Does fasting become invalid due to the intention of nullifying the fast and hesitating about it, or due to an unintentional glance at a sister's 'awrah, or unintentionally inhaling the smell of cigarettes or cleaning materials, or due to belching and a sour liquid reaching the throat and being swallowed?

As long as you intended to fast before Fajr, your fast is valid, based on the Prophet's (peace be upon him) saying: "Whoever does not make the intention to fast from the night, there is no fast for him." The aforementioned hesitation does not cause any harm. As for smelling strong odors of cigarettes, chlorine, or cleaning materials, there is no sin in it, and it does not invalidate the fast. Similarly, unintentionally looking at a sister's awrah does not invalidate the fast, but the gazer must avert his gaze, as the Prophet (peace be upon him) said: "O Ali, do not follow one gaze with another; for the first is permissible for you, but not the second." Burping during fasting is also permissible and does not invalidate the fast.