Scientific Reasoning notes

MDCAT Analytical and Logical Reasoning

Scientific reasoning uses observations, measurements, experiments, calculations and logical arguments to explain natural events. It includes identifying variables, interpreting evidence, recognising patterns, estimating quantities and applying principles from physics, chemistry, biology and everyday situations.

Scientific Method and Evidence

Scientific reasoning begins with an observation or question. A hypothesis is a testable explanation. An experiment is then designed to collect evidence. The conclusion must be based on the results rather than on personal opinion.

A good investigation changes one factor at a time and keeps other relevant conditions constant. The measured result is compared with a control or with another experimental group. A conclusion should not claim more than the evidence supports.

Observation is information obtained using the senses or measuring instruments.
A hypothesis is a possible explanation that can be tested.
The independent variable is the factor deliberately changed by the investigator.
The dependent variable is the factor measured or observed.
Controlled variables are factors kept the same in all groups.
A control group provides a comparison for the experimental group.
If constant light produces lower melatonin than a normal light-dark cycle, the supported conclusion is that melatonin production is influenced by light exposure.
If increasing doses of caffeine consistently increase heart rate, the supported conclusion is that caffeine stimulates the cardiovascular system.

Interpreting Experiments and Biological Processes

Biological experiments often compare organisms or cells under different conditions. The result must be connected to a known biological process. For example, a change in cell size, membrane position or transport speed can provide evidence about water movement or transport mechanisms.

Experimental evidence supports a conclusion only when the observations match the proposed explanation. Other explanations should be considered if the investigation does not control all variables.

Higher salt concentration outside an onion cell makes the surrounding solution hypertonic.
Water leaves the onion cell by osmosis in a hypertonic solution.
Loss of water causes the vacuole to shrink and the cell membrane to pull away from the cell wall. This is plasmolysis.
A fungal culture growing best at 25°C and poorly at 10°C and 40°C shows that the fungus has an optimum temperature for growth.
If dye reaches leaves faster in a plant kept at room temperature than in a cold plant, low temperature has slowed the transport process.
A sealed jar with a lit candle has lower oxygen and higher carbon dioxide because combustion consumes oxygen and produces carbon dioxide.
The word likely in a conclusion means that the evidence supports an explanation, but does not necessarily prove it under every condition.

Rates, Graphs and Patterns

A rate describes how quickly a quantity changes. It is calculated by dividing the amount of change by the time taken. Graphs allow comparisons between trials and help identify increasing, decreasing, constant and optimum relationships.

When a variable is changed in several trials, the correct trial is selected by matching the required condition with the recorded rate. Data should be read from the values in the table or graph, not guessed from the order of the trials.

Rate of reaction = amount of reactant consumed ÷ time taken.
The usual units of rate depend on the measured quantities, such as grams per second or cubic centimetres per second.
A steeper straight-line graph represents a greater rate of increase.
A horizontal graph shows no change in the measured quantity.
A peak in a graph may show an optimum condition, such as the temperature at which fungal growth is greatest.
A consistent pattern is stronger evidence than a single unusual reading.
For a yeast culture that doubles and then has a fixed weight removed daily, a graph must show the combined effect of multiplication and subtraction. A plot that violates this pattern cannot represent the yeast weight.
Reaction rate can depend on concentration, temperature, surface area, catalysts and the nature of the reactants.

Physical Reasoning: Motion and Inertia

Inertia is the tendency of an object to resist a change in its state of rest or uniform straight-line motion. It explains several everyday observations. A moving object does not automatically follow a curved path unless a force changes its direction.

When a car turns, the passengers tend to continue moving in a straight line. Relative to the turning car, they feel as if they are thrown away from the corner. This apparent outward effect is related to inertia, while the actual required force is directed towards the centre of the turn.

Inertia depends on mass. A more massive object has greater resistance to a change in motion.
A force is needed to change an object's speed or direction.
During a sudden turn, the car changes direction but the passenger's body tends to maintain its original straight-line motion.
When a carpet is moved suddenly, dust particles tend to remain at rest because of inertia.
The carpet moves away from the dust, separating the dust from the carpet.
Inertia is not a separate force. It is a property of matter.
Seat belts provide a force that changes the motion of passengers when a car stops suddenly.

Sound, Resonance and Gears

Sound is produced by vibrating objects. A guitar string alone moves only a small amount of air, so the sound is weak. The body of the guitar receives vibrations from the strings and causes a larger volume of air to vibrate.

Resonance occurs when an object vibrates strongly at or near its natural frequency. The soundhole and hollow body help amplify the vibration and project the sound.

A detached guitar string produces very little sound because it moves a small volume of air.
The guitar body acts as a resonating chamber.
Resonance increases the amplitude of vibration and makes the sound louder.
A soundhole helps the vibrating air communicate with the surrounding air.
In a system of touching gears, neighbouring gears rotate in opposite directions.
If two gears are connected through an odd number of gear contacts, their directions are opposite.
If two gears are connected through an even number of gear contacts, their directions are the same.
To solve a gear diagram, mark the direction of one gear and reverse the direction at every contact.

Electrical Circuits and Potential Difference

A circuit contains a source of potential difference and conducting paths. Resistance opposes the flow of current. In a series path, the same current passes through each resistor, and the total potential difference is the sum of the potential differences across the resistors.

Potential difference between two points is found from the resistance between those points and the current through that part of the circuit. Ohm's law is V = IR for an ohmic conductor at constant physical conditions.

Current is measured in amperes, resistance in ohms and potential difference in volts.
Ohm's law: potential difference = current × resistance.
For resistors in series, total resistance is R1 + R2 + R3 and so on.
For series resistors, the current is the same through every resistor.
The potential difference across a resistor is V = IR.
The potential difference between b and c is calculated using the equivalent resistance of the section between b and c, multiplied by the stated current.
In the given circuit values, the potential difference across b and c is 62 V.
Before calculating, identify whether resistors are in series, parallel or a combination of both.

Chemistry in Scientific Reasoning

Chemical reasoning uses observations such as colour changes, gas production, temperature change and formation of a solid. The reactants and products must be identified using the type of reaction involved.

Tollens' reagent is an oxidising reagent. It is commonly used to distinguish aldehydes from ketones because aldehydes are oxidised while silver ions are reduced to metallic silver.

Oxidation may involve addition of oxygen, removal of hydrogen or loss of electrons.
Reduction may involve removal of oxygen, addition of hydrogen or gain of electrons.
Tollens' reagent contains silver ions that can be reduced to silver.
An aldehyde is oxidised to the corresponding carboxylic acid by Tollens' reagent.
Propanal is oxidised to propanoic acid, while silver ions form silver metal.
The products of oxidation of propanal with Tollens' reagent are propanoic acid and Ag.
Combustion generally consumes oxygen and forms carbon dioxide and water when a hydrocarbon burns completely.
Chemical conclusions must follow the reactants, conditions and observed products.

Temperature, Pressure and Everyday Applications

Physical conditions affect the behaviour of matter. The boiling point of a liquid depends on external pressure. A liquid boils when its vapour pressure becomes equal to the surrounding pressure.

At high altitude, atmospheric pressure is lower. Water therefore boils at a temperature below 100°C, so food may take longer to cook even though the water is boiling.

At sea level, pure water boils at approximately 100°C under normal atmospheric pressure.
Boiling point decreases when external pressure decreases.
Atmospheric pressure is lower in mountainous regions than at sea level.
Lower boiling temperature means that cooking water is not as hot as boiling water at sea level.
Meat takes longer to cook in mountainous regions because the boiling point of water is lower.
Pressure cookers increase pressure above the liquid and raise the boiling point of water.
Evaporation can occur at the surface of a liquid below its boiling point.
Boiling occurs throughout the liquid, while evaporation occurs mainly at the surface.

Quantitative Estimation and Proportional Reasoning

Scientific reasoning sometimes requires an approximate answer rather than an exact measurement. A sensible estimate is made by choosing a reasonable value, multiplying the number of objects by the estimated value and checking the unit.

Calculations involving distance, price, mass and time should be organised carefully. Convert all quantities to compatible units before comparing them.

The approximate combined weight of all humans on Earth is 320 billion kg.
An estimate should be expressed with suitable significant figures because the input values are approximate.
Price per item = total price ÷ number of items.
When comparing special offers, compare the price of the same quantity, such as one 500-gram yoghurt pot.
For rectangular cutting problems, test both orientations of the smaller rectangle on the board.
A 15 cm × 20 cm board can hold three 6 cm × 10 cm rectangles when arranged efficiently, but not four because of the board dimensions.
Ten identical 6 cm × 10 cm rectangles therefore require at least three boards if the arrangement gives three rectangles per board.
For journey problems, first find the total distance and then compare the distances driven by each person.

Logic, Relationships and Classification

Logical reasoning identifies relationships from statements, diagrams or tables. A conclusion must use all the relevant information and must not assume a relationship that has not been shown.

A two-way table is useful when two characteristics are being compared. A relationship is suggested when the distribution of one characteristic changes with the other, but further investigation may be needed to establish causation.

If all observed mice are classified as fat or thin and as black-tailed or white-tailed, arrange the observations in a two-by-two table.
A link appears to exist when the tail-colour pattern is associated with mouse size in the observations.
Association does not prove that one characteristic causes the other.
In a social network diagram, each dot represents a person.
A line joining two dots represents a relationship, such as two people having met.
The number of lines connected to a dot gives the number of direct relationships of that person.
A completed graph should be checked against every stated relationship, including relationships that are absent.
An answer based on a diagram must follow the connections shown, not the visual position of the dots.

Biological Information, Organisms and Data Protection

Scientific reasoning also involves recognising biological definitions and understanding how information is preserved. DNA carries hereditary information, and recombinant DNA combines genetic material from different sources.

Classification questions require attention to defining features and habitat. Data protection questions require matching the technique with its purpose, such as recovery after data loss.

A nucleotide contains a sugar, a phosphate group and a nitrogenous base.
DNA has two nucleotide strands arranged in a double helix.
The sugar-phosphate backbone is connected to the bases by bonds between complementary base pairs.
Recombinant DNA is DNA produced using genetic material from two different organisms.
Echinoderms are marine invertebrates with a hard, often spiny covering or skin.
Echinoderms may be found in offshore mud and ooze, and many have an extensive fossil record.
Data backup means making a separate copy of data so that it can be recovered after loss or system failure.
A backup is different from data deletion, encryption or password protection because its main purpose is recovery.

Key terms

Scientific reasoning: The use of evidence, logic, measurements and scientific principles to reach a conclusion.
Hypothesis: A testable explanation proposed for an observation.
Independent variable: The factor deliberately changed in an experiment.
Dependent variable: The factor measured as a result of changing the independent variable.
Control group: A comparison group that is not exposed to the experimental change.
Rate: The amount of change divided by the time taken.
Inertia: The tendency of matter to resist a change in rest or uniform straight-line motion.
Resonance: Strong vibration produced when a system is driven near its natural frequency.
Potential difference: The electrical energy transferred per unit charge between two points, measured in volts.
Resistance: The opposition offered by a material to the flow of electric current.
Ohm's law: The relationship V = IR between potential difference, current and resistance.
Oxidation: A chemical process involving loss of electrons, addition of oxygen or removal of hydrogen.
Hypertonic solution: A solution with a higher solute concentration than the cell or solution being compared.
Plasmolysis: The pulling away of a plant cell membrane from the cell wall after water loss.
Optimum temperature: The temperature at which a process, such as growth, occurs at its greatest measured rate.
Recombinant DNA: DNA formed by combining genetic material from two different organisms.
Echinoderm: A marine invertebrate with a hard, commonly spiny body covering.
Data backup: A separate copy of data kept so the data can be restored after loss or damage.

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Analytical and Logical Reasoning shortcuts

Choosing the stronger supporting argument

Choose the option that directly explains why the claim is true. Prefer relevant evidence with a clear link to the claim, not a statement that is merely related.

Identify the claim and ask, “Why would this make the claim true?”
Reject personal opinions, unrelated facts and extreme statements.
Example: “Teachers should be paid more” is supported by “They shape future generations.”

This shortcut does not apply when the option gives direct statistical or experimental evidence that is stronger than a general reason.

Finding the pattern in letter and number series

Check letters and numbers separately. Look for equal increases, alternating changes, multiplication, or a repeating pattern.

Convert letters to positions if needed, such as A = 1 and D = 4.
Find the change in the letters and in the numbers independently.
Example: A2, D4, G6, J8 has letters increasing by 3 and numbers by 2, so the answer is M10.

Do not assume a simple addition pattern if alternating or multiplication patterns fit all the given terms.

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