CBSE Class 7 Science Revision Worksheet Set 14

Multi Choice Questions

Question 1. The present system of representing elements using letters of the English alphabet was suggested by.
(a) Berzelius
(b) John Dalton
(c) Aristotle
Answer: (a) Berzelius
Jons Jacob Berzelius proposed using one or two letters of the English name of an element as its chemical symbol.
In simple words: Berzelius suggested using alphabet letters to write element symbols.

Exam Tip: Remember that Dalton used circular symbols with drawings, whereas Berzelius introduced the modern letter-based symbols.

Question 2. The valency of which of these elements is one
(a) Sodium
(b) Calcium
(c) Oxygen
Answer: (a) Sodium
Sodium (\(\text{Na}\)) has an atomic number of 11 with electronic configuration 2, 8, 1, so it loses 1 electron to achieve stability, giving it a valency of 1. Calcium has a valency of 2 and oxygen has a valency of 2.
In simple words: Sodium has a combining capacity (valency) of 1 because it has 1 valence electron to lose.

Exam Tip: Memorize the valencies of common elements: Sodium = 1, Calcium = 2, Oxygen = 2.

Question 3. Which one of these represents the chemical formula of Nitric Acid?
(a) \(\text{HCl}\)
(b) \(\text{H}_2\text{SO}_4\)
(c) \(\text{HNO}_3\)
Answer: (c) \(\text{HNO}_3\)
\(\text{HCl}\) is Hydrochloric acid, \(\text{H}_2\text{SO}_4\) is Sulphuric acid, and \(\text{HNO}_3\) is Nitric acid.
In simple words: Nitric acid is represented by the formula \(\text{HNO}_3\).

Exam Tip: Always double-check subscript numbers in chemical formulas, such as the 4 in \(\text{H}_2\text{SO}_4\) versus the 3 in \(\text{HNO}_3\).

Question 4. The common name of Sodium hydroxide is.
(a) Caustic Soda
(b) Slaked lime
(c) Baking Soda
Answer: (a) Caustic Soda
Sodium hydroxide (\(\text{NaOH}\)) is commonly known as caustic soda. Slaked lime is calcium hydroxide, and baking soda is sodium bicarbonate.
In simple words: Sodium hydroxide is called caustic soda because it is a strong corrosive base.

Exam Tip: Match common names with chemical names carefully as this is a frequent objective question type.

Question 5. Which Salt is used in developing films in Photography?
(a) Silver Nitrate
(b) Potassium Nitrate
(c) Copper Sulphate
Answer: (a) Silver Nitrate
Silver salts like silver nitrate and silver bromide are sensitive to light and are widely used in photographic films.
In simple words: Silver nitrate reacts with light, making it useful in photography.

Exam Tip: Remember silver compounds for photography applications.

Question 6. Which acid is used in automobile batteries?
(a) Sulphuric acid
(b) Hydrochloric acid
(c) Nitric acid
Answer: (a) Sulphuric acid
Car batteries (lead-acid storage batteries) use an aqueous solution of sulphuric acid (\(\text{H}_2\text{SO}_4\)) as the electrolyte.
In simple words: Sulphuric acid is used inside car batteries to help store and generate electrical energy.

Exam Tip: Sulphuric acid is often called the king of chemicals and is essential for industrial batteries.

Question 7. Which one of these salts is used in purification of water?
(a) Potash Alum
(b) Potassium Nitrate
(c) Sodium Chloride
Answer: (a) Potash Alum
Potash alum is added to muddy water during purification to coagulate suspended impurities and settle them down (loading process).
In simple words: Potash alum cleans muddy water by making dirt particles stick together and settle at the bottom.

Exam Tip: Alum works through coagulation and sedimentation.

Question 8. Calcium Hydroxide will turn
(a) Blue litmus red
(b) Red litmus blue
(c) None of the options
Answer: (b) Red litmus blue
Calcium hydroxide is a basic substance. Bases turn red litmus paper blue.
In simple words: Since calcium hydroxide is a base, it changes red litmus to blue.

Exam Tip: Recall the acronym BRB - Bases turn Red to Blue.

Question 9. The roof like structure formed by the branches of tall trees over other plants is called
(a) Canopy
(b) Crown
(c) Understory
Answer: (a) Canopy
The branches of tall trees look like a roof over the other plants in the forest, which is known as the canopy.
In simple words: A canopy is the umbrella-like ceiling formed by tall forest trees.

Exam Tip: Do not confuse canopy (entire overhead roof) with crown (the branchy part of an individual tree).

Question 10. Among these which one is a natural indicator?
(a) Methyl orange
(b) China rose
(c) Phenolphthalein
Answer: (b) China rose
China rose is a plant-based natural indicator, whereas methyl orange and phenolphthalein are synthetic indicators.
In simple words: China rose is obtained from nature and acts as an indicator, unlike man-made chemicals.

Exam Tip: Natural indicators include turmeric, litmus, and China rose petals.

Very Short Answer Type Questions

Question Q1. Name the only base which does not have a metal atom in its molecule.
Answer: Ammonium hydroxide (\(\text{NH}_4\text{OH}\)).
In simple words: Ammonium hydroxide contains the ammonium radical instead of a metal.

Exam Tip: Ammonium (\(\text{NH}_4^+\)) acts like a metal ion in chemical compounds despite being made of non-metals.

Question Q2. Name one decomposer that is not a microorganism.
Answer: Earthworm (or mushrooms/fungi visible to the naked eye, though earthworms are classic macro-decomposers / detritivores).
In simple words: Earthworms help break down dead organic matter without being microscopic.

Exam Tip: Remember that not all decomposers are bacteria or fungi; some are animals like earthworms and dung beetles.

Question Q3. What is the specific name given to the no of hydrogen atoms that can combine with or be displaced by one atom of an element?
Answer: Valency.
In simple words: Valency is measured by how many hydrogen atoms an element can bond with or replace.

Exam Tip: This is the classical definition of valency based on hydrogen combining capacity.

Question Q4. If a certain compound has formulae H3X, what is the valency of X?
Answer: The valency of X is 3.
In simple words: Since 3 hydrogen atoms (each with valency 1) combine with X, element X must have a valency of 3.

Exam Tip: Use the cross-valency rule to quickly deduce combining capacities from chemical formulas.

Question Q5. What are scavengers?
Answer: Animals that feed on the flesh of dead animals are called scavengers (e.g., vultures, crows, hyenas).
In simple words: Scavengers clean up nature by eating dead animals.

Exam Tip: Scavengers differ from decomposers because scavengers ingest large pieces of dead flesh rather than absorbing nutrients at a microscopic level.

Question Q6. Name an indicator which turns yellow in basic medium?
Answer: Turmeric paper (in basic medium, yellow turmeric turns reddish-brown; methyl orange turns yellow in basic/neutral medium compared to red in acidic medium). Specifically, methyl orange turns yellow in basic medium.
In simple words: Methyl orange turns yellow when added to a basic solution.

Question Q7. What are monatomic elements?
Answer: Elements whose molecules consist of only a single atom are called monatomic elements (e.g., noble gases like Helium, Neon, Argon).
In simple words: Monatomic elements exist freely as individual single atoms.

Exam Tip: Noble gases are stable and do not need to bond with other atoms, hence they are monatomic.

Question Q8. Which base is used to neutralize the acidity of soil?
Answer: Slaked lime (Calcium hydroxide) or quicklime (Calcium oxide).
In simple words: Farmers add lime to acidic soil to neutralize it and make it fit for crops.

Exam Tip: Acidic soil is treated with basic substances, while basic soil is treated with organic matter (compost).

Question Q9. Name one medicinal plant.
Answer: Neem (or Tulsi, Aloe vera, Amla).
In simple words: Plants like neem and tulsi have medicinal properties used to treat illnesses.

Exam Tip: Forests are storehouses of diverse medicinal plants.

Question Q10. Which scale is used to measure the strength of an acid or an alkali?
Answer: pH scale.
In simple words: The pH scale tells us how acidic or basic a solution is.

Exam Tip: The pH scale ranges from 0 to 14, where 7 is neutral, below 7 is acidic, and above 7 is basic.

Short Answer Questions

Question 1. Helium is the second most abundant element in the universe. However there are no known compounds of helium. Why?
Answer: Helium has a completely filled outermost shell (duet rule with 2 electrons in its only shell). Because its electronic configuration is completely stable, helium is chemically inert and does not gain, lose, or share electrons to form chemical compounds.
In simple words: Helium has a full valence shell, making it completely stable and unreactive.

Exam Tip: Inertness is always related to stable electronic configurations.

Question 2. How can we control forest fires?.
Answer: Forest fires can be controlled by:
1. Creating fire breaks (clearing strips of vegetation to stop fire spread).
2. Using water tankers, fire-retardant chemicals, and sand sprayed by helicopters or ground teams.
3. Preventing human negligence like leaving campfires unattended or throwing burning cigarette stubs.
In simple words: We can stop forest fires by cutting off fuel lines, spraying water and chemicals, and preventing carelessness.

Exam Tip: Mention both preventive measures and active firefighting techniques.

Question 3. All alkalis are bases, but all bases are not alkalis. Why?
Answer: Bases are substances that can neutralize acids, which include metal oxides and metal hydroxides. An alkali is specifically a base that is soluble in water. Therefore, all water-soluble bases are alkalis, but many bases are insoluble in water and cannot be called alkalis.
In simple words: Alkalis are just the special bases that dissolve completely in water.

Exam Tip: Use the golden rule: "All alkalis are bases, but all bases are not alkalis."

Short Answer Type Questions

Question 1. How can planned harvesting help in conserving forests?
Answer: Planned harvesting (selective felling and replanting) ensures that only mature trees are cut down while young trees are left to grow. Additionally, reforestation and afforestation replace the harvested trees, maintaining the balance of the forest ecosystem.
In simple words: Cutting trees wisely and planting new ones keeps forests alive and thriving.

Exam Tip: Emphasize sustainable forestry practices like selective logging and tree plantation.

Question 2. Is the meaning of 'weak acid' same as that of 'dilute acid'?
Answer: No, the two terms have different meanings. A weak acid is an acid that dissociates only partially in water to release hydrogen ions (chemical property). A dilute acid is one that contains a large amount of water and a small amount of acid dissolved in it (concentration property).
In simple words: Weak refers to how strongly an acid ionizes, while dilute refers to how much water has been added to it.

Exam Tip: Never confuse strength (weak/strong) with concentration (dilute/concentrated).

Long Answer Type Question

Question 1. Define food web. Draw a neat and well labelled diagram of a food web.
Answer: A food web is a complex, interconnected network of multiple food chains within an ecosystem that shows the various feeding relationships and energy flow between different organisms.
In simple words: A food web is a network of many overlapping food chains in nature.

Exam Tip: Show multiple intersecting arrows originating from producers to various primary, secondary, and tertiary consumers.