Here's why:

* Ionic compound: Sodium nitrate is an ionic compound, meaning it's formed by the electrostatic attraction between positively charged sodium ions (Na⁺) and negatively charged nitrate ions (NO₃⁻).

* Strong ionic bonds: These ionic bonds are strong and hold the ions together in a tightly packed, crystalline structure.

* High melting point: To break these bonds and transition from a solid to a liquid (melt), a significant amount of energy is required. Sodium nitrate has a relatively high melting point of 308 °C (586 °F).

However, sodium nitrate is soluble in water. When dissolved in water, the ionic bonds are broken, and the ions become surrounded by water molecules (aqueous solution).