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Answers to Readers' Questions on Physics:

Pressure and States of Matter

The following 7 comments and questions have been sent in. They are listed according to date.

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Pressure and area

Question

October 22, 2010

hallow can I ask a question 1) Is pressure depend on area if yes why we say that pressure depend on 1- height 2- density of material as pressure is height times density times accelration due to gravity for aliquid 2) why pressure of the foot of human is greater than pressure of elephant's heel althouh pressure act on a unit area not surfaca area? I hope you help me

esraa - Egypt

20569

Answer

Pressure is the force divided by the area that the force is applied. The force can be the weight of a solid object or a fluid.

The height of a fluid or liquid increases its weight and thus its pressure at that depth.

The pressure applied by a man's foot can be greater than the pressure of the foot of an animal 10 times as heavy, provided that the area of the animal's foot on the ground is greater than 10 times the area of a man's foot.

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How much does water expand when it turns to steam?

Question

August 27, 2009

How many times does water expand when it turns to steam? I say 1600 a friend of mine says 1728

Dean - Canada

18219

Answer

At atmospheric pressure, the expansion ratio between liquid water and steam is about 1:1700.

Of course, it depends on at what temperature you start also, but it is a good approximation.

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What state of matter is a cloud?

Question

May 5, 2007

In what state of matter is A CLOUD on the sky
Is it liquid or gas or a mixture whatever ?
thank you

denis - GREECE [HELLAS]

13709

Answer

A cloud is typically the gaseous state of water. But often tiny water droplets can form within a cloud, especially just before it starts to rain. In same cases, solid ice particles can form in a storm cloud, resulting in hail.

So, a cloud is usually a gas, but it can be a mixture under certain circumstances.

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How many times does water expand?

Question

October 18, 2006

How many times does water expand when it turns to steam?

Yamile - USA

12195

Answer

Water expands when heated. When it turns to steam, the steam itself expands.

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Process of solid changing into gas

Question

October 11, 2006

Dear school for champions I like your site and all but I asked what is the process of solid changing into gas and didnt get an answer leaving me to continue my search so if you please add this answer to your site I would really appreciate it. Also I noticed that you didnt mension the process of evaporation heat of fusion or any of those things so I think you should also add all the things like that.

- United States Virgin Islands

12139

Answer

Sorry taking so long to answer you. We've had too many email lately.

Our lesson on Changing States of Matter shows how changing a solid to a gas is called sublimation.
See: http://www.school-for-champions.com/science/matterstates_changing.htm

In Pressure and States of Matter, the chart shows that when the pressure and temperature is low, the material will skip the liquid phase and go directly from a solid to a liquid. For example, at very high altitudes (low air pressure) ice can go directly to become steam.

We may cover evaporation heat of fusion some time soon.

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Which water freezes first?

Question

August 17, 2006

when you put cold water and hot water in an ice tray, which one freezes faster?

joanie - USA

11716

Answer

Usually the cold water freezes faster, but there are exceptions. See our lesson on the Mpemba Effect at: http://www.school-for-champions.com/science/mpemba.htm

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Compressing air to an extreme

Question

April 3, 2006

1. Is it possible to compress air till it reaches 1000kg/m3.(ie. water density).?Please justify the answer.
2.What is the maximum pressure and temperature attained before there is a phase change into water?

KALYANAKUMAR - INDIA

10739

Answer

When air is compressed and liquefied, it can reach water density. For example, liquid Nitrogen has a density of 800kg/m^3.

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