### Theory:

Let us consider a simple activity to understand the concept of density.
• Fill one of the flasks with water up to the $$250$$ ${\mathit{cm}}^{3}$ mark.
• Fill the other with kerosene to the same level of $$250$$ ${\mathit{cm}}^{3}$ mark.
• Measure them on a weighing scale and note down the values.
To understand the concept density better, let us assume that the mass of the flask is $$80$$ $$g$$.
Consider the mass of the flask with water is $$330$$ $$g$$, and the mass of the flask filled with kerosene is $$280$$ $$g$$. This shows, the mass of water alone is $$250$$ $$g$$, and kerosene is $$200$$ $$g$$.
$\begin{array}{l}\mathit{Mass}\phantom{\rule{0.147em}{0ex}}\mathit{per}\phantom{\rule{0.147em}{0ex}}\mathit{unit}\phantom{\rule{0.147em}{0ex}}\mathit{volume}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{water}\phantom{\rule{0.147em}{0ex}}=\frac{250}{250}\\ =1\phantom{\rule{0.147em}{0ex}}g/{\mathit{cm}}^{3}\end{array}$
$\begin{array}{l}\mathit{Mass}\phantom{\rule{0.147em}{0ex}}\mathit{per}\phantom{\rule{0.147em}{0ex}}\mathit{unit}\phantom{\rule{0.147em}{0ex}}\mathit{volume}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{kerosene}\phantom{\rule{0.147em}{0ex}}=\frac{250}{280}\\ =0.8\phantom{\rule{0.147em}{0ex}}g/{\mathit{cm}}^{3}\end{array}$
From the above result, the density of water and kerosene are $$1$$ $g/{\mathit{cm}}^{3}$ and $$0.8$$ $g/{\mathit{cm}}^{3}$, respectively. Therefore, the density of a substance is the mass per unit volume of a given substance. The SI unit of density is kilogram per meter cubic ($kg/{m}^{3}$) as well as gram per centimetre cubic ($g/{\mathit{cm}}^{3}$), respectively. The symbol for density is $$rho$$ ($\mathrm{\rho }$).