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:
- Observation: Noticing something and asking a question.
- Example: Kitchen cleaner X works better than kitchen cleaner Y.
- Question: Why?
- Hypothesis: Formulating a testable statement.
- Requires research.
- Example: "X may contain more sodium hydroxide than Y."
- Experiment Planning: Designing a test for the hypothesis.
- Example: Titration to test the amount of sodium hydroxide.
- Experiment Execution and Recording:
- Performing the experiment and noting results.
- Results Analysis:
- Examining the data to find trends/patterns.
- Hypothesis Support:
- Determining if the results validate the hypothesis.
- 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
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 | CaCO3 (s) + 2HCl (aq) \rightarrow CaCl2 (aq) + H2O (l) + CO2 (g) |
| Hydrogen (H2) | Pieces of zinc | Dilute hydrochloric acid | Zn (s) + 2HCl (aq) \rightarrow ZnCl2 (aq) + H2 (g) |
| Oxygen (O2) | Manganese(IV) oxide (catalyst) | Hydrogen peroxide | 2H2O2 (aq) \rightarrow 2H2O (l) + O2 (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). | NH4^+ (aq) + OH^- (aq) \rightarrow NH3 (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)