arena: used with "una"

Question

In listening exercise "Barra", I saw the following sentence: "(La playa) Tiene una arena blanca y suave" Is this a typo? or is this really how "sand" is referenced in spanish? As in, "one sand"?

Answer

Hola Shafagh
Yes, we use the article when we want to be a bit more specific about a word, so, here we are giving some specific description to the sand (blanca y suave). 
We could use the word arena without the article and say:
La playa tiene arena.
The beach has sand. 
This is using the word sand in a general way. 
You can also say:
La playa tiene arena blanca y suave.
We still omit the article just to keep this as a general description.
And we can include the article as we did in the exercise to add that nuance of being a bit more specific:
La playa tiene una arena blanca y suave...
Here, you would expect the speaker to keep describing the sand a bit more or adding more information.
The omission and presence of the articles in Spanish is one of the most difficult topics, where, unfortunately, there is hardly any rule to follow so that you always get it right. It is a question of lots of practice. : ))
Saludos