In the opening bars of this c-sharp minor nocturne, we have the famous descending scales that are played twice, first in piano then in pianissimo. Here's a snapshot ([source](http://www.free-scores.com/partitions_telecharger.php?partition=43903)) of the part in question:
![enter image description here](https://i.stack.imgur.com/ubNZ8m.png)

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**Question:**
The lower curved line connecting the two *c sharps* of the right hand, (i.e. between the 2nd and 3rd chords of right hand, similarly between 4th and 5th chords), does this line correspond to a [**slur**](https://en.wikipedia.org/wiki/Slur_(music)) in which case the two chords are played in a legato or does it correspond to a [**tie**](https://en.wikipedia.org/wiki/Tie_(music)), in which case the *c sharp* note is only played in the starting chord but held down during the following chord? 

* Finally, what is the general rule in distinguishing between slurs and ties?