Development of the Modern Freezer. Bacteria are usually the cause of food decay, but they can't grow as well or at all in freezing temperatures, which are at 32 degrees Fahrenheit (0 degrees Celsius) or lower.
HF- polar molecule: dipole-dipole forces. There are also dispersion forces and H bond between HF. BF3- nonpolar: dispersion forces. Ar- Dispersion forces between Ar atoms.
In $\ce{CF4}$, you are correct. The molecules is perfectly symmetrical, so every electron pair on each fluorine cancels out the electron pairs of every other fluorine. For this reason, this molecule is non-polar. In $\ce{CHF3}$, however, the hydrogen does not have 3 other electron clouds around it like the fluorine do.\ce{CF4}$: As you can see this molecules adopts a tetrahedral geometry which is perfectly symmetrical in every direction and so the dipoles of the four $\ce{C-F}$ bonds cancel out, leaving no overall dipole. $\ce{CHF3}$: