A substance with higher attraction will freeze faster?

  concave means having an outline or surface that curves inward like the interior of a circle or sphere. so the mirror curves inward.

Year  1936,  lehman predicted that the earths 'inner core' is solid. she observed that the seismic signals during her study were bent and did not pass through instead bouncing back. later it was confirmed that the earth's inner core was solid made up of nickel-iron and had a molten outer core.

Bis the answer, adding an indicator to a solution to test its acidity.