Is the intention valid when saying: "I will perform ablution to remove minor ritual impurity," and "I will pray four rak'ahs for Asr," and "I will perform ghusl to remove major ritual impurity to purify myself from janabah"? And must the reason be mentioned in the intention? And is it sufficient to say: "I will perform ablution" or "I will remove minor ritual impurity"?

The matter of intention for ablution (wudu) and ritual bath (ghusl) is simple. For wudu, you intend to remove the minor impurity or to make prayer permissible. For ghusl, you intend to remove the major impurity or to make prayer permissible. Any doubt about the intention after that is a whisper from Satan (waswas). If you doubt having washed a limb after finishing wudu or ghusl, do not pay attention to that doubt. However, if the doubt occurs during ghusl, or you are certain you did not wash it, a renewed intention is not required unless a long time has passed. But if a long time has passed and continuity (muwalat) is lost, a new intention is required to wash that particular limb.