Why do egg shells dissolve in hydrochloric acid?

Q . Why do egg shells dissolve in hydrochloric acid?

Answer: The main component of eggshells is Calcium carbonate, which reacts with hydrochloric acid to produce Carbon dioxide, Water, calcium chloride, and heat.

Q .Why is glass or chinaware packed with straw?

Answer The straw can be compressed easily and absorbs shocks therefore any sudden force will not cause the breakage of the glass or chinaware 

Q . Why does temperature remain constant during phase change?

Answer:The energy is all being used to change the state of the substance (ordering the molcules) instead of increasing the temperature.

In the change of state, all the heat is consumed by the system for rearranging its molecules. Nothing is left for the first part ie for changing the Internal energy of the system, which is a function of Temperature -
Q = Change in IE + Mechanical Work => c . f(T) + W where c = a constant and is = mCv dT 
Hence the temperature does not change. This process is known as Isothermal Change.

in case of Perfect gases.

Q. Prove that if a body is thrown vertically upward, the time of ascent is equal to the time of descent.

Answer: Answer: Time of ascent be t seconds and the object be thrown with an initial velocity be u m/s.

V² – u²    = -2gh

0² - u²    = - 2gh

u²    =  2gh ----(i)

Also,

t₁ = (v-u)/-g

t₁ = (0-u)/-g

t₁ = u/g  --------------(ii)

Descent:

Time of descent be t ' seconds and the object final velocity be v m/s. The distance covered will be the same as when the object is thrown upwards.

V² – u²    = -2gh

v²  - 0²    = - 2gh

v²    =   2gh ----(iii)

Also,

t₂ = (v-0)/g

t₂ = (v -0)/g

t₂ = v/g --------------(iv)

From equation (i) and (iii) we can see that u = v. -----(v)

Also, From equation (ii) ,(iv) and (v) we can see that

t₁ = t₂ , that is, time of ascent is equal to time of descent