The key difference between sodium sulphate and sodium sulphite is that the sodium sulphate has a sulphate anion consisting of one sulfur atom and four oxygen atoms whereas the sodium sulphite has a sulphite anion consisting of one sulfur atom and three oxygen atoms. Moreover, sodium sulphate is hygroscopic while sodium sulphite is less soluble in water comparatively.
Both sodium sulphate and sodium sulphite are inorganic chemical compounds. When considering the chemical formulas of these two compounds, they differ from each other from the number of oxygen atoms that they have.
