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MATSEC SEC Core Science study materials
MATSEC SEC Core Science study resources

MATSEC SEC Core Science practice questions

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Question 1Core Science (single subject; no official sub-domain breakdown published in the source)

Which part of a typical plant cell traps light energy for photosynthesis?

  1. Ribosome
  2. Chloroplast Correct answer.
  3. Nucleus
  4. Mitochondrion

Explanation

Chloroplasts contain chlorophyll, the pigment that absorbs light energy so it can be used to convert carbon dioxide and water into glucose. Mitochondria release energy from glucose instead, ribosomes build proteins and the nucleus controls the cell.

Question 2Core Science (single subject; no official sub-domain breakdown published in the source)

A student measures the mass of a metal block as 54.0 g and its volume as 20.0 cm³. What is the density of the metal?

  1. 34.0 g/cm³
  2. 0.37 g/cm³
  3. 1080 g/cm³
  4. 2.70 g/cm³ Correct answer.

Explanation

Density is mass divided by volume, so 54.0 g ÷ 20.0 cm³ = 2.70 g/cm³. The value is consistent with aluminium, which helps confirm the calculation is sensible.

Question 3Core Science (single subject; no official sub-domain breakdown published in the source)

Which statement correctly describes a chemical change rather than a physical change?

  1. The substance changes shape but keeps the same chemical identity
  2. A new substance is formed that has different properties from the starting materials Correct answer.
  3. The substance dissolves in water and can be recovered by evaporation
  4. The substance melts when heated and freezes again on cooling

Explanation

A chemical change produces one or more new substances with different properties, and it is usually difficult to reverse. Melting, dissolving and reshaping are physical changes because the original substance can be recovered unchanged.

Question 4Core Science (single subject; no official sub-domain breakdown published in the source)

Which food test gives a brick-red colour when a reducing sugar such as glucose is present?

  1. Emulsion test
  2. Benedict's test Correct answer.
  3. Biuret test
  4. Iodine test

Explanation

Benedict's solution is blue and turns brick-red when heated with a reducing sugar because the copper ions are reduced. The iodine test detects starch, the biuret test detects protein and the emulsion test detects lipids.

Question 5Core Science (single subject; no official sub-domain breakdown published in the source)

In a series circuit containing two identical lamps and one cell, what happens if one lamp is removed so the circuit is broken?

  1. The remaining lamp becomes brighter
  2. The cell voltage doubles
  3. Both lamps go out because the current path is broken Correct answer.
  4. The remaining lamp stays at the same brightness

Explanation

A series circuit provides only one path for current, so breaking it anywhere stops the current everywhere. Both lamps therefore go out, and no current flows through the cell.

Question 6Core Science (single subject; no official sub-domain breakdown published in the source)

Which process releases oxygen into the atmosphere as a product?

  1. Fermentation in yeast
  2. Photosynthesis Correct answer.
  3. Aerobic respiration
  4. Combustion of methane

Explanation

Photosynthesis uses carbon dioxide and water to make glucose and releases oxygen as a by-product. Respiration and combustion consume oxygen, and fermentation in yeast produces carbon dioxide and ethanol.

Question 7Core Science (single subject; no official sub-domain breakdown published in the source)

What is the main function of red blood cells?

  1. Clotting blood at a wound
  2. Transporting oxygen around the body Correct answer.
  3. Digesting fats in the bloodstream
  4. Producing antibodies against pathogens

Explanation

Red blood cells are packed with haemoglobin, which binds oxygen in the lungs and releases it in tissues. Antibodies are made by white blood cells and clotting involves platelets.

Question 8Core Science (single subject; no official sub-domain breakdown published in the source)

Which of these is a renewable energy resource?

  1. Coal
  2. Wind Correct answer.
  3. Diesel
  4. Natural gas

Explanation

Wind is renewable because it is continually replenished by the Sun's heating of the atmosphere. Coal, natural gas and diesel are fossil fuels that take millions of years to form and will run out.

Question 9Core Science (single subject; no official sub-domain breakdown published in the source)

A car travels 150 m in 10 s. What is its average speed?

  1. 1500 m/s
  2. 15 m/s Correct answer.
  3. 160 m/s
  4. 1.5 m/s

Explanation

Average speed equals distance divided by time, so 150 m ÷ 10 s = 15 m/s. Dividing distance by time gives metres per second when distance is in metres and time in seconds.

Question 10Core Science (single subject; no official sub-domain breakdown published in the source)

Which statement best explains why a metal spoon left in a hot drink becomes warm at the handle?

  1. Hot water evaporates and condenses on the handle
  2. Convection currents carry hot water up the spoon
  3. The spoon absorbs light and converts it to heat
  4. Heat is conducted along the metal because its delocalised electrons transfer energy Correct answer.

Explanation

Metals conduct heat well because delocalised electrons and vibrating ions pass energy along the lattice. The handle warms by conduction, not by evaporation, convection or light absorption.

Question 11Core Science (single subject; no official sub-domain breakdown published in the source)

Which ion is formed when a calcium atom loses two electrons?

  1. Ca²⁺ Correct answer.
  2. Ca²
  3. Ca⁺
  4. Ca²⁻

Explanation

Metal atoms lose electrons to form positive ions, and calcium is in Group 2 so it loses two electrons. The charge is therefore 2+ and the symbol is written Ca²⁺.

Question 12Core Science (single subject; no official sub-domain breakdown published in the source)

A solution has a pH of 3. Which statement about it is correct?

  1. It is neutral because it is close to pH 7
  2. It is acidic and contains more hydrogen ions than a solution of pH 5 Correct answer.
  3. It is acidic because it contains more hydroxide ions than hydrogen ions
  4. It is alkaline and contains hydroxide ions in excess

Explanation

A pH below 7 indicates an acidic solution, and lower pH values mean a higher concentration of hydrogen ions. So pH 3 is more acidic and has more H⁺ ions than pH 5.

Question 13Core Science (single subject; no official sub-domain breakdown published in the source)

An object is placed in front of a plane mirror. Which statement describes the image formed?

  1. It is real, the same size as the object and inverted
  2. It is virtual, the same size as the object and laterally inverted Correct answer.
  3. It is virtual, larger than the object and upright
  4. It is real, smaller than the object and upright

Explanation

A plane mirror forms a virtual image behind the mirror that is the same size as the object and laterally inverted. Virtual images cannot be projected onto a screen because the light rays do not actually meet.

Question 14Core Science (single subject; no official sub-domain breakdown published in the source)

Which statement explains why a population of bacteria can become resistant to an antibiotic over many generations?

  1. Resistance appears because the bacteria need it to survive
  2. All bacteria in the population develop resistance at the same time
  3. The antibiotic teaches each bacterium how to resist the drug
  4. Bacteria with resistance genes survive and reproduce, passing the genes to their offspring Correct answer.

Explanation

Random mutations can give some bacteria resistance, and these individuals survive treatment and reproduce. This is natural selection: the resistant genes become more common in later generations.

Question 15Core Science (single subject; no official sub-domain breakdown published in the source)

A student adds a piece of magnesium ribbon to dilute hydrochloric acid. Which observation is expected?

  1. A white precipitate of magnesium chloride forms immediately
  2. Effervescence as hydrogen gas is produced Correct answer.
  3. The solution turns blue
  4. No reaction occurs at room temperature

Explanation

Magnesium is a reactive metal above hydrogen in the reactivity series, so it reacts with dilute acid to release hydrogen gas, seen as bubbles. The magnesium chloride formed stays dissolved and colourless.

Question 16Core Science (single subject; no official sub-domain breakdown published in the source)

Why does the body need a constant supply of oxygen for aerobic respiration?

  1. Oxygen is needed to break down proteins into amino acids
  2. Oxygen is converted directly into ATP without glucose
  3. Oxygen is used to build glucose from carbon dioxide
  4. Oxygen is the final electron acceptor in the reactions that release energy from glucose Correct answer.

Explanation

Aerobic respiration uses oxygen at the end of the electron transport chain, allowing glucose to be fully broken down and much more energy to be released than in anaerobic respiration.

Question 17Core Science (single subject; no official sub-domain breakdown published in the source)

Which change would increase the rate of photosynthesis in a plant kept in bright light but at a low concentration of carbon dioxide?

  1. Decreasing the temperature to near freezing
  2. Increasing the distance from the light source
  3. Increasing the carbon dioxide concentration Correct answer.
  4. Removing the plant's leaves

Explanation

When light is already plentiful, carbon dioxide becomes the limiting factor, so raising its concentration increases the rate. Lowering temperature or removing leaves would slow photosynthesis.

Question 18Core Science (single subject; no official sub-domain breakdown published in the source)

A 12 V battery is connected to a resistor of 4 Ω. What current flows?

  1. 3 A Correct answer.
  2. 0.33 A
  3. 48 A
  4. 16 A

Explanation

Ohm's law states that current equals voltage divided by resistance, so 12 V ÷ 4 Ω = 3 A. The units are consistent because volts divided by ohms gives amperes.

Question 19Core Science (single subject; no official sub-domain breakdown published in the source)

Which statement about the transfer of energy in a food chain is correct?

  1. All the energy in a producer is transferred to the primary consumer
  2. Only a small fraction of the energy in one trophic level is passed to the next Correct answer.
  3. Energy is recycled continuously through the food chain
  4. Energy increases at each level because consumers eat many organisms

Explanation

Most energy is lost at each trophic level through respiration, movement and heat, so only about ten per cent is passed on. Energy flows through an ecosystem rather than being recycled.

Question 20Core Science (single subject; no official sub-domain breakdown published in the source)

Which property of water makes it effective at absorbing and carrying heat around the body?

  1. Its lack of cohesion between molecules
  2. Its high specific heat capacity Correct answer.
  3. Its inability to dissolve salts
  4. Its low boiling point

Explanation

Water has a high specific heat capacity, so it absorbs a lot of heat with only a small rise in temperature. This helps organisms avoid rapid temperature changes and lets blood carry heat around the body.

Question 21Core Science (single subject; no official sub-domain breakdown published in the source)

Which statement describes the difference between mass and weight?

  1. Mass changes with location, while weight stays constant
  2. Mass is a measure of the amount of matter, while weight is the force of gravity acting on that mass Correct answer.
  3. Mass is measured in newtons, while weight is measured in kilograms
  4. Mass and weight are the same quantity expressed in different units

Explanation

Mass measures how much matter an object contains and is measured in kilograms, remaining constant anywhere. Weight is the gravitational force on the mass, measured in newtons, and changes with gravitational field strength.

Question 22Core Science (single subject; no official sub-domain breakdown published in the source)

A gas is compressed at constant temperature. Which statement about the gas particles is correct?

  1. They move more slowly, so the pressure decreases
  2. They become smaller so they fit into the reduced volume
  3. They collide with the walls more frequently, so the pressure increases Correct answer.
  4. They stop moving and settle at the bottom

Explanation

At constant temperature the average kinetic energy of the particles stays the same, but they are closer together and hit the walls more often. More frequent collisions per unit area raise the pressure.

Question 23Core Science (single subject; no official sub-domain breakdown published in the source)

In a reaction between zinc and copper sulfate solution, which species is oxidised and which is reduced?

  1. Both zinc and copper ions are oxidised
  2. Zinc is oxidised and copper ions are reduced Correct answer.
  3. Neither species changes oxidation state
  4. Zinc is reduced and copper ions are oxidised

Explanation

Zinc atoms lose electrons to form Zn²⁺, so zinc is oxidised; copper ions gain those electrons to form copper atoms, so they are reduced. Oxidation is loss of electrons and reduction is gain of electrons.

Question 24Core Science (single subject; no official sub-domain breakdown published in the source)

A plant is kept in the dark for 48 hours, then a leaf is tested with iodine solution. What is the expected result and why?

  1. The leaf turns blue-black because respiration produces starch in the dark
  2. The leaf stays brown because iodine cannot enter a leaf in darkness
  3. The leaf turns blue-black because starch stored earlier is still present
  4. The leaf stays brown because no starch was made without light for photosynthesis Correct answer.

Explanation

Photosynthesis needs light, so a plant kept in darkness cannot make new glucose, and any starch already present is used up by respiration. The iodine test therefore gives no blue-black colour, showing starch is absent.

Question 25Core Science (single subject; no official sub-domain breakdown published in the source)

A student connects two identical resistors in parallel across a cell. Compared with a single resistor, what happens to the total resistance and the current drawn from the cell?

  1. Total resistance stays the same and the current stays the same
  2. Total resistance decreases and the current from the cell increases Correct answer.
  3. Total resistance decreases and the current from the cell decreases
  4. Total resistance increases and the current from the cell decreases

Explanation

Adding a parallel branch gives the current an extra path, so the total resistance is lower than either resistor. With the same cell voltage, a lower total resistance means a larger current is drawn.

Question 26Core Science (single subject; no official sub-domain breakdown published in the source)

Which part of a plant cell is responsible for absorbing light energy during photosynthesis?

  1. The cell wall
  2. The nucleus
  3. The vacuole
  4. The chloroplast Correct answer.

Explanation

Chloroplasts contain the green pigment chlorophyll, which absorbs light energy so that it can be used to convert carbon dioxide and water into glucose. The other structures listed have different roles and do not capture light.

Question 27Core Science (single subject; no official sub-domain breakdown published in the source)

Which gas makes up the largest proportion of the atmosphere by volume?

  1. Nitrogen Correct answer.
  2. Oxygen
  3. Carbon dioxide
  4. Argon

Explanation

Nitrogen makes up roughly 78% of the atmosphere by volume, far more than any other gas. Oxygen is about 21%, while carbon dioxide and argon are present only in small amounts.

Question 28Core Science (single subject; no official sub-domain breakdown published in the source)

A student measures the mass of a stone as 50 g and its volume as 20 cm³. What is the density of the stone?

  1. 2.5 g/cm³ Correct answer.
  2. 0.4 g/cm³
  3. 1000 g/cm³
  4. 70 g/cm³

Explanation

Density is calculated by dividing mass by volume, so 50 g divided by 20 cm³ gives 2.5 g/cm³. This value falls in the range expected for many common rocks.

Question 29Core Science (single subject; no official sub-domain breakdown published in the source)

Which statement describes a chemical change rather than a physical change?

  1. A new substance being formed that cannot easily be turned back Correct answer.
  2. Sugar dissolving in water
  3. Water boiling to form steam
  4. Ice melting to form water

Explanation

A chemical change produces new substances with different properties, and reversing it usually requires another chemical reaction. Melting, dissolving and boiling are physical changes in which the substance itself is unchanged.

Question 30Core Science (single subject; no official sub-domain breakdown published in the source)

Which type of force acts between two charged objects without them touching?

  1. Weight
  2. Electrostatic force Correct answer.
  3. Tension
  4. Friction

Explanation

Electrostatic force acts between charged objects across a distance, attracting opposite charges and repelling like charges. The other forces listed require contact or act due to gravity.

Question 31Core Science (single subject; no official sub-domain breakdown published in the source)

Which organ of the human digestive system is the main site where digested food is absorbed into the blood?

  1. The small intestine Correct answer.
  2. The stomach
  3. The oesophagus
  4. The liver

Explanation

The small intestine has a long length and a folded lining covered in villi, giving a large surface area for absorption of digested nutrients into the bloodstream. The stomach mainly churns food and begins protein digestion.

Question 32Core Science (single subject; no official sub-domain breakdown published in the source)

Which statement correctly describes the arrangement of particles in a solid?

  1. Particles are closely packed but free to slide past one another
  2. Particles are arranged in a regular pattern and can move freely through the whole solid
  3. Particles are closely packed in fixed positions and vibrate about them Correct answer.
  4. Particles are far apart and move randomly at high speed

Explanation

In a solid the particles are held close together in fixed positions by strong forces, and they can only vibrate about these positions. This explains why solids keep a definite shape and volume.

Question 33Core Science (single subject; no official sub-domain breakdown published in the source)

A food chain is: grass, then grasshopper, then frog, then snake. Which organism in this chain is a producer?

  1. The grass Correct answer.
  2. The grasshopper
  3. The snake
  4. The frog

Explanation

A producer is an organism that makes its own food by photosynthesis, and grass does this using sunlight. All the animals in the chain are consumers that depend on the grass for energy.

Question 34Core Science (single subject; no official sub-domain breakdown published in the source)

A student heats a beaker of water and records its temperature every minute. The temperature rises steadily, then stays constant at 100 °C while bubbles form. Which explanation is correct?

  1. The thermometer has stopped working because the temperature cannot stay constant
  2. The water is melting and the constant temperature shows the melting point
  3. The water is cooling because the heat supply has been switched off
  4. The water is boiling and the energy supplied is being used to change liquid into gas Correct answer.

Explanation

During boiling, energy supplied is used to overcome forces between particles and change the liquid into a gas rather than to raise the temperature. The temperature therefore stays constant at the boiling point until all the liquid has vaporised.

Question 35Core Science (single subject; no official sub-domain breakdown published in the source)

A 6 V cell is connected across a resistor and a current of 0.5 A flows. What is the resistance of the resistor?

  1. 12 Ω Correct answer.
  2. 3 Ω
  3. 0.08 Ω
  4. 6.5 Ω

Explanation

Using the equation resistance equals voltage divided by current, 6 V divided by 0.5 A gives 12 Ω. Rearranging Ohm's law in this way is the standard route to the answer.

Question 36Core Science (single subject; no official sub-domain breakdown published in the source)

Which statement best explains why a gas exerts pressure on the walls of its container?

  1. The gas particles attract the walls of the container
  2. The gas particles dissolve into the walls of the container
  3. The gas particles collide with the walls and change momentum, exerting a force Correct answer.
  4. The gas particles are stationary and push steadily on the walls

Explanation

Gas particles move randomly at high speed and collide with the container walls. Each collision changes the particle's momentum, and the many collisions per second produce a force spread over the wall area, which is pressure.

Question 37Core Science (single subject; no official sub-domain breakdown published in the source)

Which statement correctly describes what happens to the atoms in a chemical reaction?

  1. Atoms are created to make new products
  2. Atoms are destroyed and replaced by new atoms
  3. Atoms change into different elements during the reaction
  4. Atoms are rearranged to form new substances but the total number stays the same Correct answer.

Explanation

In a chemical reaction, bonds are broken and new bonds are formed, so atoms are rearranged into new substances. The total number of atoms of each element is conserved, which is why the equation can be balanced.

Question 38Core Science (single subject; no official sub-domain breakdown published in the source)

An athlete runs 400 m around a track and finishes at the same point where the run started. Which statement about the motion is correct?

  1. The displacement is 400 m and the distance travelled is zero
  2. Both the displacement and the distance travelled are 400 m
  3. Both the displacement and the distance travelled are zero
  4. The displacement is zero but the distance travelled is 400 m Correct answer.

Explanation

Displacement is the straight-line distance from start to finish in a stated direction, and since the athlete returns to the start it is zero. Distance is the total path length covered, which is 400 m.

Question 39Core Science (single subject; no official sub-domain breakdown published in the source)

Which feature of the alveoli makes gas exchange in the lungs efficient?

  1. They are filled with cartilage to keep them rigid
  2. They contain many blood cells that carry oxygen directly
  3. They have a thick wall that traps gases
  4. They have a large total surface area and a thin wall Correct answer.

Explanation

Millions of alveoli give a very large total surface area, and their walls are only one cell thick so gases diffuse over a short distance. A good blood supply also maintains a steep concentration gradient.

Question 40Core Science (single subject; no official sub-domain breakdown published in the source)

A solution of an acid is neutralised by adding a metal hydroxide. Which type of product is always formed in this reaction?

  1. An oxide and hydrogen
  2. A salt and water Correct answer.
  3. A metal and hydrogen
  4. A salt and carbon dioxide

Explanation

An acid reacting with a metal hydroxide is a neutralisation reaction that produces a salt and water. The hydrogen ions from the acid combine with hydroxide ions to form water.

Question 41Core Science (single subject; no official sub-domain breakdown published in the source)

A population of rabbits lives in a meadow where food is limited. Which change would most likely increase the size of the population?

  1. A decrease in the number of shelter sites
  2. A disease that spreads through the rabbits
  3. An increase in the amount of food available Correct answer.
  4. An increase in the number of predators

Explanation

Food is a limiting factor for this population, so more food allows more rabbits to survive and reproduce. The other changes would increase mortality or reduce survival, lowering population size.

Question 42Core Science (single subject; no official sub-domain breakdown published in the source)

A ray of light passes from air into a glass block at an angle. Which statement describes what happens to the ray?

  1. It slows down and bends towards the normal Correct answer.
  2. It is completely reflected back into the air
  3. It travels at the same speed and continues in a straight line
  4. It speeds up and bends away from the normal

Explanation

Light travels more slowly in glass than in air, and when it enters a denser medium at an angle it refracts towards the normal. This change in direction happens because the speed of light changes at the boundary.

Question 43Core Science (single subject; no official sub-domain breakdown published in the source)

Which statement explains why sodium chloride conducts electricity when molten but not when solid?

  1. Electrons are released when the solid melts
  2. In the solid the ions are fixed in a lattice, but when molten the ions are free to move Correct answer.
  3. In the solid the ions are free to move, but when molten they become fixed
  4. Molten sodium chloride contains metal atoms that carry the current

Explanation

Sodium chloride is an ionic compound made of charged ions held in a rigid lattice when solid, so the ions cannot move. Melting breaks down the lattice and frees the ions to move and carry charge, so the molten compound conducts.

Question 44Core Science (single subject; no official sub-domain breakdown published in the source)

A student wants to separate a mixture of sand and salt. Which sequence of steps would work?

  1. Add water, stir, then distil the whole mixture
  2. Use a magnet to remove the sand, then evaporate
  3. Add water, stir, filter, then evaporate the filtrate Correct answer.
  4. Filter the dry mixture, then heat the residue

Explanation

Salt dissolves in water while sand does not, so adding water and stirring dissolves the salt. Filtering removes the sand as residue, and evaporating the filtrate leaves the salt behind.

Question 45Core Science (single subject; no official sub-domain breakdown published in the source)

A student builds a circuit with a cell, a lamp and an ammeter connected in series. A second identical lamp is then added in series. What happens to the brightness of the lamps?

  1. Both lamps become dimmer because the total resistance increases Correct answer.
  2. Both lamps become brighter because there are more components
  3. The first lamp stays the same and the second does not light
  4. The brightness stays the same because the cell voltage is unchanged

Explanation

Adding another lamp in series increases the total resistance of the circuit, so the current from the cell falls. With less current through each lamp, both lamps are dimmer than the single lamp was.

Question 46Core Science (single subject; no official sub-domain breakdown published in the source)

Which statement correctly compares the energy transfers in an electric motor and an electric generator?

  1. A motor transfers kinetic energy to electrical energy, while a generator transfers electrical energy to kinetic energy
  2. Both devices transfer chemical energy to electrical energy
  3. A motor transfers electrical energy to kinetic energy, while a generator transfers kinetic energy to electrical energy Correct answer.
  4. Both devices transfer electrical energy to kinetic energy

Explanation

A motor uses a current in a magnetic field to produce motion, converting electrical energy into kinetic energy. A generator uses motion of a conductor in a magnetic field to induce a current, converting kinetic energy into electrical energy.

Question 47Core Science (single subject; no official sub-domain breakdown published in the source)

A solution contains 5 g of a dissolved solid in 200 cm³ of solution. What is the concentration in g/dm³?

  1. 40 g/dm³
  2. 25 g/dm³ Correct answer.
  3. 0.025 g/dm³
  4. 1000 g/dm³

Explanation

There are 1000 cm³ in 1 dm³, so 200 cm³ is 0.2 dm³. Dividing 5 g by 0.2 dm³ gives 25 g/dm³, which is the concentration of the solution.

Question 48Core Science (single subject; no official sub-domain breakdown published in the source)

Two identical cells are connected in parallel to a single lamp. Compared with using one cell, which statement about the circuit is correct?

  1. The voltage across the lamp is halved and the lamp becomes dimmer
  2. The voltage across the lamp stays the same but the cells can supply current for longer Correct answer.
  3. The voltage across the lamp is doubled and the current doubles
  4. The lamp does not light because the cells oppose each other

Explanation

Cells in parallel each provide the same voltage, so the potential difference across the lamp is unchanged and its brightness stays the same. The total current available is shared between the cells, so they last longer.

Question 49Core Science (single subject; no official sub-domain breakdown published in the source)

A plant is given water containing a dissolved mineral that cannot pass through the root cell membranes. Which prediction about the plant is most likely?

  1. The plant will grow faster because minerals are always beneficial
  2. The plant will absorb more water because the mineral attracts it
  3. The plant will lose all its chlorophyll within a few hours
  4. The plant will take up less water and may wilt because the water potential in the soil is lowered Correct answer.

Explanation

Water moves into root hair cells by osmosis from a region of higher water potential to lower water potential. Dissolved minerals lower the water potential of the soil solution, so less water enters and the plant may wilt.

Question 50Core Science (single subject; no official sub-domain breakdown published in the source)

A sealed container holds a mixture of hydrogen and oxygen at room temperature. A spark is passed and water is formed. Which statement about the reaction is correct?

  1. The mass of the contents increases because water is heavier than the gases
  2. The mass decreases because a gas is used up
  3. The total mass stays the same because atoms are conserved Correct answer.
  4. The reaction cannot occur because gases do not react in a sealed container

Explanation

The container is sealed, so no substance can enter or leave. Atoms are rearranged into water molecules but none are created or destroyed, so the total mass of the contents is unchanged.