A control is a experimental variable that is kept constant or unchanged during an experiment. During a controlled experiment, the controlled variables are held constant to minimize or stabilize their effects on the subject, its unchanging state allows the relationship between the other variables being tested to be better understood.
The embryo sac which present in egg shell protects it
Ca2+ means that the charge on the ion is positive be two units. As electrons have a negative charge of -1, when an atom loses electrons, the overall charge increases. In F- one electron has been gained. Iron (III) is slightly different, it refers to the magnitude of the oxidation number rather than being a +3 charge
