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I am reading a book and it gives the following answer:

No, the density of water changes with temperature. 10.0 mL of water at 10 °C would have a higher mass than 10.0 mL of water at 25 °C because the density is higher at 10 °C.

For me it doesn't make sense. In other part of the book it states "Mass is defined as the amount of matter in an object". How can a change in temperature change the amount of matter of an object? So mass of an object is dependent on temperature?

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    $\begingroup$ Heating up 10 g of water does not change its mass, as you say. But that's not what this question is about. This is about two different amounts of water: 10 mL of water at 10 °C (let's say in a measuring cylinder), and 10 mL of water at 25 *C (in, let's say, a different measuring cylinder). The amount of water in the two measuring cylinders is not the same. $\endgroup$ Commented Jan 27, 2021 at 15:34
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    $\begingroup$ In fact, heating up 10 mL of water does not change its mass, either. However, it will change its volume, such that at 25 °C it is no longer 10 mL. Hence, 10 mL of water at 10 °C is not the same thing as 10 mL of water at 25 °C. Hence my suggestion to think of them as completely different samples. They are not the same object. $\endgroup$ Commented Jan 27, 2021 at 15:36
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    $\begingroup$ $m_1 = \rho_1V;$ $m_2 = \rho_2V.$ When $\rho_1 > \rho_2,$ $m_1 > m_2.$ What is the problem? And, yes, comparing two identical bodies at different temperatures will result in minuscular difference in mass (hotter one will be a slight bit heavier), but this is not what your textbook (which, by the way, should be properly referenced) is trying to say. $\endgroup$
    – andselisk
    Commented Jan 27, 2021 at 15:36
  • $\begingroup$ Thanks for your comments, I understand now! $\endgroup$ Commented Jan 27, 2021 at 16:24
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    $\begingroup$ @M.Farooq why? Is the sentence too obvious? $\endgroup$
    – Alchimista
    Commented Jan 28, 2021 at 11:05

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Changing temperature doesn't change the mass of a fixed amount of a substance. But it does change the density. So a 10mL sample of water at 25 celcius will still contain the same amount of substance at 10 celcius. But it will not longer occupy exactly 10mL, but will be a slightly smaller volume.

Since you specified the conditions as having fixed volume, you would need to add a (small) extra amount of water to get 10mL at the lower temperature. So, at fixed volume, there would be more water.

Think carefully about all the conditions you specify in comparisons and be consistent.

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