General Science Previous Year Questions (PYQ) with Answers – Page 25

Without oxygen, yeast carries out alcoholic fermentation, breaking glucose into ethanol and carbon dioxide and gaining a little ATP. Glucose is first split into two molecules of pyruvate by glycolysis, which yields a net gain of two ATP.

Autotrophs take carbon dioxide from their surroundings and build their own organic food from it, so CO₂ is their main source of carbon.

When iron(III) oxide is heated with aluminium powder, aluminium takes away the oxygen, and the reaction gives out so much heat that the iron produced is molten: Fe2O3 + 2Al → 2Fe + Al2O3 + heat.

In anodizing the clean aluminium article is indeed the anode, but oxygen is released at the anode, not at the cathode; hydrogen is released at the cathode.

Arranged from most acidic to most basic, the four substances fall in exactly this order. Lemon juice, rich in citric acid, has a pH of about 2 to 2.5.

After fertilisation each structure of the flower turns into the next stage in exactly this way. When a male gamete from the pollen tube fuses with the egg cell inside the ovule, the fertilised egg becomes the zygote.

Only statement 3 is correct: the acid must be added slowly to water, and dilution lowers the concentration of hydronium ions per unit volume.

A soda-acid extinguisher uses sodium hydrogen carbonate (baking soda) solution and sulphuric acid, which react to release carbon dioxide. In this extinguisher the sulphuric acid is kept in a separate container inside a cylinder filled with sodium hydrogen carbonate solution.

The egg is fertilised to form the zygote, the zygote grows into the embryo, and the ovule carrying the embryo becomes the seed.

Pollen grains are formed by meiosis, the division that halves the chromosome number and DNA content of the parent cell.

Sound travels as compressions and rarefactions, while light is a transverse electromagnetic wave. In a longitudinal wave the particles of the medium vibrate back and forth along the direction in which the wave moves.

Stainless steel is an alloy of iron with chromium and usually nickel and a little carbon; tin is not one of its components.

Ethanol does not change the colour of either red or blue litmus, because it is neutral. Ethanol, C₂H₅OH, is an alcohol.

A hydrogen bomb fuses light hydrogen nuclei, while a uranium bomb splits heavy uranium-235 nuclei. In nuclear fission a heavy nucleus such as uranium-235 absorbs a neutron and splits into two lighter nuclei, releasing energy and two or three more neutrons.

The nucellus is the diploid tissue of the ovule itself, while the synergids, the egg and the antipodals are haploid cells of the embryo sac.

For resistors in parallel the reciprocals add: 1/11 + 1/22 + 1/33 = 11/66 = 1/6, so the equivalent resistance is 6 Ω. Taking 66 as the common denominator, 1/11 becomes 6/66, 1/22 becomes 3/66 and 1/33 becomes 2/66.

Rhizopus, the bread mould, feeds on dead and decaying organic matter, which is exactly what a saprophyte does. A saprophyte gets its food by releasing digestive enzymes onto dead matter such as stale bread, fruit or leaf litter and then absorbing the simple nutrients that result.

Goldsmiths dip gold and silver ornaments in dilute nitric acid to dissolve the dirt, oxide layers and traces of base metals stuck to them, which brings back their shine.

X-rays are not used in radar; radar works with microwaves, which have wavelengths of a few centimetres. X-rays lie between ultraviolet rays and gamma rays in the electromagnetic spectrum, with wavelengths from about 10 nm down to 0.01 nm, so about 1 Å (0.1 nm) is typical.

It is an inorganic salt made of the ammonium ion and the cyanate ion, with no carbon–hydrogen bonds. Ammonium cyanate is famous in chemistry because in 1828 Friedrich Wöhler heated it and obtained urea.