CBSE Class 10 Science Electricity, Chemical Reactions, Acids Bases Salts and Life Processes (Revision) Worksheet Set 05

Answer: Electric charge is a basic property of matter due to which it experiences a force in an electric field. Its SI unit is the coulomb (C).
1. Charges are of two kinds, positive and negative.
2. Like charges repel and unlike charges attract.
3. Charge is conserved and is quantised, so it exists in whole-number multiples of e = 1.6 × 10⁻¹⁹ C.

Teacher's Note:
1. Charge is carried by electrons and protons.
2. Electric current is the flow of charge.

Answer: (a) Current is measured by an ammeter, connected in series with the part of the circuit being measured.
(b) Potential difference is measured by a voltmeter, connected in parallel across the two points.

Teacher's Note:
1. An ammeter has very low resistance and a voltmeter has very high resistance.
2. This is why they are connected differently.

Answer: Resistance is the property of a conductor that opposes the flow of current through it: R = V/I. Its SI unit is the ohm (Ω).
Resistivity is the resistance of a conductor of unit length and unit area of cross-section: ρ = RA/l. It depends on the material and temperature only, and its SI unit is ohm-metre (Ω m).

Teacher's Note:
1. Resistance depends on the size of the conductor but resistivity does not.
2. Resistivity is a property of the material.

Answer: (a) Series: the same current I flows through all three, and the total potential difference is V = V₁ + V₂ + V₃. Using V = IR for each, IR = IR₁ + IR₂ + IR₃, so R_s = R₁ + R₂ + R₃.
(b) Parallel: the same potential difference V acts across each, and the total current is I = I₁ + I₂ + I₃. Using I = V/R, \( \dfrac{V}{R_p} = \dfrac{V}{R_1} + \dfrac{V}{R_2} + \dfrac{V}{R_3} \), so \( \dfrac{1}{R_p} = \dfrac{1}{R_1} + \dfrac{1}{R_2} + \dfrac{1}{R_3} \).

Teacher's Note:
1. In series the currents are equal and the voltages add.
2. In parallel the voltages are equal and the currents add.

Answer: 1. A bar magnet is a permanent magnet whose field is fixed; the field of a solenoid exists only while current flows.
2. The strength of a bar magnet's field cannot be changed easily, but that of a solenoid can be changed by changing the current or the number of turns.
3. A solenoid's polarity can be reversed by reversing the current; a bar magnet's poles cannot be reversed in this way.
In both cases the field lines are closed curves, and inside the solenoid they are parallel straight lines showing a uniform field.

Teacher's Note:
1. A solenoid behaves like a bar magnet only when it carries current.
2. Its field can be controlled, which is why electromagnets are so useful.

Answer: A solenoid is a long cylindrical coil of many circular turns of insulated copper wire wound closely; a coil may be a few turns of any shape and length. The field inside a solenoid is uniform and strong, while that of a short coil is not.
An electromagnet uses a core of soft iron, because it gets magnetised strongly when current flows and loses its magnetism as soon as the current is switched off.

Teacher's Note:
1. A solenoid is a special long, tightly wound coil.
2. Soft iron is chosen because it does not keep its magnetism.

Answer: Stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then the thumb points in the direction of the force on the conductor.

Teacher's Note:
1. Remember the order Field, Current, Force for forefinger, middle finger and thumb.
2. This rule gives the direction of force on a current-carrying conductor.

Answer: (a) For a straight conductor, the field increases with the current in the wire and decreases as the distance from the wire increases.
(b) For a circular coil, the field at the centre increases with the current, increases with the number of turns, and decreases as the radius of the coil increases.

Teacher's Note:
1. More current always gives a stronger field.
2. The field weakens as you move away from the wire or if the coil radius is bigger.

Answer: (a) Evolution of gas: Zn + H₂SO₄ → ZnSO₄ + H₂↑; CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑.
(b) Change in temperature: CaO + H₂O → Ca(OH)₂ + heat (temperature rises); NaOH + HCl → NaCl + H₂O + heat.
(c) Precipitate: BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl (white); AgNO₃ + NaCl → AgCl↓ + NaNO₃ (white).

Teacher's Note:
1. Gas bubbles, a temperature change and a solid appearing are signs of a chemical reaction.
2. Equations must be balanced.

Answer: 1. The blue colour of the copper sulphate solution fades and changes to light green, because iron sulphate forms.
2. A reddish-brown deposit of copper appears on the iron nail.
Fe + CuSO₄ → FeSO₄ + Cu (displacement reaction).

Teacher's Note:
1. Iron is more reactive than copper, so it displaces copper from its solution.
2. The colour change shows that a new compound formed.

Answer: Consider CuO + H₂ → Cu + H₂O (on heating).
1. CuO loses oxygen and becomes Cu, so copper oxide is reduced.
2. H₂ gains oxygen to form H₂O, so hydrogen is oxidised.
Both changes occur in the same reaction, so it is a redox reaction.

Teacher's Note:
1. Whatever substance is oxidised, another is reduced in the same step.
2. Such reactions are called redox reactions.

Answer: Curd and sour foods contain acids. The acids react with copper and brass to form poisonous copper salts, which spoil the food and can harm health.

Teacher's Note:
1. Acids react with many metals.
2. Use glass, steel or ceramic containers for sour food.

Answer: On dilution, the number of H⁺ (H₃O⁺) ions per unit volume decreases, so their concentration decreases and the acid becomes weaker in strength per unit volume. The pH of the solution rises towards 7.

Teacher's Note:
1. The same ions are spread through more water.
2. Dilution never makes the solution basic.

Answer: Dry HCl gas does not have free hydrogen ions, because HCl ionises only in the presence of water. Without H⁺ ions, it cannot show acidic behaviour, so dry blue litmus does not turn red.

Teacher's Note:
1. Acid properties are shown only when H⁺ ions are present.
2. Moist litmus paper turns red with HCl gas.

Answer: Yes, water in a basic solution gives some H⁺ ions. But the concentration of hydroxide ions (OH⁻) is much higher than that of H⁺ ions, so the solution shows basic properties.

Teacher's Note:
1. Every aqueous solution has both ions.
2. The ion present in larger amount decides whether it is acidic or basic.

Answer: 1. Fix two nails on a cork and connect them through a bulb and a switch to a 6 V battery.
2. Pour dilute HCl into the beaker with the nails: the bulb glows, showing ions in the solution.
3. Repeat with glucose solution and alcohol: the bulb does not glow.
This shows that glucose and alcohol do not give H⁺ ions in solution, so they are not acids even though they contain hydrogen.

Teacher's Note:
1. Only substances that release H⁺ ions in water are acids.
2. The glowing bulb shows the presence of ions.

Answer: The salt is lead nitrate, Pb(NO₃)₂.
2Pb(NO₃)₂ → 2PbO (yellow residue) + 4NO₂ (brown fumes) + O₂

Teacher's Note:
1. Brown fumes are nitrogen dioxide.
2. Lead oxide is yellow when hot.

Answer: The salt is ferrous sulphate crystals, FeSO₄·7H₂O.
FeSO₄·7H₂O → FeSO₄ + 7H₂O (green crystals lose water and turn white)
2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (on stronger heating, with the smell of burning sulphur).

Teacher's Note:
1. Green crystals are a clue to iron(II) sulphate.
2. The suffocating gas is sulphur dioxide.

Answer: Translocation is the transport of soluble products of photosynthesis (mainly sucrose) from the leaves to other parts of the plant. It takes place through the phloem, uses energy in the form of ATP, and can occur in both upward and downward directions.

Teacher's Note:
1. Phloem carries food, while xylem carries water and minerals.
2. Unlike xylem transport, translocation needs energy.

Answer: Guard cells control the opening and closing of stomata, and so regulate gas exchange and water loss.
When water flows into the guard cells, they swell and become curved, so the stomatal pore opens. When they lose water, they become flaccid and the pore closes.

Teacher's Note:
1. Guard cells change shape as water enters or leaves.
2. A stoma stays open in light when guard cells are turgid.

Answer: Breathing alternates between inhalation and exhalation, so it is rhythmic. However, the lungs always keep a residual volume of air that cannot be expelled, so the alveoli always contain air. Oxygen therefore keeps diffusing into the blood and carbon dioxide keeps diffusing out, without a pause.

Teacher's Note:
1. Residual air in the lungs keeps gas exchange going between breaths.
2. This also stops the alveoli from collapsing.

Answer: In humans, blood passes through the heart twice in one complete circuit.
1. Pulmonary circulation: deoxygenated blood from the right ventricle goes to the lungs, takes up oxygen and returns to the left atrium.
2. Systemic circulation: from the left atrium, blood goes to the left ventricle, which pumps oxygenated blood to the body; deoxygenated blood returns to the right atrium.
This keeps oxygenated and deoxygenated blood separate and maintains high pressure and efficient supply of oxygen, which warm-blooded animals need.

Teacher's Note:
1. The right side of the heart handles deoxygenated blood and the left side handles oxygenated blood.
2. The four-chambered heart makes this separation possible.

Answer: Cuscuta (amar-bel) and the tapeworm are parasites; Plasmodium and lice are other examples.
Fungi follow saprophytic nutrition. They secrete digestive enzymes onto dead and decaying matter, break it down outside the body, and then absorb the nutrients.

Teacher's Note:
1. A parasite lives on a living host and harms it.
2. Saprophytes use dead organic matter.

Answer: 1. Kidneys (two): filter blood and form urine, removing urea and extra salts and water.
2. Ureters (two): tubes that carry urine from the kidneys to the bladder.
3. Urinary bladder: stores urine until it is passed out.
4. Urethra: the tube that carries urine out of the body.

Teacher's Note:
1. Urine travels from kidney to ureter, bladder and urethra.
2. Each kidney has about a million filtering units called nephrons.

Answer: 1. The lungs have millions of alveoli, giving a very large surface area (about 80 m²) for gas exchange.
2. The walls of alveoli are very thin (one cell thick), so gases diffuse quickly.
3. Alveoli are richly supplied with blood capillaries and kept moist, which help gases to dissolve and be carried away.

Teacher's Note:
1. Large area, thin walls and rich blood supply make exchange fast.
2. Moist surfaces let the gases dissolve.

Answer: (a) The liver releases bile. Bile makes the food entering the small intestine alkaline so that pancreatic enzymes can act, and its salts break large fat globules into small ones (emulsification).
(b) The pancreas secretes pancreatic juice (with trypsin, amylase and lipase) through a duct into the small intestine, so it is an exocrine gland. It also secretes the hormones insulin and glucagon directly into the blood, so it is an endocrine gland.

Teacher's Note:
1. Exocrine glands have ducts while endocrine glands release hormones into blood.
2. The pancreas does both jobs.

Answer: 1. Red blood cells (RBCs) carry oxygen with the help of haemoglobin.
2. White blood cells (WBCs) fight germs and give protection against infection.
3. Platelets help in clotting of blood and stop bleeding.

Teacher's Note:
1. Plasma carries the cells and dissolved substances.
2. Each cell type has its own task.

Answer: (a) Emulsification is the breaking of large fat globules into tiny droplets by bile salts. It increases the surface area for the enzyme lipase to act upon the fat.
(b) Villi are finger-like projections in the inner lining of the small intestine. They increase the surface area for absorption of digested food, and they have many blood vessels to carry the absorbed food to all parts of the body.

Teacher's Note:
1. Small droplets give more surface for enzymes.
2. Villi make absorption in the small intestine quick and efficient.