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In saturation temperature, we have both gas and liquid. In the steam tables, I don't know if the liquid is in form of droplets inside the steam or in the form of normal liquid water at the bottom of the container. Also, the specific volume of the liquid is the volume of the liquid itself or the volume of the container over the mass of liquid? In other words, the specific volume $v_f$ and $v_g$ have the same nominator $v$ which is the volume of the container or different $v$?

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At saturation, the amount of liquid about to evaporate is balanced by the amount of vapor about to condense. For the purposes of most calculations, one can treat the amount of saturated vapor and the amount of saturated liquid which occupies a unit volume as being dispersed evenly throughout this volume. The fraction of saturated vapor to the mass of the whole saturated vapor-liquid mixture is called the quality. A quality of 100% implies that there is only the vapor phase present; 0% implies only saturated liquid is there.

The link below explains further:

Saturated Properties

Thermo properties like specific volume, specific enthalpy, and specific entropy for saturated vapor-liquid mixtures can be calculated using the quality of the mixture and the corresponding properties of the 100% saturated liquid and 100% saturated vapor phases of the fluid.

Since the specific volume of a substance is the volume occupied by a unit mass, then $v_f$ represents how many cubic meters are occupied by 1 kg of saturated liquid (0% quality), while $v_g$ is how many cubic meters are occupied by 1 kg of saturated vapor (100% quality).

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If there are liquid water and water vapor present, it doesn't matter where the liquid water is located. The volume of the container divided by the mass of water in the container is a weighted average of the specific volume of the liquid and vapor, weighted in terms of the mass fractions:$$\frac{V}{m}=x_lv_l+x_vv_v$$ where the x's are the mass fractions. The mass fraction of vapor is called the "quality." V/m is typically referred to as the average specific volume.

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  • $\begingroup$ Why do you copy textbooks and not answer my question? I asked, the vf and vg are calculated with the mass of liquid or gas respectively AT THE WHOLE VOLUME OF THE CONTAINER or AT ONLY THE VOLUME THEY OCCUPY RESPECTIVELY? in other words is the volume in the fractions vg and vf the volume of the container or they are v1 and v2 where v1+v2=the volume of the cobtainer? $\endgroup$ – ergon May 8 '16 at 13:30
  • $\begingroup$ At the volume they occupy respectively. No need to get testy. If you had stated your question more clearly, I could have given you your desired response. $\endgroup$ – Chet Miller May 8 '16 at 14:07
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The morphology of the liquid, i.e. its shape, is not important in thermodynamics. It can be liquid at the bottom of the container (due to gravity) and can be dispersed in space as droplet in non-gravity environment.

As for the volumes, when you calculate liquid specific volume, it is the liquid volume. Same is to the calculation of vapor specific volume. But not the container's volume.

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