Identifying gases and ions in the laboratory: tests, observations and safe practice.
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Gases are identified by simple tests. Hydrogen burns with a pop. Oxygen relights a glowing splint. Carbon dioxide turns lime water milky (Ca(OH)₂ + CO₂ → CaCO₃ + H₂O). Ammonia turns moist red litmus blue and gives dense white fumes with hydrogen chloride. Chlorine bleaches moist litmus and is greenish-yellow. Sulphur dioxide turns acidified potassium dichromate paper from orange to green.
A gas turns lime water milky. Name it and write the equation.
Carbon dioxide: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O.
A gas relights a glowing splint. Name it.
Oxygen.
Why does lime water become clear again on passing excess CO₂?
Soluble calcium hydrogencarbonate forms.
Adding sodium hydroxide solution: Cu²⁺ gives a pale blue precipitate, Fe²⁺ a dirty green one, Fe³⁺ a reddish-brown one and Zn²⁺ a white one that dissolves in excess. Silver nitrate with a chloride gives a white precipitate (curdy), soluble in ammonia. Barium chloride with a sulphate gives a white precipitate insoluble in acid. Flame tests: sodium golden yellow, potassium lilac, calcium brick red, copper bluish-green.
A salt solution gives a reddish-brown precipitate with NaOH. Which ion is present?
Fe³⁺ (iron(III)).
A solution gives a white precipitate with barium chloride insoluble in dilute HCl. What ion is present?
Sulphate, SO₄²⁻.
What colour does a calcium salt give in a flame test?
Brick red.
Wear safety goggles and a lab coat, never taste or directly smell chemicals (waft the vapour towards you), add acid to water and not water to acid, heat test tubes at a slant pointing away from people, and keep flammable liquids away from flames. Record observations honestly before drawing a conclusion, and wash hands after the experiment.
Why is acid added to water and not the reverse?
Dilution gives out a lot of heat; adding water to concentrated acid can spurt acid out.
How should one smell a gas in the lab?
By wafting a little vapour toward the nose with the hand.
Why are test tubes heated at a slant?
To avoid the contents shooting out at the user.
1. How is carbon dioxide tested?
2. Give the test for ammonia.
3. What does Cu²⁺ give with NaOH?
4. Name the ion that gives a reddish-brown precipitate with NaOH.
5. What is observed when AgNO₃ is added to NaCl solution?
6. What colour is the flame of potassium salts?
7. Which gas bleaches moist litmus?
8. Why is concentrated acid added to water slowly?
9. What is the colour change of K₂Cr₂O₇ paper with SO₂?
10. Name two safety precautions in a laboratory.