Class 10 Chemistry Pba Notes and Solved Model Paper (FBISE)
A. Major experiments (Section A, 6 marks each)
C-Major 1. Separation of water and alcohol by fractional distillation
- Apparatus: round-bottom flask (A mixture here), fractionating column, thermometer, Liebig condenser, receiving flask, burner or heating mantle, retort stands, anti-bumping chips (porcelain pieces).
- Principle: ethanol boils at 78 C, water at 100 C. The column has a large surface area (glass beads or rings) where vapour repeatedly condenses and re-vaporises, so the more volatile ethanol rises to the top and water is held back. This gives a much better separation than simple distillation.
- Procedure: mix water and ethanol in the flask with anti-bumping chips. Thermometer bulb at the level of the side arm to the condenser. Cooling water enters at the bottom of the condenser and leaves at the top. Heat gently. The thermometer rises and holds near 78 C: collect this fraction (ethanol). When the temperature starts to rise towards 100 C, change the receiver, the rest is water.
- Confirm ethanol: it has a characteristic smell, it burns with a pale blue flame (water does not), its boiling point is 78 C.
- Control heat: low, steady flame, 1 to 2 drops of distillate per second. Reduce the heat if the temperature jumps, because rapid heating lets water vapour pass.
C-Major 2. Paper chromatography of a mixture of inks
- Apparatus: chromatography paper, pencil, ruler, capillary tube, beaker with lid, solvent (water or alcohol mixture), glass rod or stick to hang the paper.
- Procedure: draw a pencil baseline (origin) about 2 cm from the bottom. Put a small spot of ink on it with the capillary, let it dry, repeat 2 or 3 times so the spot is concentrated but small. Hang the paper in the beaker so only the bottom dips in the solvent, with the solvent level below the baseline. Cover the beaker. When the solvent front is near the top, remove the paper and immediately mark the solvent front with a pencil, then dry it.
- Why pencil: pencil graphite is insoluble in the solvent and does not move or dissolve, while pen ink would run and form its own spots.
- Rf = distance moved by the spot / distance moved by the solvent front. Rf is always between 0 and 1, same for the same substance in the same solvent. Two spots means two components. A pure substance gives one spot.
C-Major 3. Separate Pb2+ and Cd2+ by paper chromatography
- Pb2+ and Cd2+ are colourless, so the spots cannot be seen. A locating agent (a chemical sprayed after the run that reacts with the ions to give coloured spots) is needed.
- Same procedure as inks, with a suitable solvent (acid or acetone-based). After the run, dry the paper and spray the locating agent. Mark the coloured spots, then calculate Rf for each ion and compare with known samples (identify unknowns by comparing Rf with the known substances).
- Typical locating agents: hydrogen sulfide water or sodium sulfide (Pb2+ gives a black spot of PbS, Cd2+ gives a yellow spot of CdS), or potassium iodide (Pb2+ gives yellow PbI2).
The framework names the experiment but not the exact solvent or locating agent. Use whichever your school lab manual used. The agents above are the common ones, but they are not printed in the framework.
C-Major 4. Exact molarity of NaOH by titration
- Apparatus: burette, volumetric pipette, conical flask, white tile, funnel, retort stand, wash bottle.
- Solutions: standard acid of known molarity (for example 0.1 M HCl) and the NaOH solution; indicator phenolphthalein (colourless in acid, pink in alkali) or methyl orange.
- Procedure: rinse the burette with NaOH and fill it, remove air bubbles, note the initial reading. Pipette 10 or 25 cm3 of the acid into the conical flask, add 2 or 3 drops of indicator. Add NaOH slowly while swirling, until one drop gives a permanent faint pink (end point). Note the final reading. Repeat until two readings agree within 0.10 cm3 (concordant), take the mean.
- Calculation: Ma Va / na = Mb Vb / nb where n is the number of moles in the balanced equation. For HCl + NaOH (1:1): MNaOH = MHCl x VHCl / VNaOH.
Example: 25.0 cm3 of 0.100 M HCl needs 24.5 cm3 NaOH: M = 0.100 x 25.0 / 24.5 = 0.102 M. In g/dm3: 0.102 x 40 = 4.08 g/dm3. - Chemists use cm3, g and s because the quantities in a lab are small (1 dm3 = 1000 cm3).
C-Major 5. Tests for ammonia, carbon dioxide and chlorine
| Gas | Test | Positive result | Usual preparation |
|---|---|---|---|
| Ammonia NH3 | Damp red litmus paper | Turns blue (pungent smell, alkaline gas) | Warm NH4Cl with NaOH |
| Carbon dioxide CO2 | Pass through limewater, Ca(OH)2 | Turns milky (CaCO3); clears with excess CO2 | CaCO3 + dilute HCl |
| Chlorine Cl2 | Damp (blue) litmus paper | Bleached white (first red), greenish-yellow gas | MnO2 + conc. HCl, warm |
- Other gas tests on the same SLO: hydrogen (lighted splint, squeaky pop), oxygen (glowing splint relights), sulfur dioxide (acidified potassium manganate(VII) turns from purple to colourless).
- Equations: CaCO3 + 2HCl -> CaCl2 + H2O + CO2. Ca(OH)2 + CO2 -> CaCO3 + H2O. NH4Cl + NaOH -> NaCl + H2O + NH3.
B. Minor experiments (Section B, 4 marks each)
C-Minor 1. Separate naphthalene from sand and salt by sublimation
- Put the mixture in a china dish, cover with an inverted funnel plugged with cotton wool, heat gently. Naphthalene sublimes (solid to gas, no liquid) and its vapour condenses as white crystals on the cool funnel. Sand and salt stay in the dish. Cool and scrape the naphthalene off. Salt and sand can then be separated by adding water, filtering the sand and evaporating the filtrate. Other sublimers: ammonium chloride, iodine, camphor.
C-Minor 2. Flame test for metal ions
| Ion | Flame colour |
|---|---|
| Na+ | Golden yellow |
| K+ | Lilac (pale violet) |
| Ca2+ | Brick red (orange-red) |
| Cu2+ | Blue-green |
| Ba2+ | Apple green (pale green) |
- Clean the nichrome or platinum wire by dipping in conc. HCl and heating until no colour, so earlier samples do not contaminate the next flame. Moisten with HCl, dip in the sample, hold it in the edge of the hot non-luminous (blue) Bunsen flame and watch the colour. Yellow means sodium (it also masks other colours, so sodium contamination must be avoided).
C-Minor 3. Pure crystals of CuSO4.5H2O
- Warm dilute H2SO4, add copper(II) oxide (or carbonate) in small portions until some solid stays unreacted (excess). Filter to remove the excess (residue = unreacted CuO, filtrate = blue CuSO4 solution). Heat the filtrate in an evaporating dish on a water bath until it is saturated (a drop on a glass rod forms crystals when cooled). Leave to cool slowly, filter the crystals, wash with a little cold distilled water, dry between filter papers.
- CuO + H2SO4 -> CuSO4 + H2O. Slow cooling gives large, well-formed crystals, fast cooling gives many small crystals. Do not heat to dryness, or the water of crystallisation is lost.
C-Minor 4. Melting point of naphthalene
- Fill a capillary tube with powdered naphthalene, attach it to a thermometer, heat in a water bath (or liquid paraffin), stirring. Note the temperature when it just melts. Pure naphthalene melts sharply at about 80 C. An impure sample melts below this value over a range.
C-Minor 5. Boiling point of ethyl alcohol
- Heat ethanol in a flask or test tube (water bath) with a thermometer, the bulb in the vapour just above the liquid (not touching the glass), with anti-bumping chips. The steady temperature is the boiling point, about 78 C. Repeat 3 times and take the mean.
- Accuracy vs precision: accuracy is closeness to the true value, precision is closeness of repeated readings to each other.
C-Minor 6. Metal displacement reaction in aqueous medium
- A more reactive metal displaces a less reactive metal from its salt solution. Zn + CuSO4 -> ZnSO4 + Cu. Observation: the blue colour fades, reddish-brown copper is deposited on the zinc. Cu + ZnSO4: no reaction (copper is less reactive). Reactivity order: Mg, Zn, Fe, Cu, Ag. Cu in AgNO3 gives silver and a blue solution.
C-Minor 7. Test for water with anhydrous copper(II) sulfate
- Add the liquid to white anhydrous CuSO4. It turns blue (hydrated CuSO4.5H2O) if water is present. CuSO4 + 5H2O -> CuSO4.5H2O. The test proves water is present, it does not prove the water is pure. Heating the blue crystals gives white powder again (reversible).
C-Minor 8. Purity of water by melting and boiling point
- Pure water: melts at 0 C and boils at 100 C (standard pressure), sharp and fixed. Impurities (such as salt) lower the melting point and raise the boiling point, and make them spread over a range. Distilled water has no dissolved salts, leaves no residue on evaporation and is used in labs, car batteries, irons. Tap water has dissolved minerals and chlorine and leaves scale.
C. Model paper solved (Chemistry, 20 marks)
Q.1 Paper chromatography of ink mixture X (6 marks)
a. A horizontal baseline (origin) about 2 cm from the bottom edge, above the solvent level. A pencil is used because graphite does not dissolve in the solvent and does not run up the paper, while pen ink would dissolve and move, giving false spots.
b. Touch the capillary lightly to the line for a very small spot, let it dry, repeat 2 or 3 times in the same place to concentrate it. Keep the spot small (a few mm) and on the baseline, leaving space between different spots and from the edges.
c. The solvent level must be below the baseline (otherwise the ink dissolves into the solvent). The paper must hang straight without touching the beaker walls, the beaker should be covered so the air is saturated with solvent vapour, and it must stand still.
d. The run is finished when the solvent front is near the top of the paper (about 1 cm below it) and the spots stop separating further. Immediately mark the solvent front with a pencil, because it evaporates and becomes invisible, then dry the paper.
e. Two distinct spots mean X is a mixture of two different substances (components), not a pure substance.
f. From the diagram: solvent front = 8 cm, substance A = 2.6 cm, substance B = 5.6 cm from the origin.
Rf(A) = 2.6 / 8 = 0.325 (about 0.33). Rf(B) = 5.6 / 8 = 0.70.
b. Touch the capillary lightly to the line for a very small spot, let it dry, repeat 2 or 3 times in the same place to concentrate it. Keep the spot small (a few mm) and on the baseline, leaving space between different spots and from the edges.
c. The solvent level must be below the baseline (otherwise the ink dissolves into the solvent). The paper must hang straight without touching the beaker walls, the beaker should be covered so the air is saturated with solvent vapour, and it must stand still.
d. The run is finished when the solvent front is near the top of the paper (about 1 cm below it) and the spots stop separating further. Immediately mark the solvent front with a pencil, because it evaporates and becomes invisible, then dry the paper.
e. Two distinct spots mean X is a mixture of two different substances (components), not a pure substance.
f. From the diagram: solvent front = 8 cm, substance A = 2.6 cm, substance B = 5.6 cm from the origin.
Rf(A) = 2.6 / 8 = 0.325 (about 0.33). Rf(B) = 5.6 / 8 = 0.70.
Q.2 Fractional distillation of water and alcohol (6 marks)
a. A = fractionating column, B = (Liebig) condenser, C = round-bottom flask holding the water-alcohol mixture, D = receiving (conical) flask holding the distillate.
b. The fractionating column (packed with glass beads or rings) gives a large surface area where vapours condense and evaporate repeatedly. The more volatile ethanol travels to the top and the less volatile water drains back down, so the separation is much sharper than in simple distillation.
c. Alcohol vaporises first, because its boiling point (78 C) is lower than that of water (100 C), so it is more volatile.
d. A steady 78 C means pure ethanol is distilling (the boiling point of ethanol). Ethanol is collected at this temperature. When the temperature rises above this, water is starting to come over.
e. Ethanol has a distinct smell and burns with a pale blue flame (a few drops on a watch glass catch fire), while water does not burn. Its boiling point is 78 C.
f. Heat gently and steadily with a small flame, or use a water bath or heating mantle. Aim for about 1 to 2 drops of distillate per second, lower the flame if the thermometer rises too fast, and keep the condenser water flowing (in at bottom, out at top).
b. The fractionating column (packed with glass beads or rings) gives a large surface area where vapours condense and evaporate repeatedly. The more volatile ethanol travels to the top and the less volatile water drains back down, so the separation is much sharper than in simple distillation.
c. Alcohol vaporises first, because its boiling point (78 C) is lower than that of water (100 C), so it is more volatile.
d. A steady 78 C means pure ethanol is distilling (the boiling point of ethanol). Ethanol is collected at this temperature. When the temperature rises above this, water is starting to come over.
e. Ethanol has a distinct smell and burns with a pale blue flame (a few drops on a watch glass catch fire), while water does not burn. Its boiling point is 78 C.
f. Heat gently and steadily with a small flame, or use a water bath or heating mantle. Aim for about 1 to 2 drops of distillate per second, lower the flame if the thermometer rises too fast, and keep the condenser water flowing (in at bottom, out at top).
Q.3 Flame test, accuracy and precision, sublimation (4 marks)
a. Cleaning removes traces of the previous sample. Left-over salts would give their own colours and make the result wrong. Dip in conc. HCl and heat in the flame until it burns without colour.
b. Hold the wire loop in the edge (the hot blue part) of the non-luminous Bunsen flame, so it heats evenly and the colour is clear. A persistent golden-yellow flame shows sodium, Na+.
c. The readings (70, 71, 70.5 C) are close to each other (spread 1 C), so the work is precise. The mean 70.5 C is 7.5 C away from the true value 78 C, so it is not accurate. Likely cause: a systematic error such as the thermometer bulb not in the vapour, heat loss, or an uncalibrated thermometer.
d. Put the mixture (for example naphthalene or ammonium chloride mixed with sand or salt) in a china dish. Cover with an inverted funnel plugged with cotton wool. Heat gently. The sublimable solid turns to vapour and deposits as pure crystals on the cool funnel. The other substances stay in the dish. Cool and collect the crystals.
b. Hold the wire loop in the edge (the hot blue part) of the non-luminous Bunsen flame, so it heats evenly and the colour is clear. A persistent golden-yellow flame shows sodium, Na+.
c. The readings (70, 71, 70.5 C) are close to each other (spread 1 C), so the work is precise. The mean 70.5 C is 7.5 C away from the true value 78 C, so it is not accurate. Likely cause: a systematic error such as the thermometer bulb not in the vapour, heat loss, or an uncalibrated thermometer.
d. Put the mixture (for example naphthalene or ammonium chloride mixed with sand or salt) in a china dish. Cover with an inverted funnel plugged with cotton wool. Heat gently. The sublimable solid turns to vapour and deposits as pure crystals on the cool funnel. The other substances stay in the dish. Cool and collect the crystals.
Q.4 Purity of water, displacement, crystals (4 marks)
a. Sample A melts at 0 C and boils at 100 C, exactly like pure water, so it is pure. Sample B melts at -2 C (lower) and boils at 102 C (higher), so it contains dissolved impurities. Impurities lower the melting point and raise the boiling point.
The experiment helps because a salt or solute makes the water freeze below 0 C and boil above 100 C. Any shift from these fixed values, or a range instead of a sharp point, shows contamination. The bigger the shift, the more impurity.
b. Zinc is more reactive than copper, so it displaces copper: Zn + CuSO4 -> ZnSO4 + Cu. The blue colour of the solution fades (zinc sulfate is colourless) and reddish-brown copper deposits on the zinc.
c. Slow cooling gives the particles time to arrange themselves on a few seeds, so fewer, larger and well-shaped crystals form. Fast cooling (as in icy water) forms many nuclei at once, so many small crystals.
The experiment helps because a salt or solute makes the water freeze below 0 C and boil above 100 C. Any shift from these fixed values, or a range instead of a sharp point, shows contamination. The bigger the shift, the more impurity.
b. Zinc is more reactive than copper, so it displaces copper: Zn + CuSO4 -> ZnSO4 + Cu. The blue colour of the solution fades (zinc sulfate is colourless) and reddish-brown copper deposits on the zinc.
c. Slow cooling gives the particles time to arrange themselves on a few seeds, so fewer, larger and well-shaped crystals form. Fast cooling (as in icy water) forms many nuclei at once, so many small crystals.