The hydrogen bonds can be form with molecules that have free electron pairs and hydrogen atoms. In the case of NH4 molecule, the "free" electron pair in the nitrogen is not free because there is an hydrogen occuping them, and, in fact, this molecule has a positive charge. The other acids have free electron pairs in the oxigen, nitrogen or sulfur.
Your independent variable is - The kind of nail polish used Your dependent variable is - How much the nails have grown And your constant is - Amount of time allowed for growth
