Case Based Questions: Nature of Matter: Elements, Compounds, and Mixtures | Science Curiosity Class 8 - New NCERT PDF Download

Case Study 1

Rahul was helping his mother in the kitchen. She mixed flour, water, and salt to make dough for chapatis. Rahul noticed that the dough was uniform and tasted salty throughout. Later, he saw his mother adding sugar to tea, which dissolved completely, making the tea sweet. Rahul wondered why the dough and tea were uniform mixtures, and how they differed from a salad where ingredients like tomatoes and onions remained separate. He also thought about whether air, which he couldn't see, was a mixture too.

Questions

1. What type of mixture is the dough made from flour, water, and salt? (2 marks)
Solution:
The dough is a uniform mixture because the components (flour, water, salt) are evenly distributed, and individual particles cannot be seen separately. It is non-uniform at first but becomes uniform after kneading.

2. Why did the sugar dissolve in tea, making it a uniform mixture? (2 marks)
Solution:
Sugar dissolves in tea, forming a uniform mixture where sugar particles spread evenly among tea particles. The components retain their properties, like sweetness, but can't be seen separately.

3. How does a salad differ from the dough or sugared tea in terms of mixture type, and why? (3 marks)
Solution:
A salad is a non-uniform mixture because its components, like tomatoes and onions, are visible and not evenly distributed. Unlike uniform mixtures like dough or sugared tea, where particles mix completely, salad components don't dissolve or blend uniformly. This is because salad ingredients are solids with different properties that don't interact chemically or dissolve.

4. Explain why air is considered a mixture and not a compound. (2 marks)
Solution:
Air is a uniform mixture of gases like nitrogen, oxygen, and carbon dioxide, where components retain their properties and can be separated physically. It isn't a compound because its gases aren't chemically combined in fixed ratios, and air's composition varies slightly by location.

5. How is the mixture in Rahul's tea similar to seawater? (2 marks)
Solution:
Both are uniform mixtures; sugared tea has dissolved sugar in water, like seawater has dissolved salts. Components are evenly distributed and retain properties, such as sweetness or saltiness.

Case Study 2

Ananya conducted an experiment in class where she passed electricity through water, collecting gases in test tubes. One gas made a pop sound with a flame, and the other made it burn brighter. She realized water broke into hydrogen and oxygen. At home, she heated sugar and saw it char, leaving black residue and water droplets. Ananya wondered why water and sugar behave this way, and how they differ from elements like iron.

Questions

1. What do the gases collected from water indicate about its nature? (2 marks)
Solution:
The gases are hydrogen (pop sound) and oxygen (brighter flame), showing water is a compound of these elements combined chemically in a fixed ratio.

2. Why did heating sugar produce charcoal and water? (2 marks)
Solution:
Sugar is a compound of carbon, hydrogen, and oxygen. Heating decomposes it into carbon (charcoal) and water, showing its elements separate chemically.

3. How does water as a compound differ from a mixture like air? (3 marks)
Solution:
Water is a compound where hydrogen and oxygen combine chemically in a 2:1 ratio, with properties different from its elements (e.g., extinguishes fire). Air is a mixture where gases like nitrogen and oxygen are physically mixed, retaining properties and separable physically. Compounds have fixed composition; mixtures don't.

4. Explain why iron is an element and not a compound like sugar. (3 marks)
Solution:
Iron is an element made of identical iron atoms, unbreakable into simpler substances by chemical means. Sugar is a compound of carbon, hydrogen, and oxygen, decomposable into these elements. Elements like iron are building blocks; compounds form from combined elements.

5. How are compounds like water essential in daily life? (2 marks)
Solution:
Water, a compound, is vital for drinking, cleaning, and life processes. It supports chemical reactions in bodies and environments, unlike elements alone.

Case Study 3

Vikram visited a mine and saw rocks containing shiny minerals like quartz and mica. His guide explained that some minerals are pure elements like gold, but most are compounds. At school, he learned alloys like brass are mixtures of metals. Vikram wondered why minerals are important, and how elements and compounds differ in uses.

Questions

1. What makes quartz a mineral and a compound? (2 marks)
Solution:
Quartz is a mineral, a natural solid with fixed composition (silicon and oxygen). It's a compound as these elements combine chemically.

2. Why are alloys like brass considered mixtures? (2 marks)
Solution:
Brass is a uniform mixture of copper and zinc, where metals mix physically, retaining properties, and can be separated.

3. How do minerals contribute to everyday items, and why are they compounds mostly? (3 marks)
Solution:
Minerals like calcite make cement, and talc makes powder. Most are compounds (e.g., quartz: silicon dioxide) as elements combine naturally in Earth's crust. They provide materials for buildings, electronics, and health, extracted as pure elements or used as compounds.

4. Explain the difference between native minerals like gold and compound minerals like mica. (3 marks)
Solution:
Native minerals like gold are pure elements, existing naturally without combining. Mica is a compound of elements like silicon, oxygen, and aluminum, combined chemically. Native minerals are rare metals/non-metals; compounds form most minerals with varied properties.

5. Why is understanding minerals key to innovation? (2 marks)
Solution:
Minerals provide elements/compounds for technologies like phones (lithium, cobalt) and medicines, driving advancements in materials and sustainability.

Case Study 4

Priya mixed iron filings with sulfur and heated them, forming a black mass that didn't attract to a magnet. She compared it to mixing sand with water, where sand settled. In class, she learned stainless steel is an alloy mixture. Priya wondered why heating iron and sulfur created a new substance, unlike sand in water.

Questions

1. What happened when Priya heated iron and sulfur? (2 marks)
Solution:
Iron and sulfur reacted chemically to form iron sulfide, a compound with new properties, not attracting to magnets like iron.

2. Why is sand in water a mixture but iron sulfide a compound? (2 marks)
Solution:
Sand and water mix physically, separable, retaining properties. Iron sulfide forms chemically, with fixed ratio and different properties from elements.

3. How do alloys like stainless steel show mixture properties? (3 marks)
Solution:
Stainless steel is a uniform mixture of iron, chromium, and nickel, mixed physically for enhanced strength/corrosion resistance. Components retain some properties but blend uniformly, separable by methods like melting, unlike compounds with fixed composition.

4. Explain why compounds have different properties from their elements. (3 marks)
Solution:
Compounds form when elements combine chemically, creating new particles with unique structures. E.g., sodium (reactive metal) and chlorine (toxic gas) form sodium chloride (table salt), safe and different, due to chemical bonding altering behavior.

5. How are mixtures like air useful in life? (2 marks)
Solution:
Air, a gas mixture, provides oxygen for breathing, nitrogen for plants, and carbon dioxide for photosynthesis, supporting life without fixed ratios.