Sometimes , a crackling sound is heard while taking off a sweater during winters. Explain.

Sound is heard clearly in summer than in winter Why ?

Typical 'lubb-dupp' sounds heard during heartbeat are due to

A white powder is added while baking bread and cakes to make them soft and fluffy. Write the name of the powder. Name its main ingredients. Explain the function of each ingredient. Write the chemical reaciton taking place when the powder is heated.

Lightning the thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why ?

A train is approaching towards a platform with a speed of 10 m s^(-1) while blowing a whistle of frequency 340 Hz. What is the frequency of whistle heard by a stationary observer on the platform ? Given speed of sound = 340 ms^(-1) .

When a copper wire was left in silver nitrate solution for sometime, it was observed that the solution turned bluish green. (i) Explain the observation . (ii) Write the balanced chemical equation to represent the change taking place.

a] Name the parts of the human female reproductive system where i] fertilisation takes place, ii] implantation of the fertilised egg occurs. b] Explain how the embryo gets nourishment inside the mother's body. c] Write the function of following parts in human female reproductive system: i] Ovary ii] Oviduct iii] Uterus d] Describe in brief the structure and function of placenta. OR Explain the structure and important role of placenta during gestation period of woman.