An aluminium cube 10 cm on a side at 0°C is heated from 10°C to 30°C. The change in its density is given by 
[Given: Coefficient of volume of expansion of aluminium $=7.2\times {10}^{-5}/{}^{\mathrm{o}}\mathrm{C},$Density of aluminium at $0{}^{\mathrm{o}}\mathrm{C}=2700 \mathrm{kg}/{\mathrm{m}}^3]$

volume coefficient of expansion     $\gamma =\frac{∆\rho }{\rho (∆t)}$

$$\begin{gathered} ∆\rho =\gamma \rho ∆t \\ =7.2X{10}^{-5}\times 2700\times 20=3.88 Kg{m}^{-3} \\ as {\rho }_1>{\rho }_2 ∆\rho is negative \end{gathered}$$

(negative sign indicates as temperature increaes density decreases.