Externet Posted July 27 Posted July 27 Hi. On a recent thread about microwaving steam, a graph posted has this portion : Is the word vapour valid only for the pink area left of the vertical red line by 100C ? and steam the correct word for the area at right of that vertical red line by 100C ? If I leave a glass of water under my bed for a couple of weeks; it will be dry. Water turned to vapour, was never exposed to over 100C. If the same glass of water was exposed to over 100C, it turned to steam, not vapour. Am I wrong ? Or should the graph say GAS instead; that depending on temperature it can be vapour or steam ?
exchemist Posted July 27 Posted July 27 (edited) 18 minutes ago, Externet said: Hi. On a recent thread about microwaving steam, a graph posted has this portion : Is the word vapour valid only for the pink area left of the vertical red line by 100C ? and steam the correct word for the area at right of that vertical red line by 100C ? If I leave a glass of water under my bed for a couple of weeks; it will be dry. Water turned to vapour, was never exposed to over 100C. If the same glass of water was exposed to over 100C, it turned to steam, not vapour. Am I wrong ? Or should the graph say GAS instead; that depending on temperature it can be vapour or steam ? In this context, vapour steam and gas all mean the same thing. Vapour is probably the best term, and applies to the entire coloured area to the right of the curved line. "Steam" can mean different things to different people, as in popular parlance it can refer to the visible white clouds, which are actually droplets of condensed water, true steam being a colourless vapour. "Vapour" could as you suggest be replaced by "gas", but as water is liquid at STP, vapour seems appropriate. Water "vapour" just means water in the gas phase. Edited July 27 by exchemist
sethoflagos Posted July 27 Posted July 27 (edited) 1 hour ago, Externet said: Hi. On a recent thread about microwaving steam, a graph posted has this portion : Is the word vapour valid only for the pink area left of the vertical red line by 100C ? and steam the correct word for the area at right of that vertical red line by 100C ? If I leave a glass of water under my bed for a couple of weeks; it will be dry. Water turned to vapour, was never exposed to over 100C. If the same glass of water was exposed to over 100C, it turned to steam, not vapour. Am I wrong ? Or should the graph say GAS instead; that depending on temperature it can be vapour or steam ? No, no, yes, and no. Liquid water cannot exist above its critical temperature of 647 K (374 oC) at any pressure. Above that temperature, steam is a gas: below it, a vapour. (The distinction is due to whether or not it is possible for a vapour/liquid equilibrium state to exist at a particular temperature). Edited July 27 by sethoflagos 2
studiot Posted July 27 Posted July 27 2 hours ago, exchemist said: In this context, vapour steam and gas all mean the same thing. Vapour is probably the best term, and applies to the entire coloured area to the right of the curved line. "Steam" can mean different things to different people, as in popular parlance it can refer to the visible white clouds, which are actually droplets of condensed water, true steam being a colourless vapour. "Vapour" could as you suggest be replaced by "gas", but as water is liquid at STP, vapour seems appropriate. Water "vapour" just means water in the gas phase. It is quite common to be confused about water terminology. When studying the literature you will find that some books define water in the purely gaseous state as vapour and others (particularly engineers) as steam. The gaseous state in common terms is also called more technically the gaseous phase. It is also common for water to exist as a mixture of a gas and a mist of fine liquid droplets called an aerosol. Engineers call this wet steam and the water as a gas on its own dry steam. Gaseous water is invisible to the eye, all the white mist you can see is due to the aerosol. When talking about properties you have to consider the components of the mixture separately, as they have different heat capacities etc. 1
sethoflagos Posted July 28 Posted July 28 13 hours ago, studiot said: Engineers call this wet steam and the water as a gas on its own dry steam. In my younger days, CEGB engineers called it 'ot fog.
exchemist Posted July 28 Posted July 28 19 hours ago, sethoflagos said: No, no, yes, and no. Liquid water cannot exist above its critical temperature of 647 K (374 oC) at any pressure. Above that temperature, steam is a gas: below it, a vapour. (The distinction is due to whether or not it is possible for a vapour/liquid equilibrium state to exist at a particular temperature). That's an interesting subtlety I was unaware of. But it is worth perhaps stressing that there is no difference, in terms of physical state, between vapour so defined and gas.
Sensei Posted July 28 Posted July 28 (edited) 20 hours ago, Externet said: If I leave a glass of water under my bed for a couple of weeks; it will be dry. Water turned to vapour, was never exposed to over 100C. It depends on the properties of the air. If the air is dry, water from the glass will evaporate. If the air is humid, water will condense and appear "out of nowhere" even without rain (e.g. dew on leaves). That is, the humidity of the air is a key factor in determining whether or not a glass of water will disappear. "When the temperature is high and the relative humidity is low, evaporation of water is rapid; soil dries, wet clothes hung on a line or rack dry quickly, and perspiration readily evaporates from the skin. Wooden furniture can shrink, causing the paint that covers these surfaces to fracture. When the temperature is low and the relative humidity is high, evaporation of water is slow. When relative humidity approaches 100 %, condensation can occur on surfaces, leading to problems with mold, corrosion, decay, and other moisture-related deterioration. Condensation can pose a safety risk as it can promote the growth of mold and wood rot as well as possibly freezing emergency exits shut." Edited July 28 by Sensei
sethoflagos Posted July 28 Posted July 28 (edited) 4 hours ago, exchemist said: That's an interesting subtlety I was unaware of. But it is worth perhaps stressing that there is no difference, in terms of physical state, between vapour so defined and gas. Perhaps, but it can have a considerable impact on engineering. Vapours (Tr < 1.0 in attached chart) depart from ideal gas behaviour markedly more so than gases (Tr > 1.0). Edited July 28 by sethoflagos formatting
exchemist Posted July 28 Posted July 28 1 hour ago, sethoflagos said: Perhaps, but it can have a considerable impact on engineering. Vapours (Tr < 1.0 in attached chart) depart from ideal gas behaviour markedly more so than gases (Tr > 1.0). Hmm, interesting. It figures, I suppose, in that intermolecular forces will have more effect at lower molecular speeds.
sethoflagos Posted July 28 Posted July 28 53 minutes ago, exchemist said: Hmm, interesting. It figures, I suppose, in that intermolecular forces will have more effect at lower molecular speeds. Indeed. In passing, I find it remarkable how the General Compressibilty Chart can be reasonably accurate for all materials irrespective of their individual peculiarities. I guess it must be dominated by the ratio of intermolecular bonding energy to kinetic energy or similar simple relationship.
studiot Posted July 28 Posted July 28 I'm away on holiday tonight but you folks might find this interesting reading.
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now