Which of the following rules is used to determine the direction of the force on a current-carrying conductor in an electric motor?

The correct answer is C, Both I and II. Atomicity is the number of atoms in one molecule of an element, and it is why we write nitrogen gas as N2 and not just N. Nitrogen gas exists as N2, two atoms joined by a strong triple bond, so it is diatomic, like oxygen, hydrogen and chlorine. Sulphur exists as S8, a crown-shaped ring of eight atoms, so it is polyatomic. Phosphorus, as P4, is tetra-atomic, ozone (O3) is triatomic, and noble gases such as helium and argon are monoatomic because they exist as single atoms. These examples are listed in the NCERT Class 9 chapter on atoms and molecules. Option A is wrong because it accepts only the first pair, though the sulphur pair is also right. Option B is wrong because both pairs are correct, so 'neither' cannot be the answer. Option D is wrong because it accepts only the sulphur pair and leaves out nitrogen, which is correctly diatomic. Exam tip: He mono, N2 di, O3 tri, P4 tetra, S8 poly.

The correct answer is B, H3O+. A hydrogen ion (H+) is just a bare proton, so it cannot stay alone in water. It at once attaches itself to a water molecule and forms the hydronium ion, H3O+. So when an acid such as hydrochloric acid dissolves in water, it really produces H3O+ ions, and these give the solution its acidic nature. Without water, acids do not show acidic behaviour, which is why dry HCl gas does not change the colour of dry blue litmus paper. Option A is wrong because HO- (OH-) is the hydroxide ion, which bases give in water. Option C is wrong because H2 is hydrogen gas, a neutral molecule, not an ion. Option D is wrong because H2SO4 is sulphuric acid itself, a compound that releases hydrogen ions in water. Exam tip: acids give H3O+ ions in water and bases give OH- ions; a solution with pH 7 is neutral, below 7 acidic and above 7 basic.

The correct answer is D, f-block. The inner transition elements are the lanthanides and actinides, in which electrons fill the f-orbitals, so they form the f-block of the periodic table. They are placed in two separate rows at the bottom of the table so that the table does not become too wide. The lanthanides run from cerium to lutetium and are also called rare earth elements; the actinides run from thorium to lawrencium and are all radioactive. Uranium and plutonium, used in nuclear power, are actinides. Option A is wrong because the d-block holds the transition elements such as iron, copper and zinc, in groups 3 to 12. Option B is wrong because the p-block holds groups 13 to 18, including the halogens and the noble gases. Option C is wrong because the s-block holds groups 1 and 2, the alkali and alkaline earth metals. Exam tip: the d-block has the transition elements; the f-block has the inner transition elements, the lanthanides and actinides.

The correct answer is D, Golgi apparatus. The Golgi apparatus is the main site where glycoproteins and glycolipids are formed. It is a stack of flat, disc-shaped sacs called cisternae, first described by Camillo Golgi in 1898. Proteins made on the endoplasmic reticulum reach the Golgi, where sugars are added to them, and they are then packed into vesicles and sent to places inside the cell or out of it. That is why the Golgi is called the packaging and dispatch centre of the cell, and it also helps to form lysosomes. Option A is wrong because plastids, such as chloroplasts, carry out photosynthesis or store food and pigments in plant cells. Option B is wrong because lysosomes are small sacs of digestive enzymes and are called the suicide bags of the cell. Option C is wrong because mitochondria make energy in the form of ATP and are called the powerhouse of the cell. Exam tip: Golgi means packaging and adding sugars; mitochondria mean ATP; a lysosome is the suicide bag.

The correct answer is C, Thymine. In DNA, adenine always pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three. This is called complementary base pairing, and it is why one strand of DNA can be used to copy the other. The double-helix model was proposed by James Watson and Francis Crick in 1953, using X-ray images made by Rosalind Franklin. Adenine and guanine are purines with two rings, while thymine and cytosine are pyrimidines with one ring, so each pair has the same width and the helix stays even. In RNA, thymine is replaced by uracil. Option A is wrong because guanine is a purine like adenine and pairs with cytosine. Option B is wrong because cytosine pairs with guanine through three hydrogen bonds. Option D is wrong because histone is a protein around which DNA is wound in chromosomes; it is not a base at all. Exam tip: A pairs with T by two bonds, G with C by three; RNA has U in place of T.