If you have two samples of air then add additional carbon dioxide to one of them and heat them up.
- Does the one with extra carbon dioxide increase in temperature faster?
- Does the sample with extra carbon dioxide retain its heat longer?
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If you just add $CO_2$ to a sample, the energy required to heat it will increase because there is more mass there. You have to heat the new $CO_2$ as well as the old sample. Then if heat is removed from the mixture, it will cool more slowly for the same reason. If you are thinking in terms of this with reference to the increase in $CO_2$ in the atmosphere, it is a trivial effect as we are talking about changes in the $100$ ppm area.
If you have a constant mass of air and increase the fraction of $CO_2$ relative to nitrogen/oxygen, the heat capacity of $CO_2$ is about $37$ J/mol at $15^\circ$ C compared to nitrogen at $29$ J/mol. Since $CO_2$ has a molecular weight of $44$ compared to $28$ for nitrogen, the lower heat capacity per gram of $CO_2$ will lower the heat capacity of the mix.
answered Jan 9, 2016 at 18:49
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1$\begingroup$ Typically you would keep gas at the same pressure, so mixing in CO2 would increase the heat capacity if you are replacing nitrogen mole-for-mole $\endgroup$– FlorisCommented Jan 9, 2016 at 19:13
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1$\begingroup$ @Floris: that is yet another way to understand adding extra $CO_2$ to air and you are correct. We need to know what is being done before we can answer the question. $\endgroup$ Commented Jan 9, 2016 at 19:35
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$\begingroup$ Might want to incorporate in your answer... $\endgroup$– FlorisCommented Jan 9, 2016 at 19:37