tefilla - Putting on tefillin late

For whatever reason, I was unable to put on tefillin while davening shacharit this morning. However, I have access to tefillin now, and the sun has not yet set. After I put on the tefillin, what should I do before I take them off? Consider both the case where I have and have not yet davened mincha.

inorganic chemistry - If a chemical formula for a mineral contains a comma, are there limits on the amounts of each element?

I have a chemical formula for the mineral actinolite which is: $$\rm{Ca}_2({Mg,Fe})_5{Si}_8{O}_{22}({OH})_2$$

The formula contains a "," which I understand means that the magnesium and the iron can have different combinations (up to 5 total).

The question is, if magnesium is the dominant atom, is it possible for the amount of magnesium to be less than iron and still be actinolite, or must magnesium always have the greatest amount?

Answer

Actinolite is a mineral that is part of a so-called solid-solution series, which means that you get different but related minerals depending on the amounts of (in this case) iron and magnesium that are present. The magnesium-rich end member is tremolite; the iron-rich end member is ferro-actinolite; and what lies between is actinolite.

According to Wikipedia's article on actinolite, the formula is more precisely: $$\rm Ca_{2}(Mg_{4.5-2.5}Fe^{2+}_{0.5-2.5})Si_{8}O_{22}(OH)_{2}$$

So, for your question:

is it possible for the amount of magnesium to be less than iron and still be actinolite, or must magnesium always have the greatest amount?

the answer is no: at best, Mg and Fe can be present in equal amounts for the mineral to still be called actinolite, but note that in that particular case you are on the "border" between actinolite and ferro-actinolite, so perhaps what you choose to call it is up to you. And as mentioned above, if magnesium is dominant, the mineral is tremolite.

In the case where iron is the dominant atom, the mineral is called ferro-actinolite with the formula: $$\rm Ca_{2}(Mg_{2.5-0.0}Fe^{2+}_{2.5-5.0})Si_{8}O_{22}(OH)_{2}$$

inorganic chemistry - Why do we scratch the walls of test tube in test for magnesium or calcium basic radicals?

In test for calcium ion, we add ammonium carbonate in presence of ammonium hydroxide and then white precipitate (of $\ce{CaCO3}$) is formed which is dissolved in acetic acid and then reacted with ammonium oxalate to form $\ce{CaC2O4}$. But it says you have to scratch the walls of the test tube to get the precipitate. Why is it so?

Similarly, for magnesium ion we add ammonium hydrogen phosphate to get white precipitate. For this reaction too, the walls of the test tube are to be scratched to get the precipitate.

I read somewhere that scratching creates a surface for recrystallization of the precipitate. I just don't really understand what does it mean. Can someone explain it?

Answer

Scratching causes microscopic shards of glass to chip off the scratched surface which can break the surface tension of the liquid, allowing the solute to precipitate/crystallize from of the supersaturated solution.

inorganic chemistry - Why aren't neutralisation reactions reversible?

Let's take

$$\ce{3HCl + Fe(OH)3 -> FeCl3 + 3H2O}$$ This would be a normal one-way neutralisation reaction.

How about hydrolysis of ferric chloride (technically iron(III) chloride)?

$$\ce{FeCl3 + 3H2O <=> 3HCl + Fe(OH)3}$$

As this is reverseable, we should be able to right it with the reactants and products flipped.

$$\ce{3HCl + Fe(OH)3 <=> FeCl3 + 3H2O}$$

Now that is nothing but our original reaction … Why aren't neutralisation reactions reversible?

Also, does anyone have any good links to chemical equilibrium as in comments?

Answer

Neutralization reactions are reversible. In theory, at least, even if not so much in practice, all reactions are reversible.

If you look at things on a molecular scale, there's a principle called microscopic reversibility. In effect, there's no physical reaction mechanism you can create that's a one-way door. Maxwell's demon doesn't exist, and you can't get around that no matter how many steps you chain in whatever order, or which particular reagents you use.

What you can do, though, is set things up so it's exceedingly unlikely that the reaction proceeds in the other direction. That's what's happening with reactions we normally deem "irreversible". There's something about them which makes it difficult, in practice, for products to go back to reactants.

One common feature is the size of the equilibrium constant. If a particular reaction has an equilibrium constant that's really big, the equilibrium for that reaction is going to be vastly tipped to the product side. So much so, that the reactant side of the reaction will not be present in appreciable amounts. (For example, this is why combustion is viewed as irreversible. The reaction is so favorable toward the products that only parts per million or parts per billion of the reactants will be present at equilibrium.)

Another method for promoting "irreversibility" is for the products to partition to another phase. For example, if the reaction forms a gas or a precipitate, that product is removed from the reaction volume, and the concentration of the products drops, pulling the reaction toward the products. Note that even in this multi-stage setup, each step is reversible. That evolved carbon dioxide molecule could re-dissolve in solution, and then react backwards, but it's terribly unlikely for it to do so. You've basically fixed one of the products at a very low concentration, which draws the equilibrium toward the product side.

Precipitates work much the same way, as the activity of the solid is fixed by surface behavior, and does not increase because you increase the amount of solid you add. (The rate of reaction may increase by adding more solid, but the equilibrium of the reaction does not.)

So in your examples, the reason you get hydrolysis or neutralization is due to how you set up the reaction conditions. Have a large excess of HCl? You drive things toward neutralization. Have a way of getting rid of HCl (for example by driving it off as a gas, or reacting it with a base)? You drive things toward hydrolysis.

usage - パソコンが 得意だ/わかる/上手だ

So I was studying this book called 日本語生中継, and there was this question where you're supposed to choose the inappropriate word among others. And I got stuck at this sentence:

私は、パソコンが(得意だ.わかる.上手だ.)

The book says the correct, or rather, incorrect answer, was 上手だ. But I don't seem to understand why, as all three seem correct to me.

halacha - Selling flour for Pesach

For those who do not rely on heter mechira (selling) for bal yera'eh bal yimatzeh (the prohibition to own leaven on Pesach) for chametz gamur (certain leaven), do they generally sell flour or get rid of it as well?

Answer

If water hasn't touched the grain, the it certainly is completely Kosher for Pesach.

The Talmud (Pesachim 40b) tells us that wheat (but not barley) can be tempered with water prior to milling without fear of Chametz. That said, the longstanding custom from the Geonim (recorded in Rambam Chametz 5:7, Tur OC 453) is not to do so for flour for Pesach, lest one not temper and immediately grind efficiently and the wheat will ferment and become Chametz. The Mishna Berura (453:27) rules that though one may not eat flour from tempered grain, one may own it on Pesach.

Regarding flour that you find in modern stores, it's mostly a question of how they actually do it. Do they leave it in water for a while? Do they grind it right away? Perhaps modern plants can clean and grind the grains more efficiently with even less water than the Geonim would prohibit? See this link for some modern opinions about flour from stores ranging from burn it, to sell it, to just own it (but don't eat it).

midrash - Grandsons of Keturah

In Sefer HaYashar, Perek 25, it lists the grandsons of Avraham and Keturah. However, in Tanakh (Chayei Sarah and Divrei HaYamim), it specifically choses to only tell us the names of Yokshan and Midian's children. Why does the Torah only choose the children of Yokshan and Midian if any of Keturah's grandchildren? To clarify, I am looking for reasons as to why Yokshan and Midian are so special that their sons are mentioned by the Torah as apposed to the others.