Practical work 1: preparing carbon(IV) oxide and studying its properties
Textbook: p. 192
GoalStudy the preparation of CO₂, its acidic-oxide properties and the test for carbonate in a practical work; explain the observations with equations.
New words
gas delivery tube · gaz o‘tkazgich naylime water · ohakli suvindicator · indikatorfiltrate · filtrat
Explanation
In the practical work CO₂ is first made by acting with dilute hydrochloric acid on pieces of chalk or marble: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑. When the gas is led through a delivery tube into lime water it turns cloudy (CaCO₃↓), and with continued gas the liquid clears because soluble Ca(HCO₃)₂ forms. Led into distilled water, the gas forms weak H₂CO₃, which turns blue litmus red; led into NaOH solution with phenolphthalein, the alkali is used up and the pink colour fades: CO₂ + 2NaOH → Na₂CO₃ + H₂O. Acid added to a soil sample releases bubbles if carbonates are present, and AgNO₃ added to the soil filtrate gives a white precipitate showing chloride ions: Ag⁺ + Cl⁻ → AgCl↓. Do everything with the teacher, in goggles and a lab coat; if acid or alkali touches the skin, rinse at once with plenty of water and tell the teacher.
Worked examples
Observation: lime water first turns cloudy: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O; with excess gas it clears: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂.
AgNO₃ added to the soil filtrate: NaCl + AgNO₃ → AgCl↓ + NaNO₃; net ionic: Ag⁺ + Cl⁻ → AgCl↓. The precipitate dissolves in ammonia solution and not in nitric acid — a mark of AgCl.
Class activity
Report table: columns “Experiment”, “Observation”, “Equation”, “Conclusion”. After the work (supervised by the teacher) fill in the table and check it with a partner. Do not repeat these experiments at home.
Practice
1
Write the equation for the preparation of CO₂.
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
2
Why does the colour disappear when CO₂ is passed through NaOH solution with phenolphthalein?
CO₂ reacts with and uses up the alkali: CO₂ + 2NaOH → Na₂CO₃ + H₂O, so the medium becomes less alkaline.
3
What happens if CO₂ is passed through lime water for a long time? Write the equation.
The cloudiness disappears: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂.
4
How can you test whether a soil extract contains chloride ions? What is observed?
Add AgNO₃ solution to the filtrate: Ag⁺ + Cl⁻ → AgCl↓, a white precipitate forms.