common requirement in many chemical experiments, either to start a reaction or to speed it up

• The choice of heating method depends on:

  • What is being heated

  • The level of control required

Bunsen burners

• A Bunsen burner is the most common piece of heating apparatus in a school lab

• It is used for general-purpose heating of substances that are not flammable

• For strong, direct heating, you should always use a blue flame, which is hotter and cleaner than the yellow safety flame

Electric hotplates

• An electric hotplate is a safer alternative to a Bunsen burner, particularly when heating flammable liquids like alcohols

• Since there is no open flame, the risk of the substance catching fire is eliminated

• Hotplates also provide more gentle and easily controllable heating