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### 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.
 Column A Column B Flask contains kerosene Flask contains water

You can notice that the flask filled with water seems heavier than the flask with kerosene. Why? We will get the answer by finding the mass per unit volume of kerosene and water.

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 $$200$$ $$g$$.

$\begin{array}{l}\mathit{Mass}\phantom{\rule{0.147em}{0ex}}\mathit{per}\phantom{\rule{0.147em}{0ex}}\mathit{unit}\phantom{\rule{0.147em}{0ex}}\mathit{volume}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{water}\phantom{\rule{0.147em}{0ex}}=\frac{250}{250}\\ =1\phantom{\rule{0.147em}{0ex}}g/{\mathit{cm}}^{3}\end{array}$

Similarly,
$\begin{array}{l}\mathit{Mass}\phantom{\rule{0.147em}{0ex}}\mathit{per}\phantom{\rule{0.147em}{0ex}}\mathit{unit}\phantom{\rule{0.147em}{0ex}}\mathit{volume}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{kerosene}\phantom{\rule{0.147em}{0ex}}=\frac{250}{280}\\ =0.8\phantom{\rule{0.147em}{0ex}}g/{\mathit{cm}}^{3}\end{array}$

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 $$rho$$ ($\mathrm{\rho }$).