Chemianalysis and Experimental Technique

Testing for ... well, everything.

​IGCSE Nov 2017, Question 4

​When iron nails rust, the mass increases.
Plan an experiment to determine if iron nails rusts faster in tap water or distilled water. You are given fresh iron nails and laboratory apparatus.

1. Fixing constant variable

   1. Using a fixed mass/volume/concentration

2. Comparing independent variable.

   1. Repeating the experiment using a different independent variable.

3. Time factor

   1. Measure amount of time taken

4. Experimental Technique/Precaution

   1. State method to collect the dependent variable

5. Measure the dependent variable

6. Hypothesis

​IGCSE Nov 2017, Question 4

Iron, tin and zinc react with hydrochloric acid to produce hydrogen.
Plan an experiment to determine the order of reactivity of iron, tin and zinc. You are provided with powdered sample of the metals and standard laboratory apparatus.

​IGCSE Nov 2023, Question 4

​IGCSE Nov 2017, Question 4

Potassium sulfate is formed from the reaction between potassium hydroxide and sulfuric acid.
Plan an experiment to obtain a pure sample of potassium sulfate. You are given dilute potassium hydroxide and sulfuric acid, with standard laboratory apparatus and materials.

​IGCSE June 2024, Question 4

​IGCSE Nov 2023, Question 4

​The solubility of solid sodium sulfate in water changes as the temperature of the water changes.

Plan an experiment to find out how the solubility of sodium sulfate in water changes with temperature.

You are provided with sodium sulfate, distilled water and common laboratory apparatus.

Multiple Select

What are the two reagents to test for presence of cations in a solution?

• Aqueous sodium hydroxide

• Aqueous potassium hydroxide

• Aqueous sodium chloride

• Aqueous ammonia

Match

Match the following test results with the correct cations when reacted with NaOH.

• White precipitate, redissolves in excess

• White precipitate, insoluble in excess

• Green precipitate, soluble in excess

• Green precipitate, insoluble in excess with browning at surface

• Light blue precipitate, insoluble in excess

Aluminium and Zinc

Calcium ions

Chromium ions

Iron(III) ions

Copper (II) ions

• Aluminium and Zinc

• Calcium ions

• Chromium ions

• Iron(III) ions

• Copper (II) ions

The reaction with sodium hydroxide produces precipitates because the resulting hydroxide salts are all insoluble - at first, at least.

Multiple Choice

Which ion produces a reddish-brown precipitate that is insoluble in excess NaOH solution?

• Cu²⁺

• Ag⁺

• Fe³⁺

• Pb²⁺

Open Ended

Why do you think chromium, aluminium and zinc ions redissolve in excess NaOH? Use what you know about aluminium to make a hypothesis.

Amphoteric properties of salts

Chromium, zinc and aluminium are amphoteric in nature, meaning their compounds often can act as acids and bases.

In this case, chromium hydroxides, zinc hydroxides and aluminium hydroxides react with the excess basic sodium hydroxides as acids, forming soluble sodium complex salts.

Match

Match the following test results with the correct cations in dropwise and excess ammonia.

• White precipitate, redissolves in excess

• White precipitate, insoluble in excess

• Green precipitate, insoluble in excess

• No precipitate or little white precipitate

• Light blue precipitate,dissolves in excess to form dark blue solution

Zinc

Aluminium ions

Chromium and Fe(II) ions

Calcium ions

Copper (II) ions

• Zinc

• Aluminium ions

• Chromium and Fe(II) ions

• Calcium ions

• Copper (II) ions

Multiple Choice

Which cationic test should be used to differentiate between aluminium and zinc ions?

• Aqueous ammonia

• Aqueous sodium hydroxide

• Both

• Neither

Multiple Choice

Which cationic test should be used to differentiate between iron (II) and iron (III) ions?

• Aqueous ammonia

• Aqueous sodium hydroxide

• Both

• Neither

Multiple Choice

Which cationic test should be used to differentiate between chromium and iron (II) ions?

• Aqueous ammonia

• Aqueous sodium hydroxide

• Both

• Neither

Fill in the Blank

There is one more cation that I have not asked about in the previous questions. Can you guess which is it?

Open Ended

What is the test for that particular ion?

Match

Match the following test results to the correct gases.

• Turns blue litmus red and then white

• Makes a squeaky "pop" sound

• Turns limewater cloudy or milky

• Turns red litmus blue

• Turns acidified KMnO4 colourless

Chlorine gas

Hydrogen gas

Carbon dioxide

Ammonia

Sulfur dioxide

• Chlorine gas

• Hydrogen gas

• Carbon dioxide

• Ammonia

• Sulfur dioxide

Tests for anions

Now the test for anions are more varied due to the different natures of each anion - but they are much easier to explain than cations from the perspective of an IGCSE student.

Match

Match the following anions with their required reagents for testing.

• Chloride, bromide and iodide

• Sulphate ions

• Sulfite ions

• Nitrate ions

• Carbonate ions

Acidified silver nitrate

Acidified barium nitrate

Acidified potassium manganate

Warm sodium hydroxide and aluminium

Adding dilute acid

• Acidified silver nitrate

• Acidified barium nitrate

• Acidified potassium manganate

• Warm sodium hydroxide and aluminium

• Adding dilute acid

Fill in the Blank

When reacting nitrate ions with warm sodium hydroxide and aluminium, a gas is produced. Make a guess at what that gas is. (Hint: think about what chemical elements are produced. The gas is a covalent compound.)

Open Ended

What is acidification and what is its purpose?

Acidification of solutions

Acidification is the addition of acid with the same anion as the reagent that is about to be used, usually nitric acid.

The excess protons react with interfering anions such as hydroxide ions and carbonate ions, preventing false positives with the reagent test.

Fill in the Blank

What is the important product between silver nitrate and halide ions? (Halide refers to chloride, bromide and iodide ions)

Open Ended

Write out the chemical reaction between silver nitrate and sodium bromide. Include state symbols.

Match

Match the following silver halides to their respective precipitate colors.

• Silver chloride (AgCl)

• Silver bromide (AgBr)

• Silver iodide (AgI)

White

Cream

Yellow

• White

• Cream

• Yellow

Open Ended

Write out the reaction between barium nitrate and sodium sulphate. State its importance in the testing of sulphate ions.

Fill in the Blank

What happens when we add acid to carbonate ions? It produces ...

Fill in the Blank

What type of gas is produced?

Separating substances

Open Ended

What are the various types of separating different substances?

Multiple Choice

What is the first thing to consider when selecting a method to separate substances?

• How many substances there are

• The states of matter of each substance

• The solubility of each substance

• The density of each substances

Match

Match the following separating techniques with the suitable pair of substances

• Filtration

• Simple distillation

• Crystallization

• Fractional distillation

• Chromatography

An insoluble solid from liquid

A liquid from a soluble solid

A soluble solid from liquid

Liquids with different densities

Different soluble solids in a liquid

• An insoluble solid from liquid

• A liquid from a soluble solid

• A soluble solid from liquid

• Liquids with different densities

• Different soluble solids in a liquid

Labelling

Label the following simple distillation diagram

Drag labels to their correct position on the image

To collect pure water

To remove heated water from system

To lower temperature and condense gas

maintain constant temperature gain

Open Ended

Briefly describe the method of crystallization for a solution of potassium sulphate.

Fractional Distillation

Fractional distillation works on the principle that all liquids have different melting points, and thus can be separated into different fractions.

"The higher the fraction on the distillation column, the lower the boiling point and the more flammable the fraction."

Paper Chromatography

Used to separate different soluble substances from a liquid and identify them.
The principle: each soluble substance has different solubility in water (wife) and affinity to the chromatography paper (mother).

*Pretty much a "Mother vs Wife" Scenario

Experimental Techniques

Open Ended

What do you understand about experiments?

How do we conduct experiments?

Experiments isn't just about going to laboratories, add A to B or measure C and use it on D.

Just like baking cakes or building RPG characters, there is a method to conducting experiments - and that's what we are going to be discussing.

Multiple Choice

What is the first question to be asked when planning an experiment?

• What?

• Why?

• How?

• Why not?

Purpose of Experiment

"If we do not know the purpose of studying, why should we study?"

When we do experiments, we start off with a purpose. By the end of the experiment, that purpose must be completed.

Problem Statement

A question to guide the experiment:

Problem Statement: Is iron a catalyst?
Purpose of experiment: The experiment will therefore test if iron is a catalyst or not.

Open Ended

Try to write the purpose of the experiments for the following problem statements:

1. Does the temperature change when ice melts?

2.Is oxygen produced when water is electrolyzed?

3.Does the reactivity of group 1 elements increase down the group?

Open Ended

Try to write the problem statements for the following experiments:

1. Investigating the effect of a catalyst on the rate of reaction.

2.To determine if photosynthesis produces oxygen.

3.To study the difference in temperature changes during an exothermic and endothermic reaction.

Variables

Variables come from the word "vary" - to change.

In an experiment, we need first and foremost the independent (manipulated) variable and dependent (responding) variable.

*The phrases in brackets are there for you to understand what each variable is supposed to do in the experiment.

Variables

Variables come from the word "vary" - to change.

In an experiment, we need first and foremost the independent (manipulated) variable and dependent (responding) variable.

*The phrases in brackets are there for you to understand what each variable is supposed to do in the experiment.

The variable that changes in response to changes in the manipulated variable.

The dependent variable is the results of the experiment, and must be recorded or measured after each change in the independent variable.

Dependent Variable

The variable that you manipulate or change throughout the experiment.

This variable is controlled by you, and you will use different values of the variable in the experiment.

Independent variable

Manipulated and Responding

Example of Experiment

Purpose of Experiment: To test the reactivity of Group 1 Metals with water.

Manipulated Variable: Type of Group 1 Metals

Responding Variable: Amount of hydrogen gas produced/temperature increase/intensity of sizzling or bubbling

Constant (Fixed) Variable

Now the fixed variable is just as its name calls it - fixed. The fixed variable is a variable that can be controlled directly by the scientist. However, it must remain constant during the experiment or else the results will not be valid.

When Planning Experiments

Each experiment should be based on one of the known chemical equations learnt in the chapter.

For Alkali Metals - reaction with oxygen, water or acids.

For Rate of Reaction - hydrochloric acid with marble chips/powder; HCl with sodium thiosulphate.

Transition Metals - the properties of a catalyst.

Open Ended

If you were to conduct an experiment to test the reactivity of Group I metals with air, what would be the manipulated and responding variables?

Match

Match the following variables for the following experiment:

Aim of Experiment: To measure effect of concentration on rate of reaction.

Method: Reacting hydrochloric acid with magnesium ribbon

• Independent variable

• Dependent variable

• Fixed variable

Concentration of HCl

Time taken to dissolve magnesium ribbon

Volume of HCl used

• Concentration of HCl

• Time taken to dissolve magnesium ribbon

• Volume of HCl used

Match

Match the following variables for the following experiment:

Aim of Experiment: To compare activity of Group 7 elements.

Method: Reacting halogens with different halide solutions

• Independent variable

• Dependent variable

• Fixed variable

Type of halogen

The presence of precipitate

Volume of halide solution used

• Type of halogen

• The presence of precipitate

• Volume of halide solution used

Match

Match the following variables for the following experiment:

Aim of Experiment: To compare activity of Group 1 elements.

Method: Reacting alkali metals with water

• Independent variable

• Dependent variable

• Fixed variable

Type of alkali metals

Time taken for the fizzing to stop

Volume of water used

• Type of alkali metals

• Time taken for the fizzing to stop

• Volume of water used

Match

Match the following variables for the following experiment:

Aim of Experiment: To compare activity of Group 1 elements.

Method: Reacting alkali metals with water

• Independent variable

• Dependent variable

• Fixed variable

Type of alkali metals

Intensity of flame or sizzling

Mass of metal ribbon

• Type of alkali metals

• Intensity of flame or sizzling

• Mass of metal ribbon

Match

Match the following variables for the following experiment:

Aim of Experiment: To compare activity of Group 1 elements.

Method: Reacting alkali metals with water

• Independent variable

• Dependent variable

• Fixed variable

Type of alkali metals

Time taken to collect 10 cm3 of gas

Temperature of surroundings

• Type of alkali metals

• Time taken to collect 10 cm3 of gas

• Temperature of surroundings

Match

Match the following variables for the following experiment:

Aim of Experiment: To compare melting point of covalent and ionic compounds

Method: Heating compounds to their melting points

• Independent variable

• Dependent variable

• Fixed variable

Type of compounds

Temperature at which compound melts

Mass of compound used

• Type of compounds

• Temperature at which compound melts

• Mass of compound used

Match

Match the following variables for the following experiment:

Aim of Experiment: To compare electrical conductivity of covalent and ionic compounds

Method: Connecting the solution to a circuit with a light bulb

• Independent variable

• Dependent variable

• Fixed variable

Type of compounds

The lighting of bulb

Volume of solution used

• Type of compounds

• The lighting of bulb

• Volume of solution used

Match

Match the following variables for the following experiment:

Aim of Experiment: To test if iron is a catalyst.

Method: Decomposing hydrogen peroxide to oxygen and water in the presence of iron

• Independent variable

• Dependent variable

• Fixed variable

Presence of iron

Amount of gas collected in 10 mins

Mass of iron used

• Presence of iron

• Amount of gas collected in 10 mins

• Mass of iron used

Consistency is how often you get similar results after multiple experiments.
Consistency increases when you repeat the experiments and obtain average readings.

Consistency

Accuracy is how close your results are to the theoretical value.
Accuracy increases when you use more sensitive equipment i.e. pipette is more accurate than a measuring cylinder.

Accuracy

Two Key Aspects

Open Ended

When planning an experiment, we should always state the hypothesis. What is the hypothesis?

EZ Hypothesis Formula

The hypothesis is your prediction as to the trend your experiment will go.

The easiest formula for a hypothesis is this:

The (higher/more/presence) of Independent Variable, the (more/less/faster/slower) the Dependent Variable.

Open Ended

Write a hypothesis for an experiment testing the reactivity of sodium, potassium and rubidium with water.

Open Ended

Write a hypothesis for an experiment testing the effect of temperature on the rate of reaction by testing the reaction of HCl with sodium thiosulphate to make a precipitate.

Open Ended

Write a hypothesis for an experiment testing the electrical conductivity between sodium chloride and sulfur chloride with a light bulb.

Open Ended

Write a hypothesis for an experiment testing the activity of iron as a catalyst for the decomposition of hydrogen peroxide.