(a) If in making cake baking soda is added in place of baking powder, then the cake would taste bitter. This is because sodium bicarbonate is a mild base, and we know that bases are bitter to taste. Baking soda will give soft and spongy texture to the cake, but will also impart a bitter taste. In baking powder, the tartaric acid neutralizes the bitterness produced by baking soda.
(b) Baking soda can be converted into baking powder by adding tartaric acid and starch.
(d) Tartaric acid is added to neutralize the bitterness produced by the baking powder. Also when baking powder mixes with water, then the sodium hydrogen carbonate reacts with tartaric acid to evolve carbon dioxide gas which gets trapped in the wet dough and bubbles out slowly making the cake to rise and hence 'soft and spongy' thus endowing them with a light, fluffy texture. The equation which takes place can be shown as:
NaHCO3 + H+ → Na+ + CO2 + H2O