NDA Chemistry · Free notes
Hydrogen and Water — NDA Chemistry
A small, high-recall chapter — about a dozen PYQs across 2017 to 2025, almost all 'which one of the following' or 'which statement is NOT correct' named-fact questions. There is no derivation to do here; the marks come from knowing the properties of hydrogen, the anomalous behaviour of water, and the chemistry of hard water cold. The chapter teaches in three movements, building from the element to the molecule to the applied problem of water quality: (1) Properties of Hydrogen — the lightest, colourless, diatomic gas, why it is unreactive at room temperature, the three types of hydrides, and the syngas trap; (2) Properties and Anomalous Behaviour of Water — the bent polar molecule, hydrogen bonding, and the famous anomalies (maximum density at 4 degrees C, high latent heats, ice floats); (3) Hardness and Purity of Water — temporary versus permanent hardness, the ions and salts that cause each, how to remove them, and the markers of pure drinking water. Every PYQ is tagged. Most concepts are reference tables: memorise the table, win the marks.
The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.
Subtopic notes
Properties of Hydrogen
4 PYQsHydrogen is the lightest element — a colourless, odourless, diatomic gas that is almost insoluble in water and unreactive at room temperature because the H–H bond is very strong.
Properties and Anomalous Behaviour of Water
4 PYQsWater is a bent, polar molecule held together by hydrogen bonds — and that hydrogen bonding gives it a set of famous anomalies: maximum density at 4 degrees C, very high latent heats, and solid ice that floats on liquid water.
Hardness and Purity of Water
5 PYQsHard water contains dissolved calcium and magnesium salts; temporary hardness (from bicarbonates) is removed by boiling, while permanent hardness (from sulphates and chlorides) needs chemical softening — and pure drinking water sits in a narrow pH band.
Formula & revision sheet
0 formulas · 7 reference tables · 11 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
0 formulas · 7 reference tables · 11 gotchas across all subtopics — the exam-eve cheat-sheet
Reference tables (2)
Physical properties of dihydrogen5 rows
| Property | Value / fact |
|---|---|
| Colour | Colourless (also odourless, tasteless)Q Hydrogen gas is colourless — a coloured-flame or coloured-gas option is always wrong. |
| Density vs air | Lighter than air (lightest of all gases) |
| Solubility in water | Almost insoluble |
| Molecular form | Diatomic, H₂ (dihydrogen) |
| Reactivity at room temperature | Inert — strong H–H bond, about 436 kJ/mol |
Types of hydrides and hydrogen storage3 rows
| Hydride type | Forms with | Example | Key point |
|---|---|---|---|
| Ionic / saline | Reactive metals (alkali, alkaline-earth) | NaH, CaH₂ | Contains the H⁻ ion |
| Covalent / molecular | Non-metals | CH₄, NH₃, H₂O, HCl | Shared electron pairs |
| Metallic / interstitial | Transition metals (e.g. palladium) | PdH₍ₓ₎ | Stores a very large volume of hydrogen; non-stoichiometricQ Large-volume hydrogen storage = non-stoichiometric (interstitial) hydrides, NOT hydrogen peroxide or simple hydrides. |
Watch out for (3)
- Hydrogen is colourless, not pale-blue→ Physical properties of dihydrogen
- Syngas is CO + H₂, never NO₂ + H₂→ Types of hydrides and hydrogen storage
- Storage is interstitial hydrides, not H₂O₂→ Types of hydrides and hydrogen storage
Reference tables (2)
Structure of water and hydrogen bonding5 rows
| Feature | Description |
|---|---|
| Molecular formula | H₂O — one oxygen, two hydrogens |
| Shape | Bent / angular, bond angle about 104.5 degrees |
| Polarity | Polar — negative O end, positive H ends |
| Intermolecular force | Hydrogen bonding (strong, extensive network) Hydrogen bonding is the single cause of water's anomalies — anchor every property statement to it. |
| Solvent power | Universal solvent (dissolves polar and ionic substances) |
Anomalous behaviour of water4 rows
| Anomaly | The fact | Cause |
|---|---|---|
| Maximum density | Densest at 4 degrees C (277 K) | Open hydrogen-bonded structure forms below 4 degrees CQ Maximum density of liquid water is at 4 degrees C = 277 K, NOT 0 degrees C / 273 K. |
| Ice floats | Solid ice is less dense than liquid water | Open cage structure of ice is less compact |
| Latent heats | High latent heat of fusion AND vaporisation | Hydrogen bonds must be broken to change stateQ Latent heat of fusion of water is HIGH, not low — that statement is the false one. |
| Boiling bubbles | Bubbles are water vapour | Liquid water turning to gasQ |
Watch out for (4)
- Water is bent, not linear→ Structure of water and hydrogen bonding
- Max density is at 4 degrees C, not 0 degrees C→ Anomalous behaviour of water
- Latent heats are HIGH→ Anomalous behaviour of water
- Boiling bubbles are vapour, not air→ Anomalous behaviour of water
Reference tables (3)
Temporary versus permanent hardness2 rows
| Type | Caused by | Removed by boiling? |
|---|---|---|
| Temporary | Bicarbonates (hydrogencarbonates) of Ca and Mg | YesQ Temporary hardness = hydrogencarbonates (bicarbonates) of Ca²⁺/Mg²⁺ — e.g. Mg(HCO₃)₂. |
| Permanent | Chlorides and sulphates of Ca and Mg | NoQ |
Removing hardness of water4 rows
| Method | Removes temporary? | Removes permanent? |
|---|---|---|
| Boiling | Yes | NoQ Boiling cannot remove permanent hardness — this is the most-tested single fact in the chapter. |
| Washing soda (Na₂CO₃) | Yes | Yes |
| Ion-exchange method | Yes | Yes |
| Calgon's method | Yes | Yes |
Watch out for (4)
- Hardness ions are Ca²⁺ and Mg²⁺, not Na⁺→ Temporary versus permanent hardness
- Boiling cannot remove permanent hardness→ Removing hardness of water
- Drinking-water pH is near neutral→ Purity and quality of drinking water
- Rain water, not river or sea water, is purest→ Purity and quality of drinking water
PYQ weightage by concept
7 concepts · 13 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
7 concepts · 13 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Physical properties of dihydrogen | 2 | 15% |
| Types of hydrides and hydrogen storage | 2 | 15% |
| Concept | PYQs | Share |
|---|---|---|
| Anomalous behaviour of water | 4 | 31% |
| Structure of water and hydrogen bondingfoundation | — | — |
| Concept | PYQs | Share |
|---|---|---|
| Temporary versus permanent hardness | 2 | 15% |
| Purity and quality of drinking water | 2 | 15% |
| Removing hardness of water | 1 | 8% |
Test yourself on a real paper
Sit a past NDA paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.