Which evidence is an indication that a chemical reaction has occurred? a. melting substance
b. boiling substance
c. frezzign of substance
d. formation of a gas

Answer is D formation of gas.

Explanation: I hope that helped/have a great day.

the answer is d. 74.02

answer: semi-infinite solid initially at a uniform temperature of 400 ◦c is cooled at its surface by convection. the cooling fluid at a constant temperature of 200 ◦c has a convective coefficient (ha) of 250 w/m2.k. the physical properties of the solid: k = 20 w/m.k and α = 4 * 10-5  m2/s. using δx = 0.04 m and δt = 10 s. write the finite difference equations describing the transient heat transfer of the first five δx and the temperature profile after 30 s.