Physical & Chemical Properties
NSO – Chemistry
(Study Notes – 2026)
1. Simple Explanation
| Property | What It Means | How It Shows Up |
|---|---|---|
| Physical | Something you can observe or measure without changing the substance’s identity. | Color, density, melting point, boiling point, solubility, etc. |
| Chemical | A characteristic that tells you how the substance reacts or changes into something new. | Color change, gas evolution, precipitation, pH shift, etc. |
Tip:
If you can change the substance’s appearance without altering its chemical identity, it’s a physical property.
If the substance turns into a different substance, it’s a chemical property.
2. Key Points
| Category | Key Concepts | Common Examples |
|---|---|---|
| Physical Properties | - Density (mass/volume) - Melting / Boiling Point - Color & Luster - Solubility - Conductivity (ionic/metallic) - Magnetism |
- Cu (density 8.96 g cm⁻³) - NaCl (melting 801 °C) - Fe₂O₃ (red, non‑metallic) |
| Chemical Properties | - Reactivity with acid / base - Oxidation state changes - Flammability - Corrosion - Catalytic activity |
- Zn + H₂SO₄ → ZnSO₄ + H₂↑ - Fe + O₂ → Fe₂O₃ (rust) - NaOH + H₂O → Na⁺ + OH⁻ |
| Testing & Identification | - Litmus test: acidity/basicity - Precipitation test: adding a reagent to form a solid - Titration: measure concentration via reaction - Spectroscopy: identify elements (UV‑Vis, IR) |
- NH₃ (blue litmus → red) - Precipitate of AgCl (white) from Ag⁺ + Cl⁻ |
| Safety & Practical Tips | - Wear goggles & gloves - Work in a fume hood for volatile gases - Use proper waste disposal for acids/bases |
- NaCl + H₂SO₄ → Na₂SO₄ + 2HCl (handle H₂SO₄ with care) |
3. Worked Example
Scenario:
You have a white, crystalline solid that dissolves in water to give a clear solution. When you add dilute nitric acid, the solution turns yellow and a gas is released. When you add the solution to a few drops of sodium hydroxide, a white precipitate forms that dissolves in excess NaOH.
Question: Identify the solid and explain each observation.
Step‑by‑Step Analysis
| Observation | What It Tells Us | Likely Substance |
|---|---|---|
| White, crystalline solid | Likely a salt (ionic compound). | NaNO₃, NaCl, Na₂CO₃, etc. |
| Dissolves in water → clear solution | Good ionic solubility. | All above. |
| Add dilute HNO₃ → yellow, gas evolved | Decomposition of an oxalate/permanganate? Actually NaNO₃ is non‑reactive with HNO₃. Yellow gas suggests Na₂CO₃ + HNO₃ → CO₂↑ (colorless, but could appear yellowish due to light). However, a yellow gas is NO₂ (brown), not yellow. So maybe it's NaNO₂ or Na₂CO₃? Wait: Na₂CO₃ + HNO₃ → CO₂↑ (colorless). Yellow gas is NO₂ from NaNO₂ + HNO₃? Actually NaNO₂ + HNO₃ → NaNO₃ + NO₂↑ (brown). So the gas is brownish‑yellow. | |
| Precipitate with NaOH → white, dissolves in excess | Classic carbonate behavior: Na₂CO₃ + 2NaOH → Na₂CO₃ + 2NaOH (no precip). But CaCO₃ precipitates with NaOH? Actually carbonate with NaOH gives no precip. Sulfate with NaOH gives no precip. Chloride gives no precip. Bicarbonate gives CO₂. Aluminum hydroxide forms precipitate. But white precipitate dissolving in excess NaOH means Al³⁺ (Al(OH)₃) dissolving to form aluminate. This suggests the solid contains Al³⁺ (e.g., AlCl₃). However, AlCl₃ + NaOH → Al(OH)₃ (white) + NaCl; excess NaOH dissolves Al(OH)₃ to [Al(OH)₄]⁻. | |
| Combined clues: white solid, Na₂CO₃? but Na₂CO₃ doesn’t react with HNO₃ giving gas, and doesn’t precipitate with NaOH. | Consider AlCl₃: White solid, dissolves → clear solution. With HNO₃ → forms Al(NO₃)₃ (colorless) and releases NO₂? Actually AlCl₃ + HNO₃ → Al(NO₃)₃ + HCl. No gas. | |
| Actually, the gas is NO₂ (brown). So we need a NO₂‑forming reaction. That occurs when NaNO₂ + HNO₃ → NaNO₃ + NO₂↑. The precipitate with NaOH: NaNO₂ + NaOH → NaNO₃ + NaOH (no precip). So not that. |