Biology
What organelle in plant cells captures sunlight to make energy?
chloroplast
Which type of hydrocarbon contains only single bonds?
Alkanes
A bicycle speeds up when a rider pushes harder on the pedals. What happens to the net force on the bicycle as it speeds up?
The net force increases.
Pushing harder means the forward force is now greater than the resisting forces, so the bicycle accelerates.
Why does chewing food make chemical digestion by enzymes faster?
Chewing breaks food into smaller pieces, which increases the surface area available for enzymes to act on. When the pieces are smaller, enzymes like amylase, protease, and lipase can reach and break down nutrients more quickly. This makes chemical digestion more efficient and helps the digestive system work faster.
Propene has the formula C₃H₆.
Why is it classified as an alkene and not an alkane?
Because Propene contains a carbon-carbon double bond.
Two identical balls are dropped from the same height, one in air and one in a vaccum. Which one hits the ground first, and why?
The ball in the vacuum hits first.
There is no air resistance, so it accelerates at 𝑔 the entire way.
In air, drag slows the ball slightly, so it takes longer.
A student’s breathing rate and heart rate both increase during exercise.
Using your understanding of respiration, explain why both rates rise together.
During exercise, muscles need more oxygen to release energy through respiration, and they also produce more carbon dioxide as a waste product.
Breathing rate increases to bring in more oxygen and remove extra carbon dioxide from the body.
Meanwhile, heart rate increases to deliver that oxygen-rich blood to the muscles faster and to carry carbon dioxide back to the lungs for removal.
Because oxygen delivery and carbon dioxide removal are linked, both heart rate and breathing rate rise together.
Two metals of the same mass are heated with the same flame for the same amount of time.
Metal A gets much hotter than Metal B.
Explain why.
Metal A becomes hotter because it has a lower specific heat capacity. This means it needs less energy to raise its temperature. Metal B must have a higher specific heat capacity, so even though both received the same amount of energy from the flame, Metal B’s temperature rose less.
A student pushes a box across the floor. She applies the same force over the same distance twice, but the box moves faster the second time.
Why does she do the same amount of work both times even though the outcomes look different?
Work = force × distance, so she does the same work both times.
The difference is that the second time, less energy is lost to friction, so more of the work becomes kinetic energy, making the box move faster.
A student investigates how well amylase breaks down starch in the mouth. She finds that when the mixture becomes too acidic, amylase suddenly stops working even though starch is still present.
Using enzyme knowledge, explain why amylase stops functioning in acidic conditions.
Amylase works best in the mouth, where the pH is close to neutral. If the environment becomes too acidic, the enzyme’s shape begins to change. This includes the shape of its active site, where starch particles would normally fit. When the active site no longer matches the starch molecule, the amylase cannot break it down. This loss of shape is called denaturation, and it stops the enzyme from functioning even though the starch is still there.
A student burns a small candle inside a sealed glass jar. After a few minutes, the flame becomes weak, burns with a yellow, smoky glow, and then goes out.
Without using any equations, explain why the flame changed colour and produced smoke before it extinguished.
Inside the sealed jar, oxygen is gradually used up as the candle burns. With less oxygen available, the flame can no longer carry out complete combustion and switches to incomplete combustion. Incomplete combustion produces soot (tiny carbon particles) and carbon monoxide instead of only carbon dioxide and water. The glowing carbon particles create the yellow, smoky flame. As oxygen continues to drop, even incomplete combustion cannot continue, so the flame weakens and eventually goes out.
A radioactive isotope has a half-life of 3 days. A student notices that after 6 days, the activity (count rate) is not zero — it’s just much smaller.
Using the idea of half-life, explain why the activity never reaches zero even after many half-lives
Because each half-life cuts the number of undecayed nuclei in half, not all the way to zero.
Repeated halving means the activity becomes smaller and smaller but never fully reaches zero.
During a long-distance run, a student suddenly gets a sharp cramp in her leg. Later, her teacher explains that her muscles temporarily switched to anaerobic respiration.
Using your understanding of respiration and the circulatory system, explain how both her muscle cells and her blood changed during this switch and why this led to the cramp.
During a long run, the student’s muscles use oxygen faster than her lungs and blood can supply it. When the oxygen level becomes too low, the muscle cells switch from aerobic respiration to anaerobic respiration so they can keep producing some energy. Anaerobic respiration is less efficient and produces lactic acid as a waste product. As lactic acid builds up inside the muscle cells, it lowers the pH and interferes with normal enzyme function, causing pain and cramping.
A marine scientist notices that some shells of sea creatures are thinning in areas where ocean water has absorbed large amounts of carbon dioxide from the atmosphere.
Using particle-level reasoning, explain how CO₂ in the air can eventually reduce the strength of shells made of calcium carbonate (CaCO₃).
When carbon dioxide dissolves in seawater, it reacts with water to form carbonic acid. This acid releases hydrogen ions, which make the water more acidic. These hydrogen ions react with calcium carbonate in shells, breaking it down and turning it into soluble bicarbonate ions. Because the shell material dissolves faster in acidic conditions, the shells thin and weaken over time.
A cart is rolling across a smooth floor at constant speed. A student suddenly places a heavy block on top of the cart, but the cart does not stop, it just continues moving forward, now more slowly.
Explain why the cart slows down even though no external horizontal force acted on it.
Momentum is conserved because no external horizontal force acts.
When the heavy block is added, the total mass increases.
To keep momentum the same, the speed must decrease