KHAN SCIENCE SET 5 junior high school and senior high school .pdf

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1. SCIENCE
PISA Past Practice Test

SET 5
Item #1
TOOTH DECAY
Question 1/3

Refer to the statements and diagram on the right.
Choose 1 answer

What is the role of bacteria in dental caries?
A. Bacteria produce enamel.
B. Bacteria produce sugar.
C. Bacteria produce minerals.
D. Bacteria produce acid.


TOOTH DECAY

Bacteria that live in our mouths cause dental caries (tooth decay). Caries have been a problem
since the 1700s when sugar became available from the expanding sugar cane industry.

Today, we know a lot about caries. For example:
▪ Bacteria that cause caries feed on sugar.
▪ The sugar is transformed to acid.
▪ Acid damages the surface of teeth.

CORRECT ANSWER D. Bacteria produce acid.
EXPLANATION
In response to the question on the role of bacteria in dental caries, students need to determine
the specific function that bacteria play in tooth decay. Important information is provided:
o "Bacteria that cause caries feed on sugar."
o " The sugar is converted into acid"
o " Acid deteriorates the enamel of teeth.”

2. To answer the question, students must evaluate each option:
A. Bacteria produce enamel: This is incorrect. It is not bacteria that make enamel. Enamel is
naturally produced by the body during the development of teeth.
B. Bacteria produce sugar: This is incorrect. Bacteria get their energy by consuming sugar
rather than producing it.
C. Bacteria produce minerals: This is incorrect. Bacteria do not generate minerals; rather, they
induce mineral depletion in the enamel via acid production.
D. Bacteria produce acid: This is correct. Bacteria metabolize sugar, resulting in acid
production that destroys teeth and leads to decay.

From this, it is important to understand that the primary role of bacteria in dental caries is to
decompose carbohydrates present in the oral cavity. When people ingest sugary meals, the
bacteria metabolize these sugars. Bacteria (like Streptococcus mutans) use a process known as
fermentation to change sugar into acid as part of their metabolism. This acid is a result of bacterial
metabolism. The enamel, the strong, protective outer coating of teeth, is demineralized and
weakened by the acid that bacteria create. This eventually results in the development of cavities or
caries.
Item #2
TOOTH DECAY
Question 2/3

Refer to the graph on the right.
Choose 1 answer

Which one of the following statements is
supported by the data given in the graph?
A. In some countries, people brush their
teeth more frequently than in other
countries.
B. The more sugar people eat, the more
likely they are to get caries.

TOOTH DECAY

The following graph shows the consumption of sugar and the amount of caries in different
countries.
Each country is represented by a dot in the graph.

3. C. In recent years, the rate of caries has
increased in many countries.
D. In recent years, the consumption of
sugar has increased in many
countries.


CORRECT ANSWER B. The more sugar people eat, the more likely they are to get caries.
EXPLANATION For the question on the connection between dental caries and sugar intake, students must evaluate
the information in the graph and choose the appropriate interpretation. The graph illustrates the
correlation between two variables: sugar intake (x-axis) and the incidence of caries (y-axis), with
each country represented by a dot.

To answer the question, students must evaluate each option:
A. In some countries, people brush their teeth more frequently than in other countries: This is
incorrect. The graph lacks information on tooth-brushing habits or oral hygiene routines
across different countries.
B. The more sugar people eat, the more likely they are to get caries: This is correct. The graph
illustrates a positive link between sugar intake and the incidence of caries. Countries with
elevated sugar intake have a greater incidence of caries, as seen by the rising trend in the
data.
C. In recent years, the rate of caries has increased in many countries: This is incorrect. The
graph lacks historical data and a time variable to illustrate changes over the last several years.
D. In recent years, the consumption of sugar has increased in many countries: This is incorrect.
The graph provides no data on changes in sugar consumption over time.

From this, students must recognize that increased sugar intake is significantly correlated with a
higher probability of getting dental caries. This finding is consistent with scientific information

4. indicating that oral bacteria consume sugar and convert it into acid via fermentation. The acid
generated harms tooth enamel, resulting in the development of caries. This connection is evident
from the graph, hence option B is the right response.
Item #3
TOOTH DECAY
Question 3/3

Refer to the graph on the right.

Can the following questions about tooth
decay in that country be answered by
scientific experiments?

Choose "Yes" or "No" for each question.

Can this question about tooth decay be
answered by scientific experiments?
• What would be the effect on tooth decay
of putting fluoride in the water supply?


• How much should a visit to the dentist
cost?
TOOTH DECAY

The following graph shows the consumption of sugar and the amount of caries in different
countries.

Each country is represented by a dot in the graph.

A country has a high number of decayed teeth per person.

CORRECT ANSWER Yes, No in that order.
EXPLANATION Learners must differentiate between issues that can be examined scientifically and those that cannot
in order to answer the question of whether the provided information on tooth decay can be obtained
through scientific experiments. Although the stimulus offers significant information about sugar
intake and the frequency of dental cavities, the emphasis is on the kind of questions being posed.

5. To determine whether each question can be answered by scientific experiments:
• What would be the effect on tooth decay of putting fluoride in the water supply?
The correct answer is Yes. This question may be addressed by scientific experimentation,
since it includes a testable hypothesis. Researchers might do research or experiments by
incorporating fluoride into the water supply in controlled environments and assessing its
impact on the incidence of tooth decay over time. This entails gathering and examining
evidence in order to determine the relationship between them.

• How much should a visit to the dentist cost?
The correct answer is No. This question is unanswerable by scientific experimentation since
it lacks testability and scientific foundation. The cost of a dental appointment is influenced
by economic, social, and policy considerations, rather than being a matter of scientific
inquiry. It encompasses subjective factors like cost, healthcare systems, and cultural
standards.

Item #4
HOT WORK
Question 1/2

Refer to the statements on the right.

What happens to the objects in the car?
Choose "Yes" or "No" for each statement.

• They all have the same temperature

• After some time, the water begins to boil
• After some time, the metal nails begin to
glow red.

HOT WORK


Peter is working on repairs to an old house. He has left a bottle of water, some metal nails, and a
piece of timber inside the boot of his car. After the car has been out in the sun for three hours, the
temperature inside the car reaches about 40°C.



CORRECT ANSWER Yes, No, No, in that order.
EXPLANATION For the question regarding what happens to the objects in the car, learners must assess each
statement in response to the question about what happens to the items in the car using the
information supplied and their knowledge of fundamental heat and temperature principles. The data

6. provided indicates that the cars inside temperature attains 40°C after three hours of sun exposure,
and provides information on items such as a water bottle, metal nails, and timber.

To answer the question, students must evaluate each option:
• They all have the same temperature.
The correct answer is Yes. After three hours in a closed car under sunlight, all items inside
the car, including the water bottle, metal nails, and timber, would have attained thermal
equilibrium with their environment. Thermal equilibrium is achieved when items in touch or
within the same environment transfer heat until they attain a same temperature, namely 40°C,
as noted in the stimulus.
• After some time, the water begins to boil.
The correct answer is No. Water reaches its boiling point at 100°C under normal atmospheric
pressure, which is much more than the temperature inside the vehicle (40°C). As the car's
interior temperature does not above the boiling point of water, the water in the bottle cannot
reach a boiling state, regardless of the time it stays in the vehicle.
• After some time, the metal nails begin to glow red.
The correct answer is No. The minimum temperature required for metal to glow red is 525°C,
which is far higher than the car's maximum temperature of 40°C. There is not enough heat
in the car for the metal nails to glow red or generate visible light.
Item #5
HOT WORK
Question 2/2

Refer to the statements on the right.
Choose 1 answer

What are the temperature of the coffee and
the mineral water likely to be after 10
minutes?
A. 70°C and 10°C
B. 90°C and 5°C
C. 70°C and 25°C
D. 20°C and 20°C

HOT WORK


For drink during the day, Peter has a cup of hot coffee, at a temperature of about 90°C, and a cup
of cold mineral water, with a temperature of about 5°C. The cups are of identical type and size
and the volume of each drink is the same. Peter leaves the cups sitting in a room where the
temperature is about 20°C.

7. CORRECT ANSWER A. 70 ºC and 10 ºC
EXPLANATION For the question on the temperature of coffee and mineral water, students are required to understand
the principles of heat transfer and thermal equilibrium. Key information provided is as follows:
• The coffee starts at 90°C and the mineral water at 5°C.
• Both are placed in a room where the temperature is 20°C.
• The cups are of identical type, size, and volume, meaning that they will absorb heat at a
similar rate due to their material properties.

To answer the question, students must evaluate each option:
A. 70°C and 10°C: This is correct. After 10 minutes, the coffee will cool down from its initial
temperature of 90°C, likely reaching around 70°C due to heat loss. The mineral water will
warm up from 5°C, likely reaching around 10°C as it absorbs heat from the surrounding air.
B. 90°C and 5°C: This is incorrect. The coffee and mineral water will not retain their original
temperatures after 10 minutes due to heat exchange with the surrounding environment,
resulting in a temperature change for both drinks.
C. 70°C and 25°C: This is incorrect. The mineral water, initially at 5°C, will not reach 25°C after
a just 10 minutes. It will be nearer to the room temperature, although still colder.
D. 20°C and 20°C: This is incorrect. The coffee, initially at 90°C, will not reach room
temperature (20°C) in a short period.
From this, it is essential to recognize that heat transfer among items (namely, the coffee, mineral
water, and the room) causes the temperatures of the objects to come into balance with those of the
surrounding environment. Nonetheless, this process is not immediate; after 10 minutes, both the
hot coffee and cold mineral water will have reached room temperature, but will not have totally
balanced with it.
Heat Transfer Process:
• Coffee (Hot): Since the coffee starts at a temperature higher than the room temperature
(90°C vs. 20°C), it will lose heat to the surrounding air through a process called cooling.
The cooling rate of an object is governed by Newton's Law of Cooling, which suggests that
the rate of temperature change depends on the temperature difference between the object
and the surroundings.

8. • Mineral Water (Cold): Similarly, the mineral water, which is colder than the room (5°C vs.
20°C), will warm up by absorbing heat from the surroundings.
Heat Transfer and Temperature Changes:
• Over 10 minutes, the coffee will cool down, but because it started much hotter (90°C), it
will likely stabilize near the room temperature, but still remain a bit warmer. Based on
typical cooling curves, after 10 minutes, it is reasonable to expect the coffee to be around
70°C.
• The mineral water will warm up over time, and after 10 minutes, it will absorb enough heat
to rise closer to the room temperature of 20°C, but still remain slightly cooler due to its
starting temperature (5°C). Typically, after such a duration, the water would reach a
temperature of around 10°C.
Item #6
MOUSEPOX
Question 1/2

Refer to the statements on the right.
Choose 1 answer

Critics have expressed concern that the
mousepox virus could infect species other
than mice. Which one of the following
reasons is the best explanation for this
concern?
A. The genes of smallpox virus and the
genes of modified mousepox virus are
identical.
B. A mutation in mousepox DNA might
allow the virus to infect other animals.
C. A mutation could make the mousepox
DNA identical to smallpox DNA.
D. The number of genes in mousepox
virus is the same as in other pox
viruses.

MOUSEPOX

There are many types of pox viruses that cause pox diseases in animals. Each type of virus
usually infects only one animal species. A magazine has reported that a scientist has used genetic
engineering to modify the DNA of mousepox. The altered virus kills all the mice it infects.

The scientist says research on modifying viruses is necessary in order to control pests that
damage human food. Critics of the research say viruses could escape from laboratories and infect
other animals. They are also worried that a modified pox virus for one species could infect other
species, especially humans.

Humans are infected with a pox virus called smallpox. Smallpox kills most people it infects. White
it is thought that this disease has been eliminated from the general population, smallpox virus
samples are kept in laboratories around the world.

9. CORRECT ANSWER B. A mutation in mousepox DNA might allow the virus to infect other animals.
EXPLANATION For the question about the possibility of the altered mousepox virus spreading to other species,
students must choose the most logical explanation for why critics are concerned about this
possibility. Key information provided is as follows:
• There are many types of pox viruses, each typically infecting one species.
• A scientist has genetically engineered the mousepox virus to be more lethal to mice.
• Critics are concerned that the modified virus might infect other animals, including humans.

To answer the question, students must evaluate each option:
A. The genes of smallpox virus and the genes of modified mousepox virus are identical: This
is incorrect. While smallpox and mousepox viruses are related, the statement about their
genes being identical is not true. The viruses are genetically distinct, and smallpox does not
infect mice in the same way that mousepox does.
B. A mutation in mousepox DNA might allow the virus to infect other animals: This is correct.
Viruses can mutate, and a mutation in the DNA of the modified mousepox virus could
potentially allow it to infect other species. This is the concern that critics have raised—
mutations could make the virus capable of infecting animals other than mice, which is a
significant risk.
C. A mutation could make the mousepox DNA identical to smallpox DNA: This is incorrect.
While mutations can alter the genetic makeup of a virus, the idea that mousepox could
mutate to become genetically identical to smallpox is highly unlikely. The viruses are
different, and while they share some similarities, they are not interchangeable.
D. The number of genes in mousepox virus is the same as in other pox viruses: This is
incorrect. While mousepox may share some genetic characteristics with other pox viruses,
the number of genes does not directly explain why the virus might infect other species. The
concern is more about the potential for mutations in the virus that could allow cross-
species transmission, not simply the number of genes.

From this, it is important to understand that viruses, including pox viruses, can undergo mutations
that change their ability to infect different species. Critics are concerned that the modification of
the mousepox virus could lead to such mutations, allowing it to infect not just mice, but other
species, potentially even humans. Therefore, the best explanation is that a mutation in mousepox
DNA could enable it to infect other animals.

10. Item #7
MOUSEPOX
Question 2/2

Refer to the statements on the right.

A person who criticized the research was
worried that the modified mousepox virus
might escape from a laboratory. This virus
could cause the extinction of some species of
mice.

Are the following outcomes likely if some
species of mice become extinct? Choose
between "Yes" or "No" in each case.
• Some food chains could be affected.
• Domestic cats could die for lack of
food.
• Plants whose seeds are eaten by mice
could temporarily increase in number.

MOUSEPOX

There are many types of pox viruses that cause pox diseases in animals. Each type of virus
usually infects only one animal species. A magazine has reported that a scientist has used genetic
engineering to modify the DNA of mousepox. The altered virus kills all the mice it infects.

The scientist says research on modifying viruses is necessary in order to control pests that
damage human food. Critics of the research say viruses could escape from laboratories and infect
other animals. They are also worried that a modified pox virus for one species could infect other
species, especially humans.

Humans are infected with a pox virus called smallpox. Smallpox kills most people it infects. White
it is thought that this disease has been eliminated from the general population, smallpox virus
samples are kept in laboratories around the world.

CORRECT ANSWER Yes, No, Yes in that order.
EXPLANATION For the question about the potential outcomes if some species of mice become extinct due to the
modified mousepox virus, students are required to evaluate each scenario and determine its
likelihood based on ecological principles. The stimulus provides important context:
• The modified mousepox virus kills all the mice it infects.
• Some species of mice may be at risk of extinction due to the virus.
• The question asks about potential effects on food chains, domestic cats, and plant species
that rely on mice.

To answer the question, students must evaluate each option:
• Some food chains could be affected:

11. The answer is Yes. Mice play an important role in food chains as prey for various predators.
If mice become extinct in an area, the animals that rely on them as a food source (such as
certain birds, snakes, and carnivorous mammals) could be affected. The absence of mice
could lead to a disruption in the food chain, as predators would either need to find
alternative food sources or suffer from a lack of prey.
• Domestic cats could die for lack of food:
The answer is No. While domestic cats may hunt mice, their diet is more varied, and they
can survive on other sources of food, such as commercially prepared cat food or other
small animals. Although mice may be a natural food source for cats, their extinction would
not necessarily lead to the death of domestic cats. Cats have evolved to adapt to different
diets, so they could find alternative food sources without immediate threat to their survival.
• Plants whose seeds are eaten by mice could temporarily increase in number:
The answer is Yes. Many species of mice feed on seeds, and their populations help regulate
the growth of certain plant species by consuming these seeds. If mice were to become
extinct, there would be fewer seed-eating animals, which could lead to an increase in the
number of plants whose seeds mice typically eat. This increase could be temporary, as
other animals might eventually take on the role of seed predators, but in the short term, the
lack of mice could result in more abundant plant growth.