The time period of a 1 m long pendulum approximates to

The correct answer is B, Nucleoid. In a prokaryotic cell the genetic material lies free in the cytoplasm with no nuclear membrane around it, and this poorly defined nuclear region is called the nucleoid. Bacteria and blue green algae are prokaryotes, and besides the lack of a true nucleus they have no membrane bound organelles such as mitochondria, plastids or the endoplasmic reticulum; their ribosomes are of the 70S type and their cell wall carries peptidoglycan. Eukaryotes such as fungi, plants and animals keep their DNA inside a double membrane called the nuclear envelope. A is wrong because nucleic acid is the chemical name for DNA and RNA and not a region of the cell. C is wrong because the nucleolus is a dense body inside a true nucleus where ribosomes are put together. D is wrong because a nucleosome is the bead of DNA wound round histone proteins, found in eukaryotes. Exam tip: prokaryote means a nucleoid and no membrane bound organelles.

The correct answer is A, Cell will lose water. A medium that holds less water than the cell is a hypertonic medium, so water moves out of the cell by osmosis and the cell shrinks. Osmosis is the movement of water through a semi-permeable membrane from the side where water is in higher concentration to the side where it is lower. In an animal cell this loss of water makes the cell shrivel, and the matching change in a plant cell is called plasmolysis, where the protoplasm pulls away from the cell wall. In a hypotonic medium the opposite happens and the cell swells, which is why red blood cells burst in pure water, while in an isotonic medium there is no net movement at all. B is wrong because water always moves when the two concentrations differ. C and D are wrong because gaining water and swelling happen in a hypotonic medium and not in a hypertonic one. Exam tip: hypertonic means water out and shrinking, hypotonic means water in and swelling.

The correct answer is D, Colourless. Hydrogen is a colourless, odourless and tasteless gas, so none of the colours listed fits it. It is the lightest element, with one proton and one electron, and it is the most abundant element in the universe, although on the earth it is found mostly combined in water and in organic compounds. Hydrogen exists as the diatomic molecule H2, burns in air with a pale blue flame and gives only water as the product, which is why it is studied as a clean fuel. It is used to make ammonia by the Haber process and to harden vegetable oils into vanaspati. A is wrong because no common form of hydrogen is yellow. B is wrong because orange is the colour seen in sodium vapour lamps and not in hydrogen. C is wrong because none of the gases here is black; carbon in its soot form is black, but that is a solid. Exam tip: hydrogen, oxygen and nitrogen are all colourless and odourless gases.

The correct answer is D, Next to diamond, graphite is the second hardest known substance. Graphite is soft and slippery rather than hard, so this is the statement that is not correct. The difference between the allotropes comes from the way the carbon atoms are joined. In diamond every carbon is bonded to four others in a rigid three dimensional network, which makes it the hardest natural substance. In graphite every carbon is bonded to only three others in flat sheets, and these sheets slide over one another, so graphite is soft enough to serve as a lubricant and as pencil lead. A is correct because the fourth electron of each carbon atom in graphite is free to move, which lets graphite conduct electricity. B is correct because diamond is indeed the hardest known natural substance. C is correct because fullerene, such as the sixty atom buckyball, is a third allotrope of carbon. Exam tip: same element but different structure, so diamond is hard while graphite is soft and conducting.

The correct answer is D, colloidal solution of two liquids. An emulsion is a colloid in which one liquid is spread as fine droplets through another liquid that does not mix with it. Milk is the common example, with droplets of fat spread through water, and butter is the reverse kind, with droplets of water held in fat. Soap and detergents work as emulsifying agents: they keep oil droplets suspended in water so that grease can be washed away. In an emulsion both the dispersed phase and the dispersing medium are liquids, and that is what the question is testing. A is wrong because it describes colloids in general and not an emulsion in particular. B is wrong because a true solution is homogeneous with particles of molecular size, while an emulsion is a colloid. C is wrong because distillation separates liquids by their boiling points and has nothing to do with colloids. Exam tip: an emulsion is liquid in liquid, while an aerosol is a liquid or solid in gas.