Model Test – Solved ഉത്തരങ്ങൾ

When milk sets into curd a new substance with a new taste and smell forms – a chemical change (രാസമാറ്റം). Melting, dissolving and boiling change only the state or the form; no new substance forms – physical changes (ഭൗതികമാറ്റം).

The blue solid, copper hydroxide Cu(OH)₂, does not dissolve in water and sinks – it is a precipitate. The ↓ after its formula shows this. Forming a precipitate is one of the signs of a chemical change.

Carbon and oxygen join to give one product – combination. In 2HgO → 2Hg + O₂ one compound, mercury(II) oxide, breaks into two substances, mercury and oxygen – decomposition (here on heating, a thermal decomposition).

Magnesium, an element, takes the place of hydrogen in hydrochloric acid and hydrogen is set free – pattern A + BC → AC + B: a displacement reaction (ആദേശ രാസപ്രവർത്തനം).

Only collisions between particles that have at least the threshold energy give products – these are the effective collisions.

Adding water makes the acid dilute (നേർപ്പിച്ച): fewer acid particles per unit volume → fewer collisions → the reaction becomes slower. Warming, powdering and a higher concentration all increase the rate.

An explosion (സ്ഫോടനം) is over in a fraction of a second. Ripening takes days, rusting (തുരുമ്പിക്കൽ) days to months and weathering of rocks many years.

Amylase – an enzyme (എൻസൈമുകൾ), that is, a biological catalyst. That is why chewed rice begins to taste sweet.

(a) Two substances, calcium oxide and water, join to form one new compound, calcium hydroxide – a combination reaction (സംയോജന രാസപ്രവർത്തനം). Pattern A + B → AB.

(b) The mixture becomes hot: heat is given out (an energy change). A reaction that gives out heat is an exothermic reaction (താപമോചക പ്രവർത്തനം).

(a) Two compounds react to form two new compounds – a double decomposition reaction (ദ്വിവിഘടന രാസപ്രവർത്തനം) of the precipitation kind (അവക്ഷിപ്തരൂപീകരണ രാസപ്രവർത്തനങ്ങൾ): a white precipitate of barium sulphate, BaSO₄, forms.

(b) Before: Ba²⁺ with Cl⁻, and Na⁺ with SO₄²⁻. After: Ba²⁺ with SO₄²⁻, and Na⁺ with Cl⁻. The barium ion and the sodium ions exchange partners (the chloride and sulphate ions change places) – AB + CD → AD + CB.

(a) Collision theory (കൊളിഷൻ സിദ്ധാന്തം): a reaction takes place when reactant particles collide; only the collisions in which the particles have at least the threshold energy form products – these are effective collisions (ഫലവത്തായ കൂട്ടിമുട്ടലുകൾ).

(b) At room temperature almost none of the molecules have the threshold energy, so their collisions are not effective. The spark gives some molecules enough energy; their effective collisions start the reaction (the energy it releases then keeps it going – very fast, an explosion).

(a) Temperature (താപനില).

(b) Spoiling is a set of chemical reactions. At a low temperature the particles have less kinetic energy (ഗതികോർജം), so fewer particles reach the threshold energy → fewer effective collisions → the reactions of spoiling become slower. (A refrigerator works in the same way.)

(a) The ash (its metal compounds) increases the rate of the burning of sugar – it acts as a catalyst (ഉൽപ്രേരകം). (Burning of sugar: C₁₂H₂₂O₁₁ + 12O₂ → 12CO₂ + 11H₂O.)

(b) The ash remains after the reaction – it has not undergone a permanent chemical change. This is the feature of a catalyst: it increases the rate without being used up.

(a) Colour change (നിറവ്യത്യാസം) – the blue colour fades and red-brown copper appears. Zinc takes the place of copper in copper sulphate: displacement.

(b) Heat is given out – the tube becomes warm. Two compounds exchange ions and water forms (a compound that does not split into ions): double decomposition – a neutralisation (നിർവീരീകരണം).

(c) A gas is given off (വാതകം) – effervescence of CO₂. Two compounds exchange ions: double decomposition of the gas-forming kind (വാതകരൂപീകരണ രാസപ്രവർത്തനങ്ങൾ).

(a) Al: 2 on the right → 2Al; Fe: 2 on the left → 2Fe; O 3 = 3: 2Al + Fe₂O₃ → Al₂O₃ + 2Fe – aluminium takes the place of iron: displacement (തെർമൈറ്റ്).

(b) Cl: 2 on the left, 3 on the right – the smallest common number is 6: 3Cl₂ and 2FeCl₃, then 2Fe: 2Fe + 3Cl₂ → 2FeCl₃ – two substances give one product: combination.

(c) Na: 2 on the right → 2NaHCO₃; then H 2 = 2, C 2 = 2, O 6 = 6: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑ – one compound breaks into three: decomposition (thermal). The CO₂ makes the cake rise.

(a) 1st minute: 36 − 0 = 36 mL. 4th minute: 86 − 76 = 10 mL. The reaction is fastest in the 1st minute – the steepest part of the graph. At the start the reactants are present in the largest amount (highest concentration), so there are the most effective collisions every second; as they are used up the rate falls.

(b) The marble (calcium carbonate) is completely used up (the acid is in excess), so the reaction stops and no more CO₂ forms – the graph becomes flat at 90 mL.

(c) The curve is steeper at the start and becomes flat sooner, but the final volume is the same (90 mL) because the mass of calcium carbonate is the same. Reason: the powder has a much larger surface area (പ്രതലപരപ്പളവ്) → more collisions with acid particles every second → a faster rate.

(a) Surface area (പ്രതലപരപ്പളവ്): the powder has a far larger surface → many more collisions with oxygen at the same time → it burns very fast with a brilliant flash.

(b) Concentration (ഗാഢത): pure oxygen has about five times as many oxygen molecules per unit volume as air (about 21 % oxygen) → more effective collisions → the stick burns brightly.

(c) Nature of reactants (അഭികാരകങ്ങളുടെ സ്വഭാവം): potassium gives away its single outermost electron very easily, so it is extremely reactive; copper is much less reactive and does not react with water.

(a) Two substances, sulphur dioxide and oxygen, join to form one product, sulphur trioxide – a combination reaction.

(b) V₂O₅ is a catalyst (ഉൽപ്രേരകം) – it increases the rate. At the end its amount is the same and it is chemically unchanged: a catalyst does not undergo a permanent chemical change.

(c) ⇌ shows a reversible reaction (ഉഭയദിശാ പ്രവർത്തനങ്ങൾ): SO₂ and O₂ form SO₃, and at the same time some SO₃ breaks back into SO₂ and O₂.

(d) Pressure pushes more gas molecules into each unit volume (യൂണിറ്റ് വ്യാപ്തം) – a higher concentration → more collisions → more effective collisions. High temperature gives the molecules more kinetic energy → more of them reach the threshold energy → more effective collisions. Both make the reaction faster.

(e) Haber process (ഹേബർ പ്രക്രിയ): N₂(g) + 3H₂(g) Fe⇌ 2NH₃(g) (200 atm, 700 K); the catalyst is iron (Fe). Ammonia made this way is used to make fertilisers in factories such as FACT at Udyogamandal.

(a) Any two: heat and light are given out; smoke and gases are given off; the wood turns into black charcoal and grey ash – new substances that cannot be turned back into wood.

(b) Temperature: in April the particles have more kinetic energy, so more of them reach the threshold energy and the enzyme-catalysed reactions of fermentation go faster. In July she should keep the covered batter in a warm place in the kitchen (for example near the hearth after cooking), not in the refrigerator.

(c) Enzymes (എൻസൈമുകൾ) are biological catalysts – substances made by living things that speed up reactions in them. Example: amylase in saliva speeds up the change of starch into maltose.

(d) Surface area (പ്രതലപരപ്പളവ്): splitting the log exposes much more wood surface to the air, so many more oxygen molecules collide with the wood at the same time → more effective collisions → it catches fire and burns faster.

(e) Concentration (ഗാഢത) of oxygen: blowing pushes more air, so more oxygen molecules reach the burning wood every second → more effective collisions → brighter, faster burning.