Why the coefficient of volume expansion is zero for water at $4\,^oC$?

A rod of length $20\, cm$ is made of metal. It expands by $0.075cm$ when its temperature is raised from $0°C$ to $100°C.$ Another rod of a different metal $B$ having the same length expands by $0.045 \,cm$ for the same change in temperature. $A$ third rod of the same length is composed of two parts, one of metal $A$ and the other of metal $B$. This rod expands by $0.060\, cm$ for the same change in temperature. The portion made of metal $A$ has the length …….. $cm$

The volume of the bulb of a mercury thermometer at $0^o C$ is $V_0$and cross section of the capillary is $A_0$. The coefficient of linear expansion of glass is $a_g$ $per ^o C$ and the cubical expansion of mercury $\gamma_m$ $per ^o C$. If the mercury just fills the bulb at $0^o C$, what is the length of mercury column in capillary at $T^o C.$

$Assertion :$ In pressure-temperature $(P-T)$ phase diagram of water, the slope of the melting curve is found to be negative.
$Reason :$ Ice contracts on melting to water.

Two rods are joined between fixed supports as shown in the figure. Condition for no change in the lengths of individual rods with the increase of temperature will be

( ${\alpha _1},\,{\alpha _2},$ = linear expansion coefficient

$A_1, A_2$ = Area of rods

$Y_1, Y_2$ = Young modulus)