Yes, both can be used for each other, but it's not advisable.
A comparator is specifically designed with very very high gain, and is also designed to provide a fast and "clean" transition. If you used it as an op-amp, the results are unpredictable, but most likely it will somewhat work. Since the ouput transistors are not designed to operate in the active (linear) region, the chip may overheat.
An op-amp can be used as a comparator easily. Again, you may not gat the fast and clean transition (may oscillate). That being said, I've used op-amps as comparators, but, I always provide for some positive feedback (schmitt trigger circuit) and it works fine.
Many opamp outputs can’t swing to the power rails.
If you have a dual/quad opamp and there is a spare opamp in your design, then it can be ok to use it is a comparator for low speed, low cost applications. Just beware of the non-rail-to-rail possibility.
For some people on this Forum, English may not be their first language.
One problem with using op amps as comparators is that the gain/phase relationship is not very well defined. They are made to switch reasonably fast, but may not be terribly stable when negative feedback is placed around a comparator. It might work, and it might not. A lot depends on the required closed loop gain and the particular comparator.
On the other hand, an op amp is either internally or externally compensated, so it has a constant phase/frequency roll of. This will ensure good stability in closed loop applications. Unfortunately this stability is achieved by rolling off the high frequency gain, so an op amp will not switch as fast under saturated conditions.
An op amp will have a gain bandwidth product. In other words if the GB product is 2Mhz, it will have a gain of ten at 200 Khz or a gain of one hundred at 20 Khz, or one thousand at 2 Khz.
So really, comparators are built for speed, and you usually apply hysteresis (positive feedback) to get best performance.
Op amps give good predictable stable behaviour when used with negative feedback in a linear application. They can be used as comparators as long as speed is not a prime requirement. Using a comparator as an op amp can get you into a lot of trouble sometimes.
