dttom

It's actually lower the pH, right? I was told that fluorides help increase the density of enamel.

I think there's no problem with drinking water just after exercise, while I think it's more suitable to drink water before exercise, too. As you exercise, water losses from body so drink before exercise would help.

The densities of alkali metals probably increase down the group. My text book said that it is because the increase of atomic number. And the text book also states that the large sizes of alkali metals are the reason of low densities compare with others, as there would be less atoms exist in a given volume in metal lattices. However, in alkali metals, down the group, isn't the atomic size increases? So if the atomic size increases, shouldn't the density decrease? But why the densities increase, base on the given data in text book? Is there any misunderstanding here?

According to the Pauling Scale, compounds with electronegativity difference of 1.7 or above is considered as ionic compound; compounds with electronegativity difference between 1.7 and 0.4 is considered as polar covalent compounds, those below 0.4 or 0 is non-polar covalent compounds. When I check the electronegativity difference between hydrogen and lithium, it is 1.1 which is between 1.7 and 0.4, according to the Pauling Scale, LiH should be polar covalent compound, however, it is told that LiH is in giant ionic structure and the bonding is 'appreciably ionic'(while 1.1 is not just a bit smaller than 1.7 in the scale) in the text book. The same situation is found in case of NaH. I would like to know what makes such result(and also what does the word 'appreciably ionic' mean[how ionic it is?]).

Well, then how to calculate the energy given out during combination?

And one more question, do we divide non-flowering plants into dicots and monocots?

The reaction between Na and O2 as the following: 4Na + O2 --> 2Na2O Once I check the data base, I found out that the first ionization energy of Na is +500kJ/mol; the first electron affinity of O is -142kJ/mol; the second electron affinity is +850kJ/mol, so if I add up these numbers, the result should be positive, so is that means that the reaction is an endothermic reaction?

Can we remove the green color of leave of a green plant by heating the leaf with water(boil it)? I've got a leaf, one side is green in color and the other side is red. When I boil it, I observe the green color disappears gradually. Does this mean that chlorophyll decomposes under boiling?

The first one is about refraction: when a wave cross a boundary (from a shallow area to a deeper area, why this also regard as two medium?), refraction occurs. Suppose a wave cross from an area of deeper to an area shallower, the wave length as well as the speed decrease, why it must be so (i.e. deeper the area, shorter the wave length is; shallower the area, longer the wave length is), why can't vice versa occurs? It's about diffraction: Why the wave direction can be changed during diffraction even it's speed, wave length and frequancy are all still constant?

I think I have made a silly mistake, actually I want to know the oxidation number of carbon in glucose. And also, what about the test for flammability?

I know, so it'll have an end point but why the curve get gentle first?0

The curve in the graph of the relationship between light intensity and photosynthetic rate firstly is quite steep and become gentler and gentler gradully, finally becomes horizontal as limited by other factors. At first the slope is steep as the input of light energy increases, and finally it becomes horizontal as limited by other factors such as carbon dioxide concentration, but why would it first get gentler?

1) During the cracking reaction of heavy oil, if we place some medical paraffin in the bottom of the boiling tube and place some powdered broken porous pot in the middle of the boiling tube, what'll happen if we heat the bottom of the boiling tuble (the place where medical paraffin is placed) but not the middle (where powdered porous pot placed)? 2) How to test the flammability of a substance? And what observation can be seen? 3) What's the oxidation number of glucose,C6H12O6? zero?

Anyway, thanks for your reply, and also, you said that there have been recently report about faked result in stem cell technology, could you stretch it out more clearly?

1) Can the demage of brain cells be recovered by using stem cell technology? 2) What's the composition of the lining of the stomach wall? Does it dissolve in ethanol so that ethanol cause stomach ulcer and bleeding in this way? 3) How does ethanol lead to mouth throat and esophagus cancer, does it mean ethanol is carcinogen?

As the following: Find the maximum value of: (a^2/sinb)*[cos(b-2n)-cosb] express in terms of a and b; where n is varied (find a formula in a and b that is the greatest) In fact, this question is derived from a figure, the topic is compound angles, and I would like to know if there is any way to get the answer more faster and make less error, if yes, how to do so.

Is there any one know whether Al2(SO4)3 can catalyse the yeast respiration? When I put some yeast into a test tube that contains dilute H2SO4, dilute Al2(SO4)3 and glucose solution, and compaire with putting yeast into a test tube the contains just water and glucose as a control. I find that the tube containing Al2(SO4)3 have a faster reaction... I wonder to know why.

AD=e

e,a, and b is variables; a+b=45 degree 0<=(a)<=45

http://www2.hkedcity.net/sch_files/a/kst/kst-07110/photo_album/200511/204962/507977820.JPG Addition: AD=e Anyone can help me with the above question please? P.s. If it's not shown immediately, please refresh it again.

I don't think so, we may see NaCl(s) as an unity and when it 'reacts' with water, there're Na+ and Cl-, which are not the same as NaCl(s), so I think ionization is also a chemical reaction.

In a chemical cell if we use copper as the anode and silver as the cathode and sulphuric acid as electrolyte. Then if any current occurs, the reaction will be: Cu + 2H+ -> Cu2+ + H2 but it doesn't seem logical, can Cu replace H+? Or if the set up as the above, any occur will occur and what's the reason?

Another question: is sulphur slightly soluble and form some alkaline solution? Non-metals usually form acidic compounds, but why when I add water to sulphur and test the pH of the liquid from the liquid-sulphur mixture and I get a blue colour?

I think it's something related LTP. I will be more familiar with the passages when you read more and more. The system in brain will take less time to do the same thing compare with the first time reading a passage. May being something like that, I'm not sure.

Are the products H2O and H2SO4?