PA 1 Sample Paper Class 10 Science 2026-27 (Ch 1-3) with Answer Key

Section A

Q1 — (B) Combination. Two reactants, calcium oxide and water, combine to give only one product, calcium hydroxide, which makes this a combination reaction. It is also the reaction used to make slaked lime, and it is exothermic.

Q2 — (B) Decomposition. Here a single compound, water, breaks down into two simpler substances, hydrogen and oxygen. This is a decomposition reaction. Because electricity is used to bring it about, it is called an electrolytic decomposition.

Q3 — (C) Baking soda solution. Baking soda, or sodium hydrogencarbonate, is a mild base and its solution has a pH of about 8 to 9, which is above 7. Lemon juice and vinegar are both acidic, with a pH below 7. Distilled water is neutral, with a pH of exactly 7, so it is not greater than 7.

Q4 — (B) Sodium. Sodium reacts with cold water so vigorously that the heat released sets fire to the hydrogen gas produced:
2Na + 2H₂O → 2NaOH + H₂
Iron reacts only with steam, while copper and gold do not react with water at all.

Q5 — (D) Potassium. In the reactivity series, potassium stands highest of these four metals, followed by zinc, then iron, with copper the least reactive of them.

Section B

Q6 — Balanced equation and reaction type. 2AgNO₃ + Zn → Zn(NO₃)₂ + 2Ag
Check: Ag 2 = 2, N 2 = 2, O 6 = 6, Zn 1 = 1, so the equation is balanced.
This is a displacement reaction. Zinc displaces silver from its salt solution because zinc is more reactive than silver.

Q7 — One mark for each substance, formula and use together. Baking soda — NaHCO₃ (sodium hydrogencarbonate). Use: as an antacid, to neutralise excess acid in the stomach; it is also used in baking powder, where it releases carbon dioxide gas and makes a cake rise.

Washing soda — Na₂CO₃·10H₂O (sodium carbonate decahydrate). Use: in the glass, soap and paper industries, and for removing the permanent hardness of water.

Q8 — Balanced equation and two observations. Mg + 2HCl → MgCl₂ + H₂
Check: Mg 1 = 1, H 2 = 2, Cl 2 = 2, so the equation is balanced.

Observations: (i) There is brisk effervescence around the magnesium ribbon, with bubbles of a colourless gas. (ii) The ribbon slowly dissolves and the solution becomes warm, because the reaction is exothermic.

Note: if a burning matchstick is brought near the gas, it burns with a pop sound, which confirms that the gas is hydrogen. That is the result of a confirmatory test rather than a direct observation, so write it separately if the question asks for a test.

Q9 — Two parts, 1 mark each. The reactivity series is a list of metals arranged in decreasing order of their reactivity, running from the most reactive, potassium, down to the least reactive, gold.

Gold and silver lie at the very bottom of this series, so they are the least reactive metals. They do not react easily with oxygen, water or other substances and therefore do not readily form compounds. This is why they are found in nature in the free, or native, uncombined state.

Section C

Q10 — Equation, and classification as displacement and as redox. 2Al(s) + Fe₂O₃(s) → Al₂O₃(s) + 2Fe(l) + Heat
Check: Al 2 = 2, Fe 2 = 2, O 3 = 3, so the equation is balanced. This is called the thermite reaction and it releases a very large amount of heat.

It is a displacement reaction, because aluminium displaces iron from its oxide.
It is also a redox reaction:
— Aluminium is oxidised, because it gains oxygen (Al → Al₂O₃).
— Iron(III) oxide is reduced, because it loses oxygen (Fe₂O₃ → Fe).

Q11 — Three parts, 1 mark each. The pH scale runs from 0 to 14 and tells us how acidic or how basic a solution is. A pH below 7 means the solution is acidic, a pH of exactly 7 means it is neutral, and a pH above 7 means it is basic.

Food particles left in the mouth are broken down by bacteria, and this produces acid. When the pH in the mouth falls below about 5.5, the enamel of the teeth, which is made of calcium phosphate, begins to corrode and cavities form.

Toothpaste is basic so that it neutralises this excess acid and protects the enamel, which is why it is used for cleaning teeth.

Q12 — Electron transfer, diagram and two properties. A magnesium atom has 2 valence electrons in its outermost shell and a chlorine atom has 7. To reach a stable octet:
— One Mg atom loses its 2 electrons and becomes an Mg²⁺ ion.
— Each Cl atom gains 1 electron and becomes a Cl⁻ ion, so 2 chlorine atoms are needed to accept the 2 electrons.

These oppositely charged ions, Mg²⁺ and two Cl⁻, are held together by a strong electrostatic force of attraction, forming the ionic compound MgCl₂.

Electron-dot (Lewis) diagram: show 2 dots around Mg with arrows carrying them to the two Cl atoms, each drawn with 7 dots. After the transfer, Mg is shown as Mg²⁺ and each Cl as Cl⁻ with a completed octet of 8 dots.

Two properties of ionic compounds: (i) They have very high melting and boiling points. (ii) They conduct electricity when molten or when dissolved in water, because the ions are then free to move.

Section D

Q13 (i) — The rule of displacement. A more reactive metal displaces a less reactive metal from its salt solution, and the less reactive metal is deposited in the solid form.

Q13 (ii) — Decreasing order of reactivity. A displaced B, so A is more reactive than B. B displaced C, so B is more reactive than C. C displaced D, so C is more reactive than D. The decreasing order of reactivity is therefore A > B > C > D.

Q13 (iii) — No, it will not. From the order above, D is the least reactive of the four and A is the most reactive. A less reactive metal can never displace a more reactive metal from its salt solution. This also agrees with the observation given, in which D showed no reaction in the salt solution of A.