In the reaction: hcl(g) + nh3(aq) → nh4 +(aq) + cl– (aq), hcl(g) is acting as a(n):

is acting as a Bronsted acid.

Explanation:

According to the Bronsted-Lowry conjugate acid-base theory, an acid is defined as a substance which donates protons and thus forming conjugate base and a base is defined as a substance which accepts protons and thus forming conjugate acid.

For the given chemical equation:

Here, is loosing a proton, thus it is considered as a bronsted acid and after losing a proton, it forms which is a conjugate base.

And, is gaining a proton, thus it is considered as a bronsted base and after gaining a proton, it forms which is a conjugate acid.

the mantle

explanation:

earth is divided into three layers: the crust or the outermost layer on which we live; the underlying, middle layer - the mantle - and the core (the innermost layer of earth).