If a solid iron ball is cut diametrically into two equal parts, then its surface area ______________.

If a solid iron ball is cut diametrically into two equal parts, then two solid hemispheres of equal radii are obtained.

Let the radius of the ball be r.

∴ Radius of each hemisphere = Radius of ball = r

Surface area of the ball = 4πr²

Surface area of each hemisphere = 2πr² + πr² = 3πr²

∴ Surface area of both hemispheres = 2 × 3πr² = 6πr²

Increase in surface area =  6πr² – 4πr²

Percentage change in surface area $=\frac{6{\mathrm{\pi r}}^2-4{\mathrm{\pi r}}^2}{4{\mathrm{\pi r}}^2}\times 100=50\%$

Since the surface area of the two hemispheres is greater than that of the ball, so the surface area increases by 50%.