Science, about class 10

In a school laboratory, a teacher mixes an enzyme from the gut with starch at 37 °C, and the starch breaks down. The teacher boils a second sample of the enzyme, cools it back to 37 °C and adds starch. Nothing breaks down. Why?

1. ABoiling used up the enzyme, as heat burns up a fuel.
2. BBoiling changed the shape of the active site, so the starch no longer fits.
3. CCooling it back to 37 °C slowed the enzyme too much.
4. DBoiling killed the enzyme, because an enzyme is a living thing.

On a cold morning at 2 °C, a leaf sits in full sun. The air around it has plenty of carbon dioxide. The leaf makes glucose only slowly. What limits the rate of photosynthesis?

1. ALight, because the Sun is too dim at that time.
2. BCarbon dioxide, because cold air has none in it.
3. COxygen, because the leaf needs it to make glucose.
4. DTemperature, because the enzymes in the leaf work slowly in the cold.

In pea plants, the allele for purple flowers, P, is dominant. The allele for white flowers, p, is recessive. Mei has a plant with purple flowers. Which statement must be true?

1. AIt has at least one P allele.
2. BIts genotype is PP.
3. CIt has one P allele and one p allele.
4. DBoth of its parents had purple flowers.

Kofi measures two features of 30 plants of one kind in a school garden. The first is the length of each plant's longest leaf, in cm. The second is whether its flowers are red or white, with nothing in between. Which statement is correct?

1. ABoth features show continuous variation.
2. BBoth features show discontinuous variation.
3. CLeaf length shows continuous variation, and flower colour shows discontinuous variation.
4. DLeaf length shows discontinuous variation, and flower colour shows continuous variation.

A gene in one person has one base swapped for another. The same gene in another person has one extra base added near its start. Which change is more likely to stop the protein from working?

1. AThe swap, because a swap always gives a different amino acid.
2. BThe extra base, because it shifts every group of three after it.
3. CNeither, because a change in DNA never changes a protein.
4. DBoth equally, because each changes the gene by only one base.

A field floods and stays under water for weeks. The soil has very little oxygen, and the nitrate in it falls. Which bacteria cause the loss, and what do they make?

1. ADenitrifying bacteria, which turn nitrates into nitrogen gas.
2. BNitrifying bacteria, which turn nitrates into ammonium.
3. CNitrogen fixing bacteria, which turn nitrates into proteins.
4. DDecomposers, which turn nitrates into carbon dioxide.

Two kinds of fish in a lake eat the same small insects. A hot, dry summer halves the number of insects. What is most likely to happen to the fish?

1. ANothing changes, because heat is a non living factor and fish are living things.
2. BThe two kinds of fish stop competing, because there are fewer insects.
3. CBoth kinds of fish rise, because the heat makes fish grow faster.
4. DThey compete harder for the fewer insects, so each kind may fall in number.

A new road cuts a forest into two parts. Deer on one side can no longer reach deer on the other side. Which plan helps the deer most?

1. AA seed bank that stores seeds of the forest trees.
2. BA fence along both sides of the road, all the way along.
3. CA bridge planted with trees over the road, joining the two parts.
4. DClearing a wide strip beside the road, so drivers see the deer.

Years ago, a person breathed in the tuberculosis bacterium but did not become ill. The person now has HIV, has not yet started treatment, and has had a cough for weeks. What is the most likely cause of the cough?

1. AThe bacteria slept in the body and woke as the defences weakened.
2. BHIV has slowly turned into the tuberculosis bacterium.
3. CA mosquito has carried tuberculosis to the person.
4. DHIV itself grows in the lungs and makes the person cough.

Dev took antibiotics from a health worker for chlamydia and finished the course. A test showed that he was cured. His partner felt well and was not tested. Three months later, Dev's test shows chlamydia again. What is the most likely reason?

1. AChlamydia is caused by a virus, so antibiotics could not cure it.
2. BHe caught it from a shared cup or a toilet seat.
3. CThe cure from antibiotics wears off after a few weeks.
4. DHis partner had the infection with no signs, and passed it to him again.

A rare illness stops a person's pancreas making glucagon. Insulin still works. When is the person most at risk of blood glucose that falls too low?

1. ASoon after a large sweet meal, when glucose has just entered the blood.
2. BMany hours after the last meal, when the liver must release glucose from its store.
3. CSoon after drinking water, when the blood is more dilute.
4. DWhile eating a meal, when insulin is released and cells take in glucose.

Tariq has a rare illness. His pituitary gland does not release any ADH. What does his urine look like, and why?

1. AIt is small in amount and dark, because the kidneys take back too much water.
2. BThere is no urine at all, because ADH is needed to make urine.
3. CIt is large in amount and pale, because the kidneys take back too little water.
4. DIt is normal, because the kidneys control water without any hormone.

A medicine keeps the level of progesterone high all through a cycle. What is it most likely to stop?

1. AThe lining of the uterus from thickening, because progesterone breaks the lining down.
2. BOvulation, because progesterone makes the pituitary gland release a surge of LH.
3. CNothing, because progesterone only matters in the first half of the cycle.
4. DOvulation, because progesterone holds back FSH and LH, so no egg matures.

Neha grows a bean seedling on a sunny windowsill. Every day she turns the pot half a turn. After two weeks the shoot is still nearly straight. Why?

1. ATurning the pot destroys the auxin, so no side grows longer.
2. BEach side is shaded in turn, so each side gets extra auxin in turn.
3. CTurning makes the shoot respond to gravity, so it ignores the light.
4. DShoots always grow straight up, so light from one side does nothing.

Caesium is in group 1, below sodium. Which prediction about caesium is correct?

1. AIt is more reactive than sodium, because its outer electron is further from the nucleus.
2. BIt is less reactive than sodium, because its atoms have more electrons.
3. CIt is less reactive than sodium, because it holds its outer electron more strongly.
4. DIt is as reactive as sodium, because both have one outer electron.

Chlorine gas is made of Cl₂ molecules. Each chlorine atom has 7 outer electrons. How are the two atoms in one molecule held together?

1. AOne atom gives an electron to the other, so two ions form.
2. BThey share two pairs of electrons, as the atoms in O₂ do.
3. CThey share one pair of electrons, so each atom has a full outer shell.
4. DThey share all 7 outer electrons, so each atom has 14.

Kofi takes 15 breaths per minute at rest. One minute after a hard race he takes 36 breaths per minute. What is the percentage increase in his breathing rate?

Mei counts oxygen bubbles from a water plant that is lit by a small bare lamp. She moves the lamp from 20 cm to 50 cm from the plant. What percentage of the light reaches the plant now, compared with before?

A large study follows 640 children. Both parents of each child are carriers of cystic fibrosis. About how many of the children are expected to be carriers, with no signs of the condition?

Dina catches 30 beetles in a garden. She marks each one with a dot of harmless paint and sets them free. Later she catches 40 beetles, and 5 of them have a mark. Estimate how many beetles live in the garden.

A trial tests a new medicine for a skin infection. In the medicine group, 48 of the 60 patients got better. In the placebo group, 33 of the 110 patients got better. By how many percentage points is the share who got better higher in the medicine group?

A teacher has a mass of 72 kg and a height of 1.60 m. What is the teacher's BMI, to one decimal place?

A bell is cast from bronze. This bronze is 78% copper by mass, and the rest is tin. The bell has a mass of 450 kg. How many kilograms of tin are in the bell?

A teacher wants to show how surface area changes the rate of a reaction with acid. A cube of chalk has sides of 3 cm. It is cut into small cubes with sides of 1 cm. What is the total surface area of all the small cubes, in cm²?

(a)The factory melts calcium chloride, not calcium oxide. Calcium chloride melts at 772 °C, and calcium oxide at about 2 600 °C. Both are ionic. Why does calcium oxide melt at a much higher temperature? [1 mark]

1. ACalcium oxide has covalent bonds, which are stronger than ionic bonds.
2. BAn oxygen atom is heavier than a chlorine atom, so it is harder to move.
3. COxide ions have a bigger charge than chloride ions, so the ions attract more strongly.
4. DCalcium oxide is made of small molecules that hold tightly together.

(b)What mass of calcium is made from 222 g of calcium chloride? [2 marks]

(c)A worker suggests heating calcium oxide with coke, which is nearly pure carbon, to get calcium. Use the reactivity series to explain why this will not work. [2 marks]

(d)The factory melts the calcium chloride and passes a current through it. Explain why calcium forms at the negative electrode. [2 marks]

(e)The factory makes its calcium chloride from hydrochloric acid. Which of these, added to the acid, makes calcium chloride with no gas given off? [1 mark]

1. ACalcium metal
2. BCalcium hydroxide
3. CCalcium carbonate