why is sodium hydroxide not called sodium hydrate (like calcium phosphate or sodium carbonate)

Question: why is sodium hydroxide not called sodium hydrate (like calcium phosphate or sodium carbonate)
- Keywords:
  sodium compound
Asked by maze45pen to Souvik, Sabine, Maria, John, Faye, Armin on 19 Nov 2019.
- Maria McNamara answered on 19 Nov 2019:
  
  That’s a good question. Normally in chemistry the term ‘hydrate’ refers to a compound that includes some water, e.g. CaSO4.H2O = calcium sulfate monohydrate.

Question: why is sodium hydroxide not called sodium hydrate (like calcium phosphate or sodium carbonate)