Answers to last issue's Do You Know?
1. We say water is a molecule. But isn't it also a compound because hydrogen and oxygen have been chemically combined? If so, how do you determine whether a substance is a compound or a molecule? 

Answer: Whether something is a compound or not depends on how many kinds of elements make it up. Compounds contain two or more different elements.
Whether something is a molecule or not depends on the type of bond that is formed when its atoms join together. In general, electrons can be shared between atoms (a molecular bond), or electrons can be completely removed from one atom and given to another (an ionic bond). Molecules have molecular bonds.
Water is a molecule because it contains molecular bonds. Water is also a compound because it is made from more than one kind of element (oxygen and hydrogen). We could say that water is a molecular compound.
Something like table salt (sodium chloride) is a compound because it is made from more than one kind of element (sodium and chlorine), but it is not a molecule because the bond that holds it together is an ionic bond. (Some people would label salt as an ionic compound.)
Oxygen in the atmosphere is a molecule because it contains molecular bonds. It is not a compound because it is made from atoms of only one element - oxygen. 

2. Does increasing the size of a magnet make it stronger?
Answer: Increasing the size is indeed one way to increase the strength of a magnet. However, it can be very difficult and expensive to simply make a magnet bigger. The other way is to make it better, so that with the same size, it can generate a stronger magnetic force.
What does it mean to make it better? One way is to carefully choose the material from which it is made. The standard strong magnet found in laboratories is usually made from ALNICO - a special alloy where strongly magnetic bits, made from an iron-nickel-aluminum alloy, are embedded in an iron-cobalt alloy base. If you want something lighter that can generate a strong magnetic force, then one can try ferrite (or ceramic) magnets which are made from iron oxide plus barium, strontium, or lead oxide. Like other ceramic materials (such as toilet bowls), they are brittle, so a lot of care in handling is necessary. 
The details of manufacturing permanent magnets are a trade secret. However, the general technique is to take a ferromagnetic material such as the ALNICO and expose it to a strong magnetic field, generated as very short but very powerful bursts from a nearby electromagnet. The magnetic "bits" in a ferromagnet are small collections of material (say, a millimeter or so) called domains that have a definite magnetic field with a north and south pole. Normally, these domains are oriented in random directions, thereby canceling each other out. When exposed to this powerful outside field, the domains start to orient themselves according to the direction of the strong outside field. The new alloy magnets have the added advantage that after the field is established, it tends to be more stable than other types of permanent magnets.
With electromagnets, another method to make them better is to construct a superconducting magnet. That is, the electrical coils are made of materials that will lose all electrical resistance when immersed in a tremendously cold substance, such as liquid helium. By reducing the electrical resistance, much higher amounts of electrical current can be put through the magnets, thereby generating a much stronger magnetic force.

3. How many atoms are there in the world?
Answer: This is not an easy thing to count, but it is a great question, since it assumes that we can count the number of atoms on earth -- which is what we will assume "in the world" to mean. (Now you may ask, what about atoms in the universe outside the earth? That is another great, but different, question.)
We can get an estimate of the number of atoms on earth by first knowing what its mass is. The mass of an object is a measure of how much material the object has. The mass of the earth is 5.98 * 10^27 grams. That is the scientific notation to write a large number that has a lot of zeroes. We can write the mass of the earth with all the zeros like this: 5,980,000,000,000,000,000,000,000,000 grams. (But that doesn't mean very much to us, since it is hard to imagine beyond billions; the scientific notation is so much better.)
The composition of the Earth, by mass, is about 32% iron, 30% oxygen, 15% magnesium, 13.9% sulphur, 3% nickel, 2% calcium, 1.4% aluminum by mass. There are dozens of other elements in the Earth's crust, but, since we are dealing with rough estimates, and they amount to less than 1% of the total mass, we can effectively discount them.
So that means that there is:
1.9 x 10^27 g of Iron
1.8 x 10^27 g of Oxygen
9.0 x 10^26 g of Magnesium
8.3 x 10^26 g of Sulphur
1.8 x 10^25 g of Nickel
1.2 x 10^25 g of Calcium
8.4 x 10^24 g of Aluminum
Because we know the atomic mass of each element, we can figure out how many atoms of each are contained in a sample of a given size. The atomic mass is the weight of 6.022 x 10^23 atoms of that element in grams. 6.022 x 10^23 is a quantity of material called a mole.
1 mole of:
Iron: weighs 55.8 grams
Oxygen: 16.0
Magnesium: 24.3
Sulphur: 32.1
Nickel: 58.7
Calcium: 40.1
Aluminum: 27.0
So, we can divide to determine how many mols of each element there are:
Iron: 3.4 x 10^25 moles
Oxygen: 1.1 x 10^26 moles
Magnesium: 3.7 x 10^25 moles
Suphur 2.6 x 10^25 moles
Nickel: 3.1 x 10^24 moles
Calcium: 3.0 x 10^24 moles
Aluminium 3.1 x 10^24 moles
Add all those moles up and you get a total of: 2.16 x 10^26 moles.
Multiply that by 6.022 x 10^23 and you get: 1.33 x 10^50 atoms.
If you want to write it with all the zeros it would be:
133,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000.

4. How does baking soda help in cooking chickpeas (chana/kondaikadalai)?
Answer: We usually use baking soda when cooking chickpeas or rajma (mochakottai). This is to soften the beans faster and decrease cooking time. It makes the beans slightly alkaline, which increases the softening effect. In the case of some beans, baking soda is known to aid in breaking down gas-causing sugars as well. Higher concentrations of baking soda and/or pressure cooking may be needed to make this effect significant. In most cases, an increased soaking time will have a much greater impact on gas-causing sugars, so baking soda should perhaps be reserved for situations where preparation time is limited.
Baking soda uses its sodium ions to replace the magnesium in the cell walls of plants, resulting in faster softening. The softening of the cell walls allows faster breakdown in some of the sugars with cause gas. 
But all this leads people to claim that adding baking soda "reduces gas", that it "lowers the temperature" etc. The former is true but the effect is too little to make a difference, the latter is false.
Baking soda added to water in fact raises the temperature, but only slightly. Chemical reactions are either "endothermic" or "exothermic". Endothermic means that you have to put in energy (heat) to make the reaction go on while exothermic means that there is energy (heat) left over. The left over heat will raise the temperature. Baking soda and water is exothermic and so the water gets a little warmer. This is because the binding energy of the chemical bonds of the products has an excess over the binding energy of the components. Therefore, energy is released and the water warms up.
In 1974 scientists did an elaborate study using 5 types of beans and many kinds of preparation conditions: (6-hour soak vs. 12-hour soak, soaking alone vs. boiling vs. pressure cooking, sprouting for 1-4 days, etc.). In almost all preparations the sugar contents were only decreased by a few percent with baking soda. For almost all scenarios, it appears that soaking for an extra few hours, doing a 24-hour germination before cooking, or choosing to pressure cook beans will have far greater impact than adding baking soda. 
In fact use of baking soda is known to destroy nutrients, so we should minimise its use.