Chemistry Notes: Scientific Method, Gases, and Ion Testing

The Scientific Method and Chemistry

19.1 The Scientific Method in Chemistry

Chemistry relies on real experiments conducted in labs.
Chemists, like all scientists, use the scientific method, which involves:
1. Observation: Noticing something and asking a question.
  - Example: Kitchen cleaner X works better than kitchen cleaner Y.
  - Question: Why?
2. Hypothesis: Formulating a testable statement.
  - Requires research.
  - Example: "X may contain more sodium hydroxide than Y."
3. Experiment Planning: Designing a test for the hypothesis.
  - Example: Titration to test the amount of sodium hydroxide.
4. Experiment Execution and Recording:
  - Performing the experiment and noting results.
5. Results Analysis:
  - Examining the data to find trends/patterns.
6. Hypothesis Support:
  - Determining if the results validate the hypothesis.
7. Conclusion Sharing:
  - Communicating the findings.

Variables in Experiments

Independent Variable: The factor you control/change (e.g., temperature).
Dependent Variable: The factor that changes in response to the independent variable (e.g., reaction rate).
Golden rule: Change only one variable at a time to see its effect.

19.2 Skills Used in Experiments

Thinking: Use your brain before, during, and after the experiment.
Working Accurately: Follow instructions and measure carefully.
Observing: Make discoveries through careful observation.
Doing Maths: Perform calculations and create graphs.
Using Apparatus and Techniques: Weigh substances, measure volumes, perform titrations, prepare crystals, etc.
Writing Up: Report the experiment, conclusions, and suggest improvements.

The Importance of Practice

Even when following pre-set instructions, you are still practicing the scientific method.

19.3 Comparing Kitchen Cleaners: A Practical Example

A student's hypothesis: Cleaner X contains more sodium hydroxide than cleaner Y.

Experiment Plan

Titrate each cleaner against hydrochloric acid using methyl orange as an indicator.
The sodium hydroxide neutralizes the acid, and the indicator changes color when neutralisation is complete.
To ensure a fair test, the same volume of cleaner, concentration of acid, and number of indicator drops should be used.
Safety goggles should be worn because sodium hydroxide and hydrochloric acid are corrosive.

Experiment Execution

25 cm $^3$ of cleaner X was measured into a conical flask using a pipette, and 5 drops of methyl orange were added, turning the solution yellow.
A burette was filled with 1 mol /dm $^3$ hydrochloric acid.
Acid was added to the flask while swirling until the solution turned from yellow to pink.
The final level of the acid was recorded.
The experiment was repeated with cleaner Y.

Results

For cleaner X: Initial level of acid was 0.0 cm $^3$ , the final level was 22.2 cm $^3$ , thus 22.2 cm $^3$ of acid was used.
For cleaner Y: Initial level of acid was 22.2 cm $^3$ , the final level was 37.5 cm $^3$ , thus 15.3 cm $^3$ of acid was used.

Analysis of results

The same volume of each cleaner was used. The sodium hydroxide in X neutralized 22.2 cm $^3$ of acid. The sodium hydroxide in Y neutralized 15.3 cm $^3$ of acid. This means that solution …

Conclusion

These results …

Improving Reliability

To improve the reliability of the results I would …

19.4 Working with Gases in the Lab

Gases can be collected over water using a gas jar or using upward/downward displacement.
Use a measuring cylinder to roughly measure the gas volume.
Use a gas syringe to accurately measure the gas volume.

Preparing Gases

Gases are typically prepared by displacing them from a solid or solution.
Ammonia can be made by heating an ammonium compound with a base.

To Make…	Place in flask…	Add…	Reaction
Carbon Dioxide (CO2)	Calcium carbonate (marble chips)	Dilute hydrochloric acid	$CaCO<em>3 (s) + 2HCl (aq) \rightarrow CaCl</em>2 (aq) + H<em>2O (l) + CO</em>2 (g)$
Hydrogen (H2)	Pieces of zinc	Dilute hydrochloric acid	$Zn (s) + 2HCl (aq) \rightarrow ZnCl<em>2 (aq) + H</em>2 (g)$
Oxygen (O2)	Manganese(IV) oxide (catalyst)	Hydrogen peroxide	$2H<em>2O</em>2 (aq) \rightarrow 2H<em>2O (l) + O</em>2 (g)$

Collecting Gases

Method	Use When…	Apparatus	Examples
Upward Displacement of Air	Gas is heavier than air	Gas jar	Carbon dioxide, Sulphur dioxide, Hydrogen chloride
Downward Displacement of Air	Gas is lighter than air	Gas jar	Ammonia
Over Water	Gas is sparingly soluble in water	Gas jar with water	Hydrogen , Oxygen
Gas Syringe	You want to measure the volume accurately	Gas syringe	Any gas

19.5 Tests for Gases

Gas	Properties	Test	Result
Ammonia (NH3)	Colourless, alkaline, strong smell	Hold damp indicator paper in it	Turns blue
Carbon Dioxide (CO2)	Colourless, weakly acidic	Bubble through limewater	Turns cloudy/milky
Chlorine (Cl2)	Green, poisonous, bleaches dyes	Hold damp indicator paper in a fume cupboard	Turns white
Hydrogen (H2)	Colourless, combines violently with oxygen when lit	Collect gas in tube, hold lighted splint to it	Burns with a squeaky pop
Oxygen (O2)	Colourless	Collect gas in test-tube, hold glowing splint to it	Splint bursts into flame

19.6 Testing for Ions in the Lab

Positive ions = cations, negative ions = anions
Tests involve precipitate formation or gas evolution.

Tests for Cations

Ammonium ions can be tested using solid or aqueous solutions.
Metal ions tested using aqueous solutions.
Sodium hydroxide or ammonia solution is used to provide hydroxide ions.

Cation	Test	If Present	Ionic Equation
Ammonium (NH4+)	Add dilute sodium hydroxide, heat gently.	Ammonia gas given off (turns litmus red).	$NH<em>4^+ (aq) + OH^- (aq) \rightarrow NH</em>3 (g) + H_2O (l)$
Copper(II) (Cu2+)	Add dilute sodium hydroxide or ammonia.	Pale blue precipitate forms. Dissolves in excess ammonia, forming deep blue solution.	$Cu^{2+} (aq) + 2OH^- (aq) \rightarrow Cu(OH)_2 (s)$
Iron(II) (Fe2+)	Add dilute sodium hydroxide or ammonia.	Pale green precipitate forms.	$Fe^{2+} (aq) + 2OH^- (aq) \rightarrow Fe(OH)_2 (s)$
Iron(III) (Fe3+)	Add dilute sodium hydroxide or ammonia.	Red-brown precipitate forms.	$Fe^{3+} (aq) + 3OH^- (aq) \rightarrow Fe(OH)_3 (s)$
Aluminium (Al3+)	Add dilute sodium hydroxide or ammonia.	White precipitate forms. Dissolves in excess sodium hydroxide but not in excess ammonia.	$Al^{3+} (aq) + 3OH^- (aq) \rightarrow Al(OH)_3 (s)$
Zinc (Zn2+)	Add dilute sodium hydroxide or ammonia.	White precipitate forms. Dissolves in excess sodium hydroxide or ammonia, giving a colourless solution.	$Zn^{2+} (aq) + 2OH^- (aq) \rightarrow Zn(OH)_2 (s)$
Calcium (Ca2+)	Add dilute sodium hydroxide solution.	White precipitate forms. Does not dissolve in excess sodium hydroxide.	$Ca^{2+} (aq) + 2OH^- (aq) \rightarrow Ca(OH)_2 (s)$
	Add dilute ammonia solution	No precipitate or very slight white precipitate

Tests for Anions

Halide ions (Cl$^-$, Br$^-$, I$^-$)

To a small amount of the solution, add an equal volume of dilute nitric acid. Then add silver nitrate solution.
Silver halides are insoluble. So if halide ions are present a precipitate will form. The colour tells you which one. Look at this table:

Precipitate	Indicates presence of …	Ionic equation for the reaction
white	chloride ions, Cl$^- $	Ag$^+$(aq) + Cl$^-$(aq) → AgCl(s)
cream	bromide ions, Br$^- $	Ag$^+$(aq) + Br$^-$(aq) → AgBr(s)
yellow	iodide ions, I$^- $	Ag$^+$(aq) + I$^-$(aq) → AgI(s)

Sulfate ions (SO₄²⁻)

To a small amount of the solution add an equal volume of dilute hydrochloric acid. Then add barium nitrate solution.
Barium sulfate is insoluble. So if sulfate ions are present a white precipitate will form.
The ionic equation for the reaction is: $Ba^{2+}(aq) + SO4^{2−} (aq) → BaSO4(s)$

Nitrate ions (NO₃⁻)

To a small amount of the unknown solid or solution, add a little sodium hydroxide solution. Then add some small pieces of aluminium foil, and heat gently.
If ammonia gas is given off, the unknown substance contained nitrate ions.
The ionic equation for the reaction is: $8Al(s) + 3NO3^− (aq) + 5OH^− (aq) + 2H2O(l) → 3NH3 (g) + 8AlO2^− (aq)$

Carbonate ions (CO₃²⁻)

To a small amount of the unknown solid or solution, add a little dilute hydrochloric acid.
If the mixture bubbles and gives off a gas that turns limewater milky, the unknown substance contained carbonate ions. The gas is carbon dioxide.
The ionic equation for the reaction is : $2H^+ (aq) + CO3^{2−} (aq) → CO2 (g) + H_2O(l)$