Worksheet Solutions: Nature of Matter: Elements, Compounds, and Mixtures | Science Curiosity Class 8 - New NCERT PDF Download

Multiple Choice Questions

Q1. Which of the following is a uniform mixture?
(a) Sprout salad
(b) Sand and water
(c) Stainless steel
(d) Poha
Ans: (c) Stainless steel
Explanation: Stainless steel is a uniform mixture because its components (iron, nickel, chromium, carbon) are evenly mixed and cannot be seen separately, even with a microscope. Sprout salad, sand and water, and poha are non-uniform mixtures as their components are visible.

Q2. What happens when lime water is exposed to air?
(a) It turns green
(b) It becomes milky
(c) It evaporates
(d) It remains unchanged
Ans: (b) It becomes milky
Explanation: Lime water turns milky when exposed to air because carbon dioxide in the air reacts with calcium hydroxide to form calcium carbonate, an insoluble white substance, as shown in the lime water test.

Q3. Which substance is an element?
(a) Water
(b) Sodium chloride
(c) Sulfur
(d) Iron sulfide
Ans: (c) Sulfur
Explanation: Sulfur is an element because it is made of identical atoms and cannot be broken down into simpler substances. Water, sodium chloride, and iron sulfide are compounds made of multiple elements.

Q4. Why can’t water be separated into hydrogen and oxygen by physical methods?
(a) It is a uniform mixture
(b) It is a compound
(c) It is an element
(d) It is a non-uniform mixture
Ans: (b) It is a compound
Explanation: Water is a compound where hydrogen and oxygen are chemically bonded in a 2:1 ratio, so physical methods like filtration cannot separate them. Only chemical processes, like electrolysis, can break it down.

Q5. Which mixture can be separated using a magnet?
(a) Sugar and water
(b) Iron filings and sulfur
(c) Air
(d) Seawater
Ans: (b) Iron filings and sulfur
Explanation: Iron filings and sulfur can be separated using a magnet because iron is magnetic and sulfur is not, allowing iron to be attracted while sulfur remains, as demonstrated in the iron-sulfur mixture activity.

Fill in the Blanks

Q1. A _________ consists of two or more substances that retain their individual properties.
Ans: Mixture
Explanation: A mixture is formed when two or more substances are combined, and each keeps its own properties, like in poha or sprout salad.

Q2. Air is a _________ mixture of gases like nitrogen and oxygen.
Ans: Uniform
Explanation: Air is a uniform mixture because its gases are evenly distributed and cannot be seen separately, even with a microscope.

Q3. A pure substance made of one type of atom is called an _________.
Ans: Element
Explanation: Elements, like gold or sulfur, consist of identical atoms and cannot be broken down further.

Q4. Water is a _________ made of hydrogen and oxygen in a fixed ratio.
Ans: Compound
Explanation: Water is a compound because hydrogen and oxygen are chemically combined in a 2:1 ratio, with properties different from its elements.

Q5. _________ in air reacts with lime water to form calcium carbonate.
Ans: Carbon dioxide
Explanation: Carbon dioxide in air causes lime water to turn milky by forming insoluble calcium carbonate.

Q6. Bronze is an _________ of copper and tin.
Ans: Alloy
Explanation: Bronze is an alloy, a uniform mixture of copper and tin, with enhanced properties like strength.

Q7. _________ particles in air are considered pollutants, not part of its natural composition.
Ans: Dust
Explanation: Dust particles, visible in a beam of sunlight, are pollutants in air, varying by place and time.

Q8. Heating sugar produces _________ and water droplets.
Ans: Carbon
Explanation: When sugar is heated, it decomposes into black carbon (charcoal) and water droplets, showing it is a compound.

Q9. Iron sulfide is a _________ formed by heating iron and sulfur.
Ans: Compound
Explanation: Iron sulfide is a compound created when iron and sulfur chemically combine, forming a new substance with different properties.

Q10. _________ is used to make buildings and vehicles due to its strength and light weight.
Ans: Aluminium
Explanation: Aluminium, an element, is used in construction and vehicles for its strength and low weight.

Very Short Answer Questions

Q1. Why is poha a non-uniform mixture?
Ans: Poha is a non-uniform mixture because its components, like flattened rice, peanuts, and spices, are visible and not evenly mixed. Each ingredient retains its own properties.

Q2. What gases are produced when electricity is passed through water?
Ans: Hydrogen and oxygen gases are produced when electricity is passed through water, in a 2:1 ratio, showing water is a compound.

Q3. How is a compound different from its elements?
Ans: A compound, like water, has different properties from its elements (hydrogen and oxygen) because the elements are chemically combined in a fixed ratio.

Q4. Name one use of the compound sodium chloride in daily life.
Ans: Sodium chloride, or common salt, is used to enhance the taste of food.

Q5. Why are native minerals considered elements?
Ans: Native minerals, like gold or sulfur, are considered elements because they consist of only one type of atom and are not compounds.

Short Answer Type Questions

Q1. How can you confirm the presence of carbon dioxide in air?
Ans: To confirm carbon dioxide in air, expose lime water (calcium hydroxide solution) to air. It turns milky due to the formation of calcium carbonate, an insoluble white substance, when carbon dioxide reacts with it. This reaction shows carbon dioxide is present in air.

Q2. Why is stainless steel stronger than pure iron?
Ans: Stainless steel is a uniform mixture of iron, nickel, chromium, and carbon, blended evenly to enhance strength and durability. Unlike pure iron, which is softer, the alloy’s components work together to create a stronger material. This makes it ideal for utensils and construction.

Q3. What happens when iron and sulfur are heated together, and how does it differ from their mixture?
Ans: When iron and sulfur are heated, they form iron sulfide, a black compound with new properties, like not being magnetic. In their mixture, iron is magnetic and sulfur is yellow, and they can be separated physically. The compound cannot be separated by physical methods, showing a chemical change.

Q4. Why is sugar considered a compound and not an element?
Ans: Sugar is a compound because it is made of carbon, hydrogen, and oxygen chemically combined, as shown when it decomposes into carbon and water upon heating. An element, like carbon, has only one type of atom and cannot be broken down. Sugar’s multiple elements and distinct properties confirm it is a compound.

Q5. How are elements like aluminium and compounds like water used in daily life?
Ans: Aluminium, an element, is used to make vehicles and buildings due to its strength and light weight. Water, a compound, is essential for drinking, cooking, and cleaning. Both are vital because of their unique properties.