If NH4 ion is an acid and OH ion is a base why they doesn't directly react together?

Actually, NH₄⁺ and OH⁻ ions do react. For example, when warning the mixture of NH₄NO₃ and NaOH solution, NH₄⁺ ions react with OH⁻ ion to form NH₃ gas. The net ionic equation is :
NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)

The question is raised may be due to the compound "ammonium hydroxide". "Ammonium hydroxide" does not exist, and it is actually ammonia solution. In ammonia solution, a reversible reaction occurs in which molecular NH₃ dissociates to form NH₄⁺ and OH⁻ ions.
NH₃(g) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
The term "ammonium hydroxide" is incorrect because over 95% of NH₃ molecules do not dissociate in the solution. Besides, pure compound of NH₄OH is never found.

The NH4+ ion is not a strong acid. As others have said it will donate only a small amount of H+ to from water since the rest remains part of the NH3/water equilibrium. The equilibrium can be driven to the right in a neutralization type of reaction by adding more NH4+ or OH-, creating more NH3.

This page looks at the reactions of the oxides of Period 3 elements (sodium to chlorine) with water, and with acids or bases where relevant. Argon is obviously omitted because it doesn't form an oxide.


A quick summary of the trend

The oxides

The oxides we'll be looking at are:

Na2O MgO Al2O3 SiO2 P4O10 SO3 Cl2O7
P4O6 SO2 Cl2O

The trend in acid-base behaviour

The trend in acid-base behaviour is shown in various reactions, but as a simple summary:

The trend is from strongly basic oxides on the left-hand side to strongly acidic ones on the right, via an amphoteric oxide (aluminium oxide) in the middle. An amphoteric oxide is one which shows both acidic and basic properties.

For this simple trend, you have to be looking only at the highest oxides of the individual elements. Those are the ones on the top row above, and are where the element is in its highest possible oxidation state. The pattern isn't so simple if you include the other oxides as well.

For the non-metal oxides, their acidity is usually thought of in terms of the acidic solutions formed when they react with water - for example, sulphur trioxide reacting to give sulphuric acid. They will, however, all react with bases such as sodium hydroxide to form salts such as sodium sulphate.

These reactions are all explored in detail on the rest of this page.