Ask Miss Science

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posted by Ask Miss Science @ 5:16 PM 1 comments

How does soap work?

Alan says:

explain again how soap works

One of the main things soap does is to force water molecules to quit being so cliquish with each other.

You see, water molecules stick together something fierce. Much of their stickiness comes from the fact that each water molecule has an electric dipole. In other words, one end of the water molecule is positively charged and the other end is negative. The difference in charge is due to an unequal distribution of electrons in the molecule. You may have heard of this before if you ever heard water described as being a polar molecule.

Of course, as you know, water molecules don't keep completely to themselves; they do associate with certain other types of particles. Two examples are the ions in table salt and molecules of sugar.

However, dirt and grease aren't on water's "cool" list. Most dirt, oil and grease are made of molecules that do not possess significant electric dipoles. In other words, molecules in those substances are non-polar.

When water molecules come near these non-polar substances, they don't even consider unclustering. Perhaps you've seen water bead on a waxy surface? The reason it does that is because the water molecules stay crowded together as tightly as they can rather than interact with the waxy surface.

The net effect of all this is that, as you know, plain water doesn't do a very good job of cleaning grease and dirt off of things.

Enter soap.

Soap molecules have split personalities. On one end of a soap molecule, there is an electric dipole. Water molecules are very attracted to this end of the soap molecule. But, soap molecules also contain a long "tail" that has a structure much more similar to dirt or grease.

Once the soap molecules mix in with the water molecules, the long tails keep the water molecules from being able to crowd together so well. In technical terms, it is said that soap "reduces the surface tension" of water.

Since the water molecules cannot cluster together so tightly, they are more likely to slip into crevaces in the dirt or grime. Also, the long soap tails, not being so snobbish, can attract the grime via a much weaker, but still electrostatic, interaction called "London" or "van der Waals."

With the water and soap molecules now mixed in with the dirt, a little
agitation, such as rubbing your hands together, can loosen the dirt so that it can be rinsed away.

posted by Ask Miss Science @ 11:46 PM 14 comments

What's up with adding comments?

I've told Blogger about this...

I set the comments to do these things:

[1] Allow anonymous posts

[2] Send me an email when a comment is posted

So far, it does that. But, the comment doesn't show up on the blog. I'll copy comments into the questions, as usual.

Right now, I have two questions in queue, in case you're curious. Feel free to send more.

posted by Ask Miss Science @ 12:03 AM 2 comments

Is my dishwasher outgassing?

I just bought a new dishwasher and it smells funny. I remember once you taught me the word "outgassing." What does that mean again? Is that what my dishwasher is doing?

You are correct!  Your dishwasher is outgassing.



In certain contexts, the term "outgassing" can have fairly specialized
meanings.  But, in this context, it is a general term used to
indicate the issuance of a gas from some substance.



Typically, the term is used when:

• the nature of the gas or the reason for its presence is unknown
  or unimportant.


• the substance releasing the gas is a solid.  That is, the
  term is usually not applied to, say, the evaporation of water from
  syrup or of solvent from a can of paint
  
  


• the presence of the gas does not necessarily imply a significant change in the nature of the substance.  For example, it is not typically used for the gases released when vegetables left sitting on the counter for a week begin to decompose, but it is used to describe the gases
  given off by carpeting and other environmental materials.
  
  


• the gases in question are released when a substance is heated or placed in a vacuum (for example, when used in manufacturing spacecraft).

The fact that you smell the dishwasher means that there is a gas coming from it: a molecule must be able to get into your nose for you to smell it.  So, unless you ground part of the dishwasher up and inserted the powder nasally, I'm assuming the molecules made it to your nose because they were gases and travelled through the air. 



The main reason that every-day objects might be outgassing is, simply put, that substances within the object are slowly drying out.  For example, consider a newly painted object.  Even after the paint is dry to touch, you can still smell it.  You can smell the paint because molecules of the thinner (aka solvent, the liquid part of the wet paint) are still escaping from the paint.  Eventually, the solvent molecules all escape and the paint no longer smells.



This is most likely what is happening with your new dishwasher.  A large part of your dishwasher is probably made of some form or other of plastic.  Some plastics are shaped by dissolving, suspending or softening them in a liquid (solvent) and then pouring them into a mold to dry into shape.  Other plastics are formed within a solvent.  Some produce a gas or liquid as they form.



It is likely that the plastic parts of your dishwasher -- and other parts that may be manufactured in similar ways -- have become dry enough to be functional, but haven't dried out completely.  In other words, you are probably smelling one or more solvents used and/or produced in the manufacture of the dishwasher.

posted by Ask Miss Science @ 8:54 PM 0 comments

How do they measure the mass of Saturn?

The question is:

Hey, what a great idea! Here's a question for you. i came across this sentence when reading some basic information about Saturn (the planet, not the car brand): Saturn's mass is about 5.69 x 10(to the)26th kg. How do they know?

Good question, Nancy! This is really a physics and astronomy question, but it's one that I can answer.

Masses in the solar system (and, indeed, beyond) are measured by observing their gravitational pull (force) on each other. The equation describing the force, F, between two objects is:

F = G m1 m2 / r 2

Equation 1

In this equation:

• F is the force (the "pull", essentially) between two objects
  
• G is the universal gravitational constant (so far, only members of the Q, a fictional race of omnipotent beings have been known to be able to be able to alter G, and they do so intuitively, seemingly unable to explain the process to us)
  
• m₁ is the mass of one of the two objects
  
• m₂ is the mass of the other object
  
• r is the distance between the two objects
  

So, if we can somehow figure out F, m₁ and r (remember, G is a constant), we can easily calculate the size of m₂. The equation above can be rearranged to make this easier:

m2 = F r2 / (G m1)

Equation 1.1

This is, indeed, one way the Earth's mass is measured. The only problem with using this equation for things that are far away is that F is not easy to determine.
Fortunately, a man named Kepler had worked out a relationship that can be used to side-step the issue of determining F. Newton, the man who worked out
Equation 1 for us, combined Equation 1 with Kepler's third law to get the following equation describing the motion of one body circling another (say, the Earth around the Sun or Io around Jupiter):

p2 = 4 pi r3 / (G(m1 + m2))

Equation 2

The new letters in this equation are:

• pi is what it typically means: the ratio of a circle's circumference to its diameter
  
• p is the period of the revolution (for example, the period for the Earth's revolution is about 365.25 days)
  

One simple way the mass of Saturn can be determined using Equation 2 is to observe the motion of one of Saturn's satellites. It will be best to choose a small satellite.
If you choose a satellite small enough that m_satellite is much, much smaller than m_Saturn, then you can say that adding the mass of the satellite to the mass of Saturn doesn't make much difference in the final answer. As an analogy, consider adding the mass of a small dog to the mass of an automobile: the additional mass doesn't change things by very much. So, now we can say:

m_satellite + m_Saturn ~ m_Saturn

Now, Equation 2 is easy to solve! We can rearrange it like so:

mSaturn = 4 pi r3 / (G p2)

Equation 2.1

All we need is:

• The distance from one of Saturn's moons to Saturn's center (r)
  
• The period in which the moon revolves around Saturn (p)
  
• The universal gravitation constant, G (presuming, of course, that the Q haven't been tampering with it..)
  
• And, of course, 4 and pi
  

posted by Ask Miss Science @ 8:01 PM 2 comments

Who is Miss Science?

Miss Science is a professor of chemistry. She has a master's degree (MEd) in science education and a doctorate (PhD) in chemistry. So, technically, she's Dr. Science, but that name's already taken.

Miss Science will answer general science questions for a general audience. Her specialty is, of course, chemistry. But, she will occasionally entertain questions from other disciplines as her knowledge permits.

posted by Ask Miss Science @ 4:23 PM 4 comments