General Science Previous Year Questions (PYQ) with Answers – Page 75

Temporary hardness is caused by the bicarbonates of magnesium and calcium, Mg(HCO3)2 and Ca(HCO3)2. It is called temporary because simple boiling removes it: on heating, the soluble bicarbonates break down into insoluble carbonate or hydroxide, carbon dioxide and water, and the solid settles out.

The gap between an object and its real image is smallest when the object is placed at 2F, twice the focal length from the lens.

Birds, blowing dust, rain and wind can all carry the agents of plant disease from one plant to another. Plant pathogens such as fungi, bacteria and viruses cannot move on their own over long distances, so they depend on carriers.

A compound always has a fixed composition by mass, so a variable composition is a property of mixtures, not compounds.

When the block is released suddenly it falls freely at first, so at the lowest point its speed is zero and all the gravitational potential energy it has lost has gone into the spring.

Once a ductile specimen passes its ultimate tensile strength, the deformation stops being uniform and concentrates in one short length, which draws out into a narrow waist.

The law of conservation of mass, stated by Antoine Lavoisier in 1789, says that in a chemical reaction mass is neither created nor destroyed.

Ammonium sulphide is an unstable, foul-smelling compound that breaks down to release ammonia and toxic hydrogen sulphide, so it is not used as a fertiliser.

When magnesium burns, magnesium loses electrons and is oxidised, while oxygen gains them and is reduced, so oxidation and reduction happen together.

A photographer's flash tube is filled with xenon, which gives an intense burst of white light when a high-voltage spark passes through it.

Ferric chloride is a salt of a weak base, iron(III) hydroxide, and a strong acid, hydrochloric acid, so its solution is acidic and has a pH below 7.

In dinitrogen pentoxide, nitrogen has an oxidation state of +5, the highest it can reach, because a nitrogen atom has five electrons in its outer shell.

The formula of ammonia is NH3, so its molecular mass is the atomic mass of one nitrogen atom added to the atomic masses of three hydrogen atoms, that is fourteen plus three times one, which equals seventeen unified atomic mass units; one mole of ammonia therefore weighs seventeen grams.

The equivalent weight of a base is its molecular weight divided by its acidity, which is the number of replaceable hydroxide ions in one molecule.

The acid-secreting cells of the stomach, called oxyntic or parietal cells, release hydrochloric acid, which protein digestion in the stomach depends on.

Muscle cells are packed with the contractile proteins actin and myosin, whose filaments slide over each other to shorten the cell and produce movement.

In 1665 Hooke looked at a thin slice of cork under his self-made microscope, saw tiny box-like compartments and named them cells, from the Latin word cella, meaning a small room.

Nerve impulses are carried by the movement of sodium and potassium ions across the membrane of the nerve cell, so sodium is essential.

Carbon monoxide binds to the haemoglobin of red blood cells to form carboxyhaemoglobin, so the blood can no longer carry oxygen to the tissues.

Fullerenes are made of carbon atoms alone, with no surface to trap other substances, so they are counted as the purest form of carbon.