This is a special reagent used to convert aldehydes and ketones (>C=O) into alkane (>CH2). Aldehydes and ketones are converted into alkane by refluxing them with amalgamated zinc and concentrated HCl. This conversion of carbonyl group (>C=O) into methylene (>CH2) group is called Clemmensen Reduction. Please note that this method is avoided to convert >C=O into >CH2 if compound has any acid sensitive group like -OH, it can get protonated by HCl and -OH can get replaced with -Cl. Another reagent which can be used to convert >C=O into >CH2 is NH2-NH2/KOH (Wolff-Kishner Reduction).