Grade 8 Science: 10 Free Practice Questions

Answer: 50 g
The product has a mass of 50 g. Mass is conserved in a chemical reaction, so the mass of the product equals the mass of the reactants added together: 20 g + 30 g = 50 g. Subtracting gives 10 g, averaging gives 25 g, and using only A gives 20 g, but none of those follow the rule. In a closed system, the total mass before always equals the total mass after.

Answer: They vibrate faster and move further apart.
Heating transfers energy to the particles, so they vibrate faster and push each other slightly further apart. The particles themselves do not get bigger - the substance expands because the spacing increases, not the particle size. That is the single most common misconception in this topic. Nor do the particles change identity: melting is a physical change, so the same particles simply arrange themselves differently.

Answer: Crushing a piece of chalk into powder
Crushing chalk changes only the size of the pieces - it is still chalk, so no new substance forms, making it a physical change. Burning, rusting and the vinegar reaction all make new substances with different properties. The clue is whether anything about the material itself changed, or only its shape and size.

Answer: Its potential energy at A was enough to reach 14 m
To arrive at a 14 m hill the car needs at least enough stored energy for that height, which its 20 m start provides. Its total at A cannot be less than at C, since energy is conserved and no push is mentioned. The push option is tempting but unnecessary: the extra 6 m of starting height is the whole explanation.

Answer: Air resistance up equals her weight down
Her constant speed means the net force on her is zero, so the upward air resistance exactly balances her downward weight. This is called terminal velocity. If you chose that air resistance is greater, she would be slowing down. If you chose that weight is greater, she would still be speeding up. If you chose that her weight is zero, you confused falling with weightlessness. Constant velocity always means balanced forces.

Answer: Streams fill with sediment
After rain washes soil downhill, the soil particles end up in streams as sediment. Sediment clouds the water and settles on the bottom, where it can bury fish eggs and plant roots, which leads to the next step. Soil does not move uphill, and bare slopes do not make streams clearer. Follow the cause-and-effect chain one link at a time.

Answer: The design is promising, but full-size tests are needed
A model can show that a shape works and help compare designs, so the result is a promising sign. But loads and material behavior do not scale in a simple way, so the full-size design needs its own engineering analysis and tests. The result is not meaningless, and it does not guarantee a particular load.

Answer: Gravitational potential energy
Being held high gives the ball gravitational potential energy, which depends on its mass and its height. It has no kinetic energy while it is held still, because kinetic energy needs motion, and that is the tempting wrong answer. Release the ball and the potential energy converts into kinetic energy as it falls and speeds up.

Answer: 6 N to the right
The net force is 6 N to the right. The 20 N up and 20 N down forces are equal, so they cancel. Horizontally, the pull of 15 N to the right minus the 9 N to the left leaves 6 N to the right. If you chose 24 N, you added the horizontal forces. If you chose 0 N, you assumed everything cancels. Resolve each direction separately before combining.

Answer: The car at 60 km/h
Kinetic energy grows with speed, so at the same mass the faster car has more of it, and considerably more because speed counts twice over in the calculation. Thinking a flat road means no kinetic energy confuses height with motion: potential energy depends on height, but kinetic energy only needs movement.