Strictly, and I mean super super super de duper strictly speaking, an object will be heavier when it is hotter. But this will not be by any means you can practically measure.
Even if you do not change anything else about the object (i.e. quantity of molecules), it will become slightly heavier when heating it.
The deal is that mass and energy are two sides of the same coin, as determined by Einstein.
In the classical understanding, mass of an object only changes if you change the amount of stuff in it, and heating it will not change the mass. Heating the object was classically understood to only change the object's internal energy.
BUT, from a "mass is energy and energy is mass" perspective, that energy itself must also manifest as mass. And hence, heating an object will increase its mass.
Of course, this is by no means measurable. The ultimate unit translation factor between mass and energy is the square of the speed of light. That is a 9 followed by 16 zeros of Joules of energy for every kilogram of mass.
For objects of about mass of 1 kg, the energy transformations involved are usually on the order of kilojoules or Megajoules, and are completely insignificant fractions of 9*10^16 Joules. Hence the mass gain due to energy gain is an immeasurable amount.
As a specific example, consider 1 kg of ice, ready to melt. In order to detect the gain of mass (gained from the melting energy source) of this ice as it melts to water, you'd need a mass measurement device accurate to the nanogram.
Have you ever seen a mass measurement device (or a weight measurement device) that can accurately display 12 digits worth of data? Because that is what you'd need to be able to measure these mass gains.
Even if you do not change anything else about the object (i.e. quantity of molecules), it will become slightly heavier when heating it.
The deal is that mass and energy are two sides of the same coin, as determined by Einstein.
In the classical understanding, mass of an object only changes if you change the amount of stuff in it, and heating it will not change the mass. Heating the object was classically understood to only change the object's internal energy.
BUT, from a "mass is energy and energy is mass" perspective, that energy itself must also manifest as mass. And hence, heating an object will increase its mass.
Of course, this is by no means measurable. The ultimate unit translation factor between mass and energy is the square of the speed of light. That is a 9 followed by 16 zeros of Joules of energy for every kilogram of mass.
For objects of about mass of 1 kg, the energy transformations involved are usually on the order of kilojoules or Megajoules, and are completely insignificant fractions of 9*10^16 Joules. Hence the mass gain due to energy gain is an immeasurable amount.
As a specific example, consider 1 kg of ice, ready to melt. In order to detect the gain of mass (gained from the melting energy source) of this ice as it melts to water, you'd need a mass measurement device accurate to the nanogram.
Have you ever seen a mass measurement device (or a weight measurement device) that can accurately display 12 digits worth of data? Because that is what you'd need to be able to measure these mass gains.
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The density (mass per unit of volume) can change, but the mass - the weight - stays the same.
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It is the same the mass(space) if the object may stay the same but the weight doesn't.