No, calcium nitrate itself does not evaporate under typical conditions where solvents like water would. When you hear about "evaporating a calcium nitrate solution," it is the water (or other solvent) that evaporates, leaving the calcium nitrate behind as a solid.
Understanding Evaporation and Calcium Nitrate
Evaporation is the process where a liquid turns into a gas, typically at a temperature below its boiling point. For a substance to evaporate, it must have a significant vapor pressure. Calcium nitrate, being an ionic compound, exists as a solid at standard room temperature and pressure. Its strong ionic bonds require a very large amount of energy to break, meaning it has an extremely high melting point and decomposes before it reaches a true boiling point where it would turn into a gas.
The provided reference clearly illustrates this: "Calcium nitrate is very soluble and forms a tetrahydrate, Ca(NO3) 2·4H2O if the solution is evaporated to dryness." This statement confirms that when the solvent (water) is removed through evaporation, the calcium nitrate does not vanish; instead, it remains as a solid, specifically forming a hydrated salt.
What Happens When a Calcium Nitrate Solution Dries?
When a solution containing dissolved calcium nitrate (Ca(NO₃)₂) is exposed to air or heated, the solvent molecules (usually water) gain enough energy to escape into the atmosphere as a gas. This is the process of evaporation. As the water leaves, the dissolved calcium nitrate solute becomes more concentrated. Eventually, if all the water evaporates, the calcium nitrate will crystallize out of the solution.
Specifically, as the reference highlights, it often crystallizes as a tetrahydrate, Ca(NO₃)₂·4H₂O. This means that for every molecule of calcium nitrate, four molecules of water are incorporated into its crystal structure, even after the bulk of the solvent has evaporated. The CAS number of the anhydrate (without water) is 10124-37-5, while that of the tetrahydrate is 13477-34-4.
Key Differences: Water vs. Calcium Nitrate
To further clarify, consider the fundamental differences in properties between water and calcium nitrate regarding evaporation:
| Property | Water (H₂O) | Calcium Nitrate (Ca(NO₃)₂) |
|---|---|---|
| State at Room Temp | Liquid | Solid (usually hygroscopic, forms hydrates) |
| Primary Bond Type | Covalent (with hydrogen bonding) | Ionic |
| Ease of Evaporation | Evaporates readily at room temperature | Does not evaporate; very low vapor pressure |
| Boiling Point | 100°C (212°F) at standard pressure | Decomposes around 560°C (1040°F) before boiling |
| What happens upon drying solution? | It leaves the system as vapor | It remains as a solid (often a hydrate) |
Practical Implications
Understanding that calcium nitrate does not evaporate is crucial in various applications:
- Fertilizers: Calcium nitrate is a common fertilizer. Its non-volatile nature means it stays in the soil for plants to absorb, rather than evaporating away.
- Industrial Processes: In industrial settings, solutions of calcium nitrate are concentrated or crystallized to recover the solid compound. This process relies on evaporating the solvent, not the calcium nitrate.
- Storage and Handling: Because calcium nitrate is very soluble and often forms hydrates, it must be stored in dry conditions to prevent it from absorbing moisture from the air and forming a solution, which could then "dry out" and leave the solid again.
In summary, while the water in a calcium nitrate solution will evaporate, the calcium nitrate itself will remain, often transitioning into its solid tetrahydrate form.
