Arrange the following in increasing order of acidity: water, ammonia, ethyne and ethane
The order of acidic nature would be: water > ethyne > ammonia > ethane.

Since oxygen has highest electronegativity amongst the atoms of the compounds, so its hydrogen will be most acidic and hence will be best acid among them.

Ethyne contains $\mathrm{sp}$-carbon and as a result the electronegativity of the carbon in ethyne is more than $\mathrm{sp^3}$ due to increased $\mathrm{s}$-character (50%) in the bonds between the carbon atoms.

The electronegativity of the carbon increases to a great extent when  and turns out to be 3.2, which is greater than the electronegativity of nitrogen which is 3.04. Hence the hydrogen of ethyne is more acidic than that of $\ce{NH3}.$

Ethane does not possess any acidic hydrogen hence it is least acidic.