Question: In the dormitory, we are supplied with hot water at certain times. When there is no hot water, we do not have the chance to make a ghusl with cold water because the bathrooms are locked. Since we do not have any other opportunity, can we perform namāz by making tayammum?
It is not an excuse to be unable to find water in a city. You can perform ghusl in the restroom with the spring water you will buy. Your case is not an excuse for making a tayammum. If you cannot find any water, only then it is permissible to make a tayammum.
[tayammum: it is a simple procedure that you follow as a substitute for the ablution and ghusl and which is permissible in want of water. You do it by rubbing your arms and your face with your hands dusted with clean soil.]
A person who is unable to perform a ghusl
Question: The place I have been in is ruled with firm discipline. The bathroom is not opened until the sun rises, and there is no other place to perform a ghusl, either. Is it permissible to perform namāz by making a tayammum?
If you do not have the opportunity to perform the ghusl even in a restroom, you can perform namāz by making tayammum. But when you make a ghusl, you make iāda [to perform a namāz for the second time for any reason whatsoever] of that namāz.
A person who could not find a place to perform ghusl
Question: If a woman whose menstrual bleeding is over during a journey cannot find a place to perform the ghusl, what should she do?
First she should look for all the ways and means to perform the ghusl. She explains the situation to a suitable person, and she goes to a public bath for women if there is. If she is still unable to find a place, she can delay the ghusl on the condition that she can combine two namāzes [making jam]. If it does not work, either, she makes a tayammum instead of the ghusl and performs an ablution with water. If people cannot find a place to perform ghusl during journeys or when they are on buses and also if they do not have the chance to make jam of two namāzes, they can make tayammum when they stop off.