CBSE Class 10 Science Electricity, Chemical Reactions, Acids Bases Salts and Life Processes Assignment Set 08

Answer: Electric current is the rate of flow of electric charge through a conductor: \( I = \dfrac{Q}{t} \). Its SI unit is the ampere (A), where 1 A = 1 coulomb per second.

Teacher's Note:
1. Current measures how much charge passes a point each second.
2. It is measured in amperes using an ammeter.

Answer: Charge on one electron e = 1.6 × 10⁻¹⁹ C.
Number of electrons = \( \dfrac{1}{1.6 \times 10^{-19}} \) = 6.25 × 10¹⁸.
So one coulomb contains about 6.25 × 10¹⁸ electrons.

Teacher's Note:
1. Use n = Q/e with the charge of one electron.
2. Write the answer in standard form.

Answer: The resistance of a conductor depends on:
1. its length (R is directly proportional to length),
2. its area of cross-section (R is inversely proportional to area),
3. the nature (material) of the conductor, and
4. its temperature.
Together, \( R = \rho \dfrac{l}{A} \), where ρ is the resistivity of the material.

Teacher's Note:
1. Resistance rises with length and falls with thickness.
2. The material enters through its resistivity.

Answer: 1. Field lines emerge from the north pole and merge at the south pole outside the magnet; inside the magnet they run from south to north, forming closed curves.
2. The tangent to a field line at any point gives the direction of the field there.
3. A crowded set of lines shows a stronger field; widely spaced lines show a weaker field.
4. Two field lines never cross each other, because the field would then have two directions at the crossing point.

Teacher's Note:
1. Field lines are imaginary paths showing direction and strength of the field.
2. They never intersect and always form closed loops.

Answer: A fuse is connected in series with the live wire of the circuit (and with the appliance it protects). When the current exceeds the safe limit, the fuse wire melts and breaks the circuit.

Teacher's Note:
1. Being in series, all the current of the circuit must pass through the fuse.
2. This is why its melting protects the appliance.

Answer: A fuse wire must have a low melting point and fairly high resistance. Tin and tin-copper alloys have a low melting point, so the wire melts quickly when excess current heats it and the circuit is broken.

Teacher's Note:
1. The heating of a thin wire by large current is what makes a fuse work.
2. A low melting point allows it to cut off the current in time.

Answer: A fuse works on the heating effect of electric current. Heat produced is H = I²Rt, so a larger current heats the fuse wire more until it melts.

Teacher's Note:
1. More current gives much more heat because heat depends on I².
2. The same effect is used in heaters and electric irons.

Answer: A force acts on the conductor. The force is largest when the current is perpendicular to the field and zero when the current is parallel to it. Its direction is given by Fleming's left-hand rule: with the forefinger along the field and the middle finger along the current (held mutually perpendicular), the thumb gives the direction of the force.

Teacher's Note:
1. The magnetic field of the magnet interacts with the field of the current.
2. Reversing either the current or the field reverses the force.

Answer: Iron reacts with oxygen and moisture of the air and gets covered with a dull, reddish-brown layer of hydrated iron(III) oxide called rust. This slow oxidation (corrosion) destroys the shine of the surface.

Teacher's Note:
1. Rusting is a slow oxidation reaction needing both air and water.
2. The coating is brittle and flakes off, exposing fresh iron.

Answer: Neutralisation is used in the manufacture of antacid medicines: magnesium hydroxide (milk of magnesia) neutralises the excess hydrochloric acid in the stomach. It is also used to treat acidic industrial waste with lime or other bases before discharge.

Teacher's Note:
1. In a neutralisation reaction an acid and a base form a salt and water.
2. Any one correct application is acceptable.

Answer: It is a photochemical decomposition reaction: 2AgBr → 2Ag + Br₂ (in sunlight). Silver bromide turns grey because silver metal is formed; this is why it is used in black-and-white photography.

Teacher's Note:
1. A compound breaking down with the help of light is a photolytic decomposition.
2. Silver chloride behaves in the same way.

Answer: Curd and sour substances contain acids. The acids react with the copper and brass of the vessel to form toxic copper salts, which spoil the food and can cause food poisoning.

Teacher's Note:
1. Acids attack many metals and form harmful compounds.
2. Stainless steel or glass vessels are safe for such foods.

Answer: In water, an acid dissociates to give hydrogen ions (H⁺, present as H₃O⁺) and negative ions, for example HCl → H⁺ + Cl⁻. These mobile ions carry the current through the solution.

Teacher's Note:
1. Electric current in a solution is carried by moving ions.
2. A dry acid has no free ions, so it does not conduct.

Answer: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
Plaster of Paris sets to a hard mass of gypsum, which is why it is used for plaster casts and moulds.

Teacher's Note:
1. Plaster of Paris takes up water of crystallisation and hardens.
2. The formula is usually written as (CaSO₄)₂·H₂O + 3H₂O → 2CaSO₄·2H₂O.

Answer: Dry slaked lime, Ca(OH)₂, reacts with chlorine to form bleaching powder: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.

Teacher's Note:
1. Bleaching powder is made by passing chlorine over dry slaked lime.
2. Its formula is CaOCl₂.

Answer: Calcium sulphate hemihydrate, CaSO₄·½H₂O, is commonly known as plaster of Paris. It is obtained by heating gypsum at 373 K.

Teacher's Note:
1. Hemihydrate means half a molecule of water per formula unit.
2. Gypsum is the dihydrate, CaSO₄·2H₂O.

Answer: Life processes are the basic functions that all living organisms carry out to stay alive and maintain their body structure. Examples are nutrition, respiration, transport of materials, and excretion.

Teacher's Note:
1. These processes need a continuous supply of energy.
2. Their failure would lead to the death of the organism.

Answer: Nutrition is the process of obtaining food and using it for energy, growth and repair of body parts.
There are two main modes:
1. Autotrophic nutrition: the organism makes its own food from simple inorganic substances, as green plants do by photosynthesis.
2. Heterotrophic nutrition: the organism depends on other organisms for food. It includes holozoic (humans, amoeba), saprophytic (fungi like bread mould) and parasitic (Cuscuta, tapeworm) nutrition.

Teacher's Note:
1. Autotrophs prepare their own food while heterotrophs depend on others.
2. Heterotrophic nutrition has three main types.

Answer: Air enters through the nostrils, passes through the nasal cavity (where it is filtered and warmed), pharynx, larynx and trachea.
1. The trachea is kept open by rings of cartilage and divides into two bronchi, one for each lung.
2. Each bronchus branches into smaller bronchioles that end in balloon-like alveoli.
3. Alveoli have very thin walls with rich blood capillaries, where oxygen diffuses into the blood and carbon dioxide diffuses out.
4. Breathing is helped by the ribs and the diaphragm, which change the volume of the chest cavity.

Teacher's Note:
1. Gas exchange takes place only in the alveoli.
2. Cartilage rings prevent the windpipe from collapsing.

Answer: 1. The kidneys filter nitrogenous wastes such as urea and uric acid from the blood and form urine.
2. They regulate the amount of water and salts in the body.
3. They help maintain the balance of acids and bases in the blood.
Urine passes through the ureters to the urinary bladder and is expelled through the urethra.

Teacher's Note:
1. The nephron is the filtering unit of the kidney.
2. Besides excretion, kidneys keep the body's water and salt balance.

Answer: An artificial kidney does haemodialysis for patients whose kidneys have failed.
1. Blood from an artery is passed into a machine with tubes made of a semi-permeable membrane, which are surrounded by dialysing fluid.
2. The fluid has nearly the same osmotic pressure as blood but contains no nitrogenous wastes.
3. Urea and other wastes diffuse from the blood into the fluid.
4. The cleaned blood is returned to a vein of the patient after anti-clotting treatment.

Teacher's Note:
1. Wastes move out of blood by diffusion across a membrane.
2. Unlike a real kidney, an artificial kidney has no reabsorption.

Answer: 1. It is the source of food for almost all living organisms, because it makes carbohydrates from carbon dioxide and water using sunlight.
2. It releases oxygen, which is needed for the respiration of living things.
3. It removes carbon dioxide from the air and helps balance the gases of the atmosphere.
4. It converts light energy into chemical energy stored in food, which is also the origin of fuels such as coal and wood.

Teacher's Note:
1. Nearly all life depends on photosynthesis directly or indirectly.
2. It maintains the oxygen and carbon dioxide balance of air.