CBSE Class 8 Science Measurement Density and Floatation Worksheet Set 05

Answer: (a) Eureka can: fill the can with water up to the spout, place a measuring cylinder under the spout, and lower the object (tied with a thread) gently. The water that overflows equals the volume of the object. Find the mass with a balance and use density = mass / volume. (b) Measuring cylinder: pour water into the cylinder and note the initial volume, lower the object in, and note the final volume. Volume of the object = final volume minus initial volume. Then density = mass / volume.

Teacher's Note: Use the key term in your answer.

Answer: An object floats when the weight of the liquid it displaces is equal to its own weight. A nail is solid and dense, so it displaces little water and sinks. A ship is hollow and spread out, so it displaces a large amount of water whose weight equals the ship's weight, so it floats.

Teacher's Note: Give an example if the question allows.

Answer: Ice is less dense than sea water (about 920 kg per cubic metre against 1,025), so an iceberg floats with about nine-tenths of it below the surface. Densities differ because the atoms are packed tightly in solids, loosely in liquids and far apart in gases. An object with density less than the liquid floats, and one with greater density sinks.

Teacher's Note: Write a short, clear answer.

Answer: When heated, the particles move apart, the volume increases and the density falls. When cooled, the particles come closer and the density rises. The change is large in gases, smaller in liquids and very small in solids.

Teacher's Note: Remember this for your exam.

Answer: Ocean currents form because water warmed by the Sun becomes less dense and rises, while cold, dense water sinks and takes its place. In the atmosphere, warm air near the ground becomes lighter and rises, and cooler, denser air moves in, causing winds and convection currents.

Teacher's Note: Learn this point and write it in your own words.

Answer: Density: mass per unit volume of a substance (unit kg per cubic metre). Buoyancy: the upward force a liquid exerts on an object placed in it.

Teacher's Note: Use the key term in your answer.

Answer: (i) (d) all of the above; (ii) (b) subtract initial volume from final volume; (iii) (c) both a and b; (iv) (b) decreases; (v) (c) floating or sinking of an object in water; (vi) (d) lead; (vii) (a) solid (its density changes very little); (viii) (c) low density; (ix) (b) high speed; (x) (a) convection current.

Teacher's Note: Give an example if the question allows.

Answer: 1. density; 2. sink; 3. density; 4. solid; 5. freezes (turns into a solid); 6. Dense; 7. Convection currents; 8. temperature; 9. water; 10. volume.

Teacher's Note: Write a short, clear answer.

Answer: 1. True. 2. False: solids are generally denser than liquids. 3. False: atoms in a solid are tightly packed. 4. False: density decreases. 5. False: cubes of different materials have different masses. 6. True. 7. False: the volume can be found by displacement. 8. True. 9. False: there should be no spillage in a graduated cylinder. 10. True.

Teacher's Note: Remember this for your exam.

Answer:

Teacher's Note: Learn this point and write it in your own words.

Answer: Fill the eureka can with water until it flows from the spout. Place a measuring cylinder under the spout. Tie the stone with a thread and lower it slowly into the can. The water that comes out of the spout is collected in the cylinder, and its volume equals the volume of the stone.

Teacher's Note: Use the key term in your answer.

Answer: Solids have the highest density because the particles are tightly packed. Liquids are a little less dense as the particles are close but can move. Gases have very low density because the particles are far apart and move freely. Water is an exception: ice is less dense than liquid water.

Teacher's Note: Give an example if the question allows.

Answer: Ocean current: a steady flow of sea water caused by differences in temperature and density and by the heat of the Sun. Convection current: the movement of a fluid (a liquid or gas) in which hot parts rise and cold parts sink, because of changes in density.

Teacher's Note: Write a short, clear answer.

Answer: Heavy objects such as ships float when they are shaped to displace a large volume of water, so the upward buoyant force equals their weight and their average density is less than that of water. Floating or sinking depends on the density of the object, the density of the liquid and the amount of liquid displaced.

Teacher's Note: Remember this for your exam.