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1. Sodium Hydrogencarbonate (NaHCO₃)

Model Answer for Exam:

• Condensation / colourless liquid droplets form on the cooler, upper inner walls of the test tube.
• White solid residue remains.

Explanation: Thermal decomposition of a Group 1 hydrogencarbonate. It breaks down into sodium carbonate, water, and carbon dioxide: . The water vapour condenses on the cool glass. (Note: Even though the mark scheme only explicitly looks for water here, testing for with limewater is always a good reflex!).

2. Potassium Manganate (VII) (KMnO₄)

Model Answer for Exam:

• Starting solid is purple crystals.
• Upon heating, the solid “jumps” or moves around in the test tube.
• A black solid residue is formed.
• Gas produced relights a glowing splint (proving oxygen is evolved).

Explanation: is an oxidizing agent that undergoes thermal decomposition. The reaction is . The escaping oxygen gas forces the solid particles to bounce around (fluidisation). The black residue is Manganese (IV) oxide ().

3. Basic Copper (II) Carbonate / Basic Zinc Carbonate / Zinc Carbonate

Model Answer for Exam (Copper):

• Initial light green / blue-green powder.
• Solid fluidises / jumps about during heating.
• Condensation / water droplets form on the sides of the tube.
• Solid turns into a black powder/residue.
• Gas produced gives a white precipitate when bubbled through limewater ().

Model Answer for Exam (Zinc):

• Initial white powder.
• Condensation / water droplets form.
• Solid turns yellow when hot, and goes paler or white upon cooling.
• Gas produced gives a white precipitate with limewater ().

Explanation: “Basic” carbonates contain both the carbonate and the hydroxide (e.g., ). Heating breaks down the hydroxide into water (condensation) and the carbonate into (limewater test) and a metal oxide. Copper(II) oxide () is black. Zinc oxide () is an entirely different beast: it is thermochromic—it temporarily changes crystal lattice structure when heated, turning yellow, but reverts to white when cooled.

4. Ammonium Carbonate ((NH₄)₂CO₃)

Model Answer for Exam:

• Initial white solid / colourless crystals.
• Condensation / water vapour forms upon gentle heating.
• Solid completely disappears / sublimes / evaporates (no residue left at the end).
• Gas produced turns moist red litmus paper blue ( present).

Explanation: Ammonium salts undergo thermal decomposition. Here: . Because all three products are gases/vapours at high temperatures, the solid completely vanishes. The ammonia () is alkaline, hence the litmus test. Examiner tip: Always write “damp” or “moist” litmus paper, as water is needed for the ions to interact with the dye.

5. Mixture: Sodium Carbonate + Sodium Hydrogensulfate

Model Answer for Exam:

• Condensation / steamy fumes produced.
• Gas produced gives a white precipitate with limewater ().
• White residue remains upon cooling.

Explanation: This is a solid-state acid-base reaction. Sodium hydrogensulfate () contains an acidic hydrogen. When heated with a carbonate, it acts as an acid, reacting to form a salt (sodium sulfate, the white residue), water, and carbon dioxide.

6. Sodium Nitrate (NaNO₃)

Model Answer for Exam:

• Solid melts / dissolves into a colourless liquid.
• Fizzing / bubbling / effervescence is seen in the liquid.
• Liquid turns yellow when hot.
• Gas produced relights a glowing splint ().
• Pale yellow solid residue formed on cooling.

Explanation: Group 1 nitrates melt first, then decompose. . The bubbling is the oxygen gas escaping the molten liquid. The yellow residue is sodium nitrite ().

7. Ammonium Sulfate & Ammonium Chloride

Model Answer for Exam (Ammonium Sulfate):

• Solid melts and froths.
• White solid condenses/reforms on the cooler upper sides of the tube (sublimation).
• Gas turns moist red litmus blue first, then later fumes turn blue litmus red.

Model Answer for Exam (Ammonium Chloride):

• White smoke/fumes produced.
• Solid sublimes (white solid forms near the top of the tube, no residue at bottom).
• Gas turns moist red litmus blue first, then later fumes turn blue litmus red.

Explanation: This is a classic Cambridge trick! Thermal dissociation of ammonium halides/sulfates breaks them into (alkaline) and or (acidic). Ammonia (, Mr = 17) is a lighter, faster-moving gas than (Mr = 36.5). Therefore, the reaches the litmus paper first, turning it blue. A few seconds later, the slower, denser acidic gas arrives, turning it back to red. Further up the tube where it is cool, the two gases recombine to form a white solid. You must state the litmus changes in this exact order to get the marks.

8. Hydrated Magnesium Sulfate (MgSO₄ · 7H₂O)

Model Answer for Exam:

• Steam / condensation / water droplets form on upper part of tube.
• White residue / solid remains.

Explanation: This is simple dehydration. The heat drives off the water of crystallisation. . No chemical decomposition of the sulfate ion occurs because Group 2 sulfates are very thermally stable.

9. Manganese (II) Chloride · 4H₂O

Model Answer for Exam:

• Initially pink crystals.
• On gentle heating, solid turns white/paler and melts/dissolves in its own water.
• Condensation / misty fumes produced.
• Gas turns damp blue litmus red (acidic gas).
• Residue turns dark brown / black solid.

Explanation: First, water of crystallisation is lost, so the pink hexaaqua/tetraaqua manganese complex turns white and dissolves in its own departing water. However, heating hydrated transition metal chlorides often leads to hydrolysis—they don’t just lose water, the water reacts with the chloride to release Hydrogen Chloride gas (), which is the misty acidic fume that turns litmus red. The remaining metal is oxidized by air during heating to form manganese oxides (like ), which are black/brown.

10. Aluminium Ammonium Sulfate (Alum)

Model Answer for Exam:

• Solid melts.
• Condensation / moisture on the walls of the tube.
• White smoke / fumes.
• Gas turns moist red litmus blue, followed by gas turning blue litmus red.
• White residue remains.

Explanation: Alums are double salts highly laden with water of crystallisation (e.g., ). It melts in this water. Heating breaks down the ammonium ion (releasing → red litmus blue). The intense heat then causes hydrolysis of the aluminium sulfate or thermal breakdown of the sulfate ion, releasing acidic sulfur oxides or sulfuric acid droplets (blue litmus red). The white residue left is highly stable aluminium oxide ().

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📝 Final Examiner Tips for Paper 3:

1. Never write “bubbles” for a dry solid. Solids don’t bubble. Liquids bubble. If a solid is releasing gas, write “solid fluidises”, “solid jumps around”, or “effervescence observed once the solid melted”.
2. Always prepare both litmus papers. If you are asked to heat an unknown solid, have a piece of damp red AND damp blue litmus paper sitting at the mouth of the test tube.
3. Check the top of the tube. Condensation is almost always worth a mark.
4. Wait for it to cool. Colour changes on cooling (like Zinc) are highly specific marks. Don’t throw the hot test tube away immediately; observe it for 3-4 minutes after turning off the Bunsen burner.