Experimental Chemistry: measurement, experimental techniques, separation and purification Flashcards
Radioactive
Avoid being near these substances
Toxic or poisonous substances
Do not touch, swallow or breathe in such substances
Caution
Handle such substances with care
Irritating or harmful substances
Do not breathe in the vapour or touch these substances with bare skin
Corrosive substances
Do not have direct contact with these substances. Wear gloves when handling. Rinse with water quickly if it spills onto skin
Explosive
Place these substances away from heat sources, sparks, friction and shock
Oxidising agent
Do not expose these substances to the air or place them close to reducing agents
Flammable substances
Keep substances away from flames or sources of heat
Displacement of water
This method is used to collect insoluble or slightly soluble gases. Examples are Hydrogen, Oxygen, Carbon dioxide
Gas released is bubbled into an inverted test tube filled with water. Gas will take up the volume and push down the water level in the gas jar ( since it is unable to dissolve in water )
Downward delivery
This method is used to collect gases that are denser than air and soluble in water. Examples are Chlorine, Hydrogen Chloride
The gas collected will sink in the test tube and displace the air. Gas jar must not be sealed in order for the air to be pushed out by the gas so that the gas can enter
Upward delivery
This method is used to collect gases that are less dense than air and soluble in water. Example is Ammonia
The gas collected will rise up the gas jar displace the air inside. Similarly, the gas jar must not be sealed in order for the displaced air to exit and the gas to enter
Uses of a gas syringe
This method is used to collect any type of gas. Gas produced will be collected in the gas syringe and it pushes the plunger. The volume of gas collected can be measure directly
Concentrated sulfuric acid
Used to dry acidic gases such as hydrogen chloride. The tube introducing the gas is immersed in the acid while the exiting tube is not
Quicklime ( calcium oxide )
Used to dry alkaline gases such as ammonia
Fused calcium chloride ( anhydrous calcium chloride )
Used to dry neutral gases such as hydrogen
Mixture
A substance that is made up of two or more substances that are not chemically combined
Pure substance
Made up of one single element or compound. It is not mixed with any other substance
liquid-liquid mixture
A liquid mixed with another liquid
Solid-solid mixture
A solid mixed with another solid
Solid-liquid
A solid mixed with a liquid
Test for purity
Melting point of a solid ( impurities lower the melting point ) - mixture melts over a range of temperatures
Boiling point of a solid ( impurities increase the boiling point ) - Pure liquid distills at a fixed temperature
Chromatography
Filtration
To separate insoluble solid particles from a liquid
The liquid that passes through the filter paper is called the filtrate
The solid that remains on the filter paper is called the residue
Evaporation to dryness
Used to obtain a soluble solid from a solution by heating the solution until all the water has boiled off
Decomposition
Break down of a compound into 2 or more substances
Crystallisation
Used to obtain a pure solid sample from its solution
Many substances decompose when they are heated strongly
Most crystals, such as copper (II) sulfate crystals, give off water to form powders when heated.
Why doesn’t the compound remain dissolved in the water when the saturated solution is cooled?
When the temperature decreases, it lowers the solubility of the compound and crystals appear
Using a suitable solvent
To separate mixtures of two or more solids, we need to use suitable solvent. The common solvents are water and ethanol
To separate a mixture of two solids, we use a solvent in which only one solid is soluble while the other solid is insoluble
Sublimation
Some substance sublime on heating while other substances like sand do not.
Using a magnet
Some metals are magnetic while other substances such as sulfur and aluminium are non-magnetic
A magnet can be used to separate a magnetic substance from a non-magnetic substance
Distillation
Used to separate a pure solvent from a solution. It is a process of boiling a liquid and condensing water vapour
Why is the bulb of the thermometer placed beside the side arm of the distillation flask?
This ensures that the thermometer measures the boiling point of the substance
Why is cold running water allowed to enter the water jacket from the bottom of the condenser and leave from the top instead of the other way round?
This is to prevent the water from coming out
Fractional distillation
Used to separate miscible liquids with different boiling points
A fractionating column is attached to the round-bottomed flash and the condenser. Glass beads in the column provide a large surface area for vapour to condense on
The vapours of the liquids will condense along the fractionating column and fall back into the round-bottomed flask
The liquid with the lowest boiling point will distil over first
Uses of fractional distillation
(i) Fractional distillation of liquid air to obtain argon, oxygen and nitrogen
(ii) Fractional distillation of crude oil to obtain useful fractions
(iii) Fractional distillation of fermentation mixture of glucose solution to obtain ethanol to manufacture alcoholic beverages or cosmetics
Separating funnel
Used to separate immiscible liquids. When immiscible liquids are shaken, they form an emulsion. The emulsion eventually separates into different layers of liquids
Paper chromatography
Used to separate two or more components that dissolve in the same solvent
The components in the food colouring mixture can be separated and identified using a suitable solvent due to the differences in solubilities of the different components in the chosen solvent
The more soluble the substance is in the solvent used, the further it will travel up the chromatogram. Identical components will travel the same distance
What can be used to identify colourless substances on the chromatogram?
Locating agent
Uses of paper chromatography
(i) Separate the components in a sample
(ii) Identify components in a sample - traces of banned substances
(iii) Identify substances ( poisons, drugs and pesticides )
(iv) Determine whether a sample is pure
Why must the start line be drawn in pencil?
Pen ink can be dissolved
Rf value ( Retention Factor )
Rf value = distance travelled by the substance / distance travelled by the solvent
