Half Yearly Class 7 Science Set 3 (Solutions)

Time: 3 Hours
Maximum Marks: 80
General Instructions:
(i) The question paper consists of 34 questions divided into four sections: A, B, C, and D.
(ii) All questions are compulsory.
(iii) Section A: Questions 1 to 15 are multiple-choice questions, carrying 1 mark each. Select the most appropriate response.
(iv) Section B: Questions 16 to 22 are short-answer questions, carrying 2 marks each.
(v) Section C: Questions 23 to 31 are short-answer questions, carrying 4 marks each.
(vi) Section D: Questions 32 to 34 are short-answer questions, carrying 5 marks each.
(vii) Answer the questions in the space provided or on a separate answer sheet as instructed.
(viii) Use of calculators is not permitted.

​Syllabus: The half-yearly question paper is framed from the following chapters : The Ever-Evolving World of Science, Exploring Substances: Acidic, Basic, and Neutral, Electricity: Circuits and their Components, The World of Metals and Non-metals,  Changes Around Us: Physical and Chemical, Adolescence: A Stage of Growth and Change.

Section A : Multiple Choice Questions

Q1. Which natural indicator is suitable for testing basic substances but cannot differentiate between acidic and neutral substances?
(a) China rose extract
(b) Turmeric
(c) Litmus
(d) Methyl orange

Ans: Turmeric.
The yellow color of turmeric turns red only in basic solutions but remains unchanged in acidic and neutral solutions.

Q2. Changes in adolescence are primarily controlled by certain chemicals produced in the body called:
(a) Enzymes
(b) Vitamins
(c) Hormones
(d) Proteins

Ans: Hormones.
Changes that take place in the body during adolescence are controlled by hormones produced within the body.

Q3. The property of materials that allows them to be drawn into thin wires is called:
(a) Malleability
(b) Ductility
(c) Sonority
(d) Conductivity

Ans: Ductility.
This property allows materials to be stretched into wires; most metals possess ductility.

Q4. To treat soil that is too acidic for plant growth, a farmer should add:
(a) Vinegar
(b) Lime (a base)
(c) Salt solution
(d) Carbon dioxide

Ans: Lime (a base).
Lime (calcium hydroxide) neutralizes acidic soil and helps maintain soil balance for plant growth.

Q5. The terminal of an electric cell represented by the small protruding metal cap is the:
(a) Negative terminal
(b) Positive terminal
(c) Neutral terminal
(d) Earthing point

Ans: Positive terminal.
The small protruding metal cap on a cell is its positive terminal.

Q6. Which is an essential requirement for the process of combustion, besides fuel and oxygen?
(a) Water
(b) Heat (to reach ignition temperature)
(c) Carbon dioxide
(d) Nitrogen

Ans: Heat (to reach ignition temperature).
Combustion requires a combustible substance, oxygen, and sufficient heat.

Q7. When red rose extract is added to a substance, the extract turns green. The substance is:
(a) Acidic
(b) Basic
(c) Neutral
(d) Salty

Ans: Basic.
Red rose extract changes to green in the presence of a base.

Q8. Which metal reacts vigorously with oxygen and water, requiring it to be stored in kerosene?
(a) Copper
(b) Sodium
(c) Iron
(d) Zinc

Ans: Sodium.
Sodium reacts violently with water and air, so it is stored under kerosene.

Q9. Why must experiments with electricity never use household wall sockets?
(a) They give weak current
(b) They supply low voltage
(c) Electricity from wall sockets can be dangerous
(d) They cannot connect to circuits

Ans: Electricity from wall sockets can be dangerous.
Household electricity has a high voltage that can cause electric shock, so experiments must use only cells or batteries.

Q10. Which change is generally considered a reversible physical change?
(a) Burning of paper
(b) Rusting of iron
(c) Melting of ice cubes
(d) Cooking of rice

Ans: Melting of ice cubes.
The melting of ice is reversible, as water can refreeze into ice.

Q11. The growth of the voice box in adolescent boys is seen as a bump in the throat region called the:
(a) Thyroid gland
(b) Adam's apple
(c) Larynx
(d) Windpipe

Ans: Adam's apple.
The enlarged voice box in boys is visible as the Adam's apple.

Q12. Wires are covered with plastic or rubber primarily because these materials are:
(a) Conductors of heat
(b) Conductors of electricity
(c) Electrical insulators
(d) Magnetic materials

Ans: Electrical insulators.
Plastic and rubber prevent electric shocks because they are poor conductors.

Q13. The main product formed when an acid is mixed with a base is:
(a) Only water
(b) Only salt
(c) Salt and water
(d) Carbon dioxide and water

Ans: Salt and water.
Acid and base react in a neutralization reaction forming salt and water.

Q14. Materials that are generally dull, brittle, and poor conductors are classified as:
(a) Metals
(b) Non-metals
(c) Alloys
(d) Minerals

Ans: Non-metals.
Non-metals lack metallic properties like lustre, malleability, and conductivity.

Q15. Erosion caused by flowing water and wind is classified as what kind of change?
(a) Chemical change
(b) Physical change
(c) Irreversible change
(d) Permanent change

Ans: Physical change.
Erosion is a physical process involving movement of soil and rock particles.

Ans: Secondary sexual characteristics : These are characteristic changes that help distinguish males from females but are not directly involved in the process of reproduction.
 Importance: They are natural signs that the body is preparing for adulthood and they mark the onset of puberty.

Ans: Silica gel is placed in glass bottle 'A' to make the air dry. The purpose is to test the condition where the iron nail comes into contact with dry air alone (without moisture).

Ans: The burning of wax vapor is a chemical change because the wax vapor reacts with oxygen (combustion) to produce a flame, resulting in the formation of new substances such as carbon dioxide and water vapor.

Ans: Non-metals like phosphorus are stored in water because phosphorus catches fire when exposed to atmospheric air. Storing it in water prevents its contact with air.

Ans: Physical weathering : This involves large rocks breaking up into smaller pieces due to physical forces like temperature changes, growing tree roots, or water freezing within cracks, without changing the chemical composition. 
Chemical weathering : This occurs when water or chemicals present in water react with the rocks, causing a change in their chemical composition (e.g., iron content changing to iron oxide).

Ans: Silver and gold are the best electrical conductors, but for making electrical wires, copper is mainly used due to its comparatively lower cost and abundant supply.

Q22. Which substances are classified into Group C when testing with litmus paper, and what is their nature? (2 marks) 

Ans: Substances in Group C: These are samples (like tap water, sugar solution, and salt solution) that do not affect either the blue or red litmus papers. 
Nature: These substances are said to be neutral because they are neither acidic nor basic.

Section C: Short Answer Questions 

Q23. Compare the reaction of magnesium oxide solution and sulfur dioxide solution on litmus paper, explaining the nature of the products formed. (4 marks) 

Ans:

1. Magnesium Oxide (Metal Oxide): Magnesium reacts with oxygen to form magnesium oxide. When tested with litmus, the solution of magnesium oxide changes the color of red litmus paper to blue. This indicates that oxides of metals are generally basic in nature.

2. Sulfur Dioxide (Non-metal Oxide): Sulfur reacts with oxygen to form sulfur dioxide gas. When sulfur dioxide is dissolved in water, sulfurous acid is formed. The solution is acidic in nature and changes the color of blue litmus paper to red.

Q24. Explain the concept of ignition temperature using the example of paper and a lighted matchstick. (4 marks) 

Ans:

1. ​Ignition temperature is the minimum temperature at which a combustible substance catches fire and begins to burn.

2. Paper and Matchstick: Paper is a combustible substance, but when kept in air, it does not spontaneously catch fire because the air temperature is below its ignition temperature. When a lighted matchstick (which is already hotter than the ignition temperature of the paper) is brought to the paper, the paper quickly reaches its ignition temperature and catches fire almost immediately.

Q25. Explain why wires are covered with plastic or rubber, and why wood is used for the handle of a screwdriver. (4 marks) 

Ans:

1. Wire Covering: Electric wires are covered with plastic or rubber because these materials are electrical insulators. Insulators are poor conductors through which current cannot pass easily. The covering protects people from electric shocks by preventing direct contact with the metal conductor inside.

2. Screwdriver Handles: The handles of screwdrivers are made with wood, plastic, or rubber because these are insulators. The use of insulating materials on the handle protects the electrician from electric shock while working on electrical circuits.

Q26. Explain why consuming a nutritious diet and maintaining good personal hygiene are crucially important during adolescence. (4 marks) 

Ans:

1. Nutritious Diet: Adolescence is a period of rapid growth and development. A nutritious diet is of utmost importance because it provides essential nutrients (like proteins, carbohydrates, fats, vitamins, and minerals) needed for proper growth, optimal bone growth, gaining strength, and improving energy levels.

2. Personal Hygiene: During adolescence, oily secretions and sweat increase. Maintaining body hygiene, particularly in the armpits and the pubic region, is crucial to protect the individual from potential infections.

 Ans:

1. Initial State: When a drop of blue litmus solution is added to the lemon juice solution, the colour of the solution initially turns red because lemon juice is acidic.

2. Neutralization: As lime water (a base) is slowly added, a neutralization reaction occurs, where the acid and base mix. The solution eventually becomes neither acidic nor basic (neutralized).

3. Final Change: Since lime water is added slowly, the color of the solution eventually changes from red to blue, showing that the solution is no longer acidic and the effect of the acid has been neutralised or surpassed by the base.

Ans:

1. Dissolving Sugar (Physical Change): When sugar dissolves in water, only the physical state (solid to dissolved) changes, and no new substance is formed. The original substance (sugar) can be recovered by evaporating the water, showing the change is reversible.

2. Making Popcorn (Chemical Change): Making popcorn from corn kernels involves a process where one or more new substances are formed. This change is irreversible because the original corn kernel cannot be retrieved from the popcorn, indicating a chemical change has occurred.

Ans:

1. Current Flow : The setup forms an electrical circuit, which provides a complete path for electric current to flow through the lamp. When current passes through the circuit, it flows through the filament of the lamp, causing the filament to get hot and glow to produce light.

2. Direction: The direction of electric current in a closed electrical circuit is conventionally taken to be from the positive to the negative terminal of the electric cell.

 Ans:

1. Corrosion Definition: Corrosion is the gradual deterioration of metal surfaces caused by exposure to air, water, or other substances in the atmosphere.

2. Problem in India: The rusting of iron (a form of corrosion) is a serious problem in India because every year, an enormous amount of money is spent to replace or repair iron structures damaged due to rusting.

3. Prevention Method: Rusting can be prevented by several methods, such as painting, oiling, greasing, or applying a protective layer of zinc metal (galvanisation) on iron.

Ans:

1. Three Uses of electricity include: Communication (Mobile phone, Internet), Transportation (Train, bus, car, lift), and Others (Water pump, crane, computer).

2. Safety Justification : The danger signs on electric poles and appliances warn that electricity can be dangerous if not carefully handled. Therefore, one must never perform experiments with the power supply at home or school, or even with portable generators; only batteries or cells should be used.

Section D: Long Answer Questions 

Ans: The experiment uses three setups to isolate the conditions required for the formation of brown deposits (rust) on iron nails.

1. Glass Bottle A: Contains an iron nail exposed only to dry air (moisture removed by silica gel). Observation: The iron nail does not show any brown deposits. This indicates that dry air alone is not sufficient for rusting.

2. Glass Bottle B: Contains an iron nail exposed only to water alone (dissolved air removed by boiling, oil layer prevents atmospheric air contact). Observation: The iron nail does not show any brown deposits. This indicates that water alone is not sufficient for rusting.

3. Glass Bottle C : Contains an iron nail partially dipped in unboiled water, exposed to both air and water. Observation: The iron nail shows the presence of brown deposits (rust). Conclusion: The experiment confirms that the presence of both water and air (moist air) is essential for the process of rusting to occur.

Ans:

1. Strong Emotions and Mood Swings:

    ◦ Experience: Adolescence is often marked by stronger emotions than childhood, leading to rapid mood swings. An adolescent may feel very energetic and happy on some days, while feeling low on others.

    ◦ Coping/Growth: To manage these fluctuating moods, adolescents can be guided to engage in varied activities like music, dance, or sports.

2. Increased Sensitivity and Self-Exploration :

    ◦ Experience: Adolescents experience increased sensitivity and may show a deeper interest in new areas. These emotions might affect their behavior, leading to self-exploration.

    ◦ Coping/Growth: They should be encouraged in self-exploration and involvement in activities that may lead to creativity and innovation, helping them make better choices and respond to situations thoughtfully.

Ans:

1. Reaction and New Substance Formation : When vinegar is mixed with a pinch of baking soda (sodium hydrogen carbonate), a chemical change occurs. An audible fizzing bubbling sound is heard, and gas bubbles are seen forming. The reaction produces carbon dioxide gas and other substances.

2. Testing the Resulting Gas : To test the identity of the gas formed:

    ◦ The gas is passed through freshly prepared lime water (calcium hydroxide solution) kept in another test tube.

    ◦ The lime water turns milky.

    ◦ This turning of lime water milky confirms that the gas formed by mixing vinegar and baking soda is carbon dioxide.

    ◦ Word Equation: Vinegar + Baking soda → Carbon dioxide + Other substances.