The focal length of a lens of refractive index $\mu$ is $f$. If the lens is immersed in a liquid of refractive index $\mu$ then the focal length of lens is 

$\frac{1}{f}=(n-1)(\frac{1}{R_1}-\frac{1}{R_2})$ 

where, $n$ is refractive index of lens material with respect to its surrounding medium

If lens is immersed in the liquid of refractive index equal to its own then $n=1$ and hence above equation becomes: $\frac{1}{f}=0$

$\Rightarrow f=\infty$