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Sri Lanka Scientific Service Exam study materials
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Sri Lanka Scientific Service Exam practice questions

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Question 1General Scientific Knowledge and Aptitude

In the SI system, which base unit is used to express the amount of substance in a sample analysed in a laboratory?

  1. Kelvin
  2. Mole Correct answer.
  3. Candela
  4. Kilogram

Explanation

The mole is the SI base unit for amount of substance, defined by a fixed Avogadro number of elementary entities. It is distinct from mass, luminous intensity, and thermodynamic temperature, which have their own base units.

Question 2General Scientific Knowledge and Aptitude

A scientific report states a length as 0.00450 m. How many significant figures does this measurement contain?

  1. Six
  2. Three Correct answer.
  3. Two
  4. Five

Explanation

Leading zeros are never significant, but trailing zeros after a decimal point are significant. Counting the digits 4, 5 and the final 0 gives three significant figures.

Question 3General Scientific Knowledge and Aptitude

Which particle carries a negative charge and orbits the nucleus of an atom?

  1. Proton
  2. Positron
  3. Electron Correct answer.
  4. Neutron

Explanation

Electrons are negatively charged and occupy regions around the nucleus. Protons are positive, neutrons are neutral, and positrons are positive antimatter particles not found in ordinary atoms.

Question 4General Scientific Knowledge and Aptitude

A car travels 150 km in 2.5 hours. What is its average speed?

  1. 75 km/h
  2. 375 km/h
  3. 50 km/h
  4. 60 km/h Correct answer.

Explanation

Average speed equals total distance divided by total time, so 150 km divided by 2.5 h gives 60 km/h. The calculation uses consistent units throughout.

Question 5General Scientific Knowledge and Aptitude

Which gas is the most abundant by volume in the Earth's lower atmosphere?

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

Explanation

Nitrogen makes up roughly 78 percent of the lower atmosphere by volume, far exceeding other gases. Oxygen is second at about 21 percent, while argon and carbon dioxide are minor components.

Question 6General Scientific Knowledge and Aptitude

In a laboratory, which piece of glassware is designed to deliver a single fixed volume with the highest precision?

  1. Conical flask
  2. Measuring cylinder
  3. Volumetric flask Correct answer.
  4. Beaker

Explanation

A volumetric flask is calibrated to contain one exact volume at a stated temperature and is used for preparing standard solutions. Beakers and conical flasks are only approximate, and measuring cylinders are less precise than volumetric glassware.

Question 7General Scientific Knowledge and Aptitude

Which of the following is a fundamental characteristic that distinguishes living organisms from non-living matter?

  1. Capacity for metabolism and reproduction Correct answer.
  2. Fixed crystalline structure
  3. Presence of carbon atoms
  4. Ability to conduct electricity

Explanation

Metabolism and reproduction are core properties of living systems, enabling growth, energy use and continuity of the species. Carbon presence, conductivity and crystallinity occur in many non-living materials as well.

Question 8General Scientific Knowledge and Aptitude

What is the value of 6 × 10³ expressed in ordinary decimal notation?

  1. 600,000
  2. 60,000
  3. 6,000 Correct answer.
  4. 600

Explanation

The factor 10³ equals 1,000, so multiplying by 6 gives 6,000. Standard form converts directly by moving the decimal point three places to the right.

Question 9General Scientific Knowledge and Aptitude

A solution has a hydrogen ion concentration of 1 × 10⁻⁴ mol dm⁻³. What is its pH?

  1. 10
  2. 4 Correct answer.
  3. 14
  4. 2

Explanation

pH is the negative base-10 logarithm of hydrogen ion concentration, so −log(10⁻⁴) equals 4. The solution is therefore weakly acidic.

Question 10General Scientific Knowledge and Aptitude

A gas occupies 300 cm³ at 300 K. Assuming constant pressure, what volume will it occupy at 600 K?

  1. 300 cm³
  2. 600 cm³ Correct answer.
  3. 900 cm³
  4. 150 cm³

Explanation

Charles's law states that volume is directly proportional to absolute temperature at constant pressure. Doubling the temperature from 300 K to 600 K doubles the volume to 600 cm³.

Question 11General Scientific Knowledge and Aptitude

Which type of chemical bond involves the electrostatic attraction between oppositely charged ions?

  1. Metallic bond
  2. Hydrogen bond
  3. Covalent bond
  4. Ionic bond Correct answer.

Explanation

An ionic bond forms when electrons are transferred and the resulting oppositely charged ions attract electrostatically. Covalent bonds share electrons, metallic bonds involve delocalised electrons, and hydrogen bonds are weaker intermolecular attractions.

Question 12General Scientific Knowledge and Aptitude

In a food chain, what term describes organisms that manufacture their own food using sunlight?

  1. Tertiary consumers
  2. Primary consumers
  3. Producers Correct answer.
  4. Decomposers

Explanation

Producers, such as green plants, use photosynthesis to convert light energy into chemical energy. Consumers obtain energy by eating other organisms, and decomposers break down dead matter.

Question 13General Scientific Knowledge and Aptitude

A 2 kg object accelerates at 3 m/s². What net force acts on it?

  1. 1.5 N
  2. 5 N
  3. 12 N
  4. 6 N Correct answer.

Explanation

Newton's second law gives force as mass multiplied by acceleration, so 2 kg × 3 m/s² equals 6 N. The newton is the SI unit of force.

Question 14General Scientific Knowledge and Aptitude

Which organelle is the principal site of ATP synthesis in a eukaryotic cell?

  1. Ribosome
  2. Mitochondrion Correct answer.
  3. Golgi apparatus
  4. Lysosome

Explanation

Mitochondria host oxidative phosphorylation, generating most of the cell's ATP. Ribosomes build proteins, the Golgi apparatus modifies and packages them, and lysosomes carry out intracellular digestion.

Question 15General Scientific Knowledge and Aptitude

A student measures the mass of a sample as 5.20 g, but the accepted value is 5.00 g. What is the percentage error?

  1. 4.0% Correct answer.
  2. 20%
  3. 2.0%
  4. 5.0%

Explanation

Percentage error is the absolute difference divided by the accepted value, multiplied by 100. Here 0.20 g divided by 5.00 g gives 0.04, which is 4.0 percent.

Question 16General Scientific Knowledge and Aptitude

Which process moves water molecules across a selectively permeable membrane from a region of higher water potential to lower water potential?

  1. Phagocytosis
  2. Osmosis Correct answer.
  3. Diffusion of solutes
  4. Active transport

Explanation

Osmosis is the net movement of water across a selectively permeable membrane down a water potential gradient. It requires no energy input and differs from solute diffusion and active transport.

Question 17General Scientific Knowledge and Aptitude

Which statement correctly describes an exothermic reaction?

  1. It proceeds only when light is present.
  2. It always produces a gas.
  3. It releases heat to the surroundings and feels warm. Correct answer.
  4. It absorbs heat from the surroundings and feels cold.

Explanation

In an exothermic reaction, the products have lower enthalpy than the reactants, so excess energy is released as heat to the surroundings. Endothermic reactions absorb heat and feel cold.

Question 18General Scientific Knowledge and Aptitude

A body of mass 10 kg is lifted vertically through 2 m. Taking gravitational acceleration as 10 m/s², what is the gain in gravitational potential energy?

  1. 100 J
  2. 500 J
  3. 200 J Correct answer.
  4. 20 J

Explanation

Gravitational potential energy gained equals mass × gravitational acceleration × height, so 10 × 10 × 2 gives 200 J. This follows from the work done against gravity.

Question 19General Scientific Knowledge and Aptitude

In a titration, 25.0 cm³ of 0.10 mol dm⁻³ sodium hydroxide is neutralised by hydrochloric acid of the same concentration. What volume of acid is required?

  1. 12.5 cm³
  2. 25.0 cm³ Correct answer.
  3. 100.0 cm³
  4. 50.0 cm³

Explanation

Sodium hydroxide and hydrochloric acid react in a 1:1 mole ratio. Equal concentrations therefore require equal volumes, so 25.0 cm³ of acid neutralises the alkali.

Question 20General Scientific Knowledge and Aptitude

Which observation provides the strongest evidence that a chemical change has occurred?

  1. A liquid is heated and begins to boil.
  2. A mixture is stirred and becomes cloudy.
  3. A new gas is formed with a distinct smell. Correct answer.
  4. A solid dissolves in water.

Explanation

Formation of a new substance with properties unlike the reactants indicates a chemical change. Dissolving, boiling and temporary cloudiness are usually physical changes that do not create new substances.

Question 21General Scientific Knowledge and Aptitude

A population of 500 bacteria doubles every 3 hours. Starting from this population, how many bacteria are present after 12 hours?

  1. 2,000
  2. 8,000 Correct answer.
  3. 4,000
  4. 1,000

Explanation

Twelve hours contains four doubling intervals of three hours each. Multiplying 500 by 2⁴, which is 16, gives 8,000 bacteria.

Question 22General Scientific Knowledge and Aptitude

Two resistors of 6 Ω and 3 Ω are connected in parallel. What is the effective resistance of the combination?

  1. 2 Ω Correct answer.
  2. 1 Ω
  3. 9 Ω
  4. 4.5 Ω

Explanation

For parallel resistors, the reciprocal of total resistance equals the sum of reciprocals, so 1/6 + 1/3 = 1/2, giving 2 Ω. The result is always less than the smallest individual resistance.

Question 23General Scientific Knowledge and Aptitude

A reaction has an activation energy of 60 kJ mol⁻¹ without a catalyst. A catalyst lowers this to 40 kJ mol⁻¹. What effect does the catalyst have on the equilibrium position of a reversible reaction?

  1. It shifts equilibrium towards the reactants.
  2. It changes the equilibrium constant to a higher value.
  3. It leaves the equilibrium position unchanged while speeding both directions. Correct answer.
  4. It shifts equilibrium towards the products.

Explanation

A catalyst lowers the activation energy for both forward and reverse reactions equally, so equilibrium is reached faster but the position and equilibrium constant remain unchanged. Only temperature alters the equilibrium constant for a given reaction.

Question 24General Scientific Knowledge and Aptitude

A sample of an element contains two isotopes with masses 10 u and 11 u in the abundance ratio 1:4. What is the relative atomic mass of the element?

  1. 10.2
  2. 10.8 Correct answer.
  3. 10.5
  4. 11.0

Explanation

The weighted average is (10 × 1 + 11 × 4) divided by 5, which equals 54/5 = 10.8. The weighted mean lies closer to the more abundant isotope.

Question 25General Scientific Knowledge and Aptitude

In a genetics cross between two heterozygous individuals, Tt × Tt, what proportion of the offspring is expected to show the recessive phenotype?

  1. 1/4 Correct answer.
  2. 0
  3. 3/4
  4. 1/2

Explanation

A Tt × Tt cross yields genotypes TT, Tt, Tt and tt in equal proportions. Only tt expresses the recessive phenotype, so one quarter of the offspring are expected to show it.

Question 26General Scientific Knowledge and Aptitude

A Sri Lankan researcher records a measurement as 0.00450 m. How many significant figures does this value contain?

  1. 6
  2. 5
  3. 2
  4. 3 Correct answer.

Explanation

Significant figures count all digits from the first non-zero digit onward. In 0.00450 the leading zeros are placeholders, so the significant digits are 4, 5 and the trailing zero, giving three.

Question 27General Scientific Knowledge and Aptitude

Which SI base unit is used to express the amount of substance in a sample?

  1. kelvin
  2. candela
  3. mole Correct answer.
  4. ampere

Explanation

The mole is the SI base unit for amount of substance and is defined through the fixed numerical value of the Avogadro constant. The other units measure luminous intensity, thermodynamic temperature and electric current respectively.

Question 28General Scientific Knowledge and Aptitude

A microscope image of a specimen is magnified 400 times, and the specimen appears 20 mm long in the image. What is the actual length of the specimen?

  1. 0.5 mm
  2. 5 mm
  3. 80 mm
  4. 0.05 mm Correct answer.

Explanation

Actual size equals image size divided by magnification, so 20 mm ÷ 400 = 0.05 mm. This conversion between image and actual dimensions is a standard microscopy skill.

Question 29General Scientific Knowledge and Aptitude

In a scientific report, a measurement is quoted as 12.3 cm ± 0.1 cm. What does the ± 0.1 cm term represent?

  1. The percentage error of the instrument
  2. The mean of repeated readings
  3. The uncertainty associated with the measurement Correct answer.
  4. The resolution of the observer's eye

Explanation

The ± term expresses the estimated uncertainty or range within which the true value is likely to lie. It is not itself a percentage error or a mean, though it may be used to calculate those quantities.

Question 30General Scientific Knowledge and Aptitude

Which statistical measure is most appropriate for describing the spread of a data set that contains one extreme outlier?

  1. Interquartile range Correct answer.
  2. Range
  3. Mean
  4. Standard deviation

Explanation

The interquartile range describes the spread of the middle 50% of values and is resistant to extreme outliers. Standard deviation and range are both strongly inflated by a single outlying value.

Question 31General Scientific Knowledge and Aptitude

A student plots a graph of extension against load for a spring and obtains a straight line through the origin. What does the gradient of this line represent?

  1. The spring constant
  2. The total work done on the spring
  3. The reciprocal of the spring constant Correct answer.
  4. The elastic potential energy stored

Explanation

Hooke's law gives extension = (1/k) × load, so plotting extension on the vertical axis against load gives a gradient equal to 1/k. The spring constant itself would be the gradient only if load were plotted against extension.

Question 32General Scientific Knowledge and Aptitude

Which of the following quantities is a vector?

  1. Temperature
  2. Displacement Correct answer.
  3. Speed
  4. Mass

Explanation

A vector has both magnitude and direction, and displacement satisfies both requirements. Speed, mass and temperature are scalars described fully by magnitude alone.

Question 33General Scientific Knowledge and Aptitude

A car travels 150 m in 10 s and then 150 m in 20 s. What is its average speed over the whole journey?

  1. 7.5 m/s
  2. 10 m/s Correct answer.
  3. 15 m/s
  4. 12.5 m/s

Explanation

Average speed is total distance divided by total time: 300 m ÷ 30 s = 10 m/s. Averaging the two speeds directly would give a misleading value because the time intervals differ.

Question 34General Scientific Knowledge and Aptitude

Which statement describes the function of a buffer solution?

  1. It neutralises any quantity of acid added to it completely
  2. It increases the solubility of gases in water
  3. It resists changes in pH when small amounts of acid or alkali are added Correct answer.
  4. It changes colour sharply at the equivalence point of a titration

Explanation

A buffer contains a weak acid and its conjugate base, or a weak base and its conjugate acid, and resists pH change on addition of small amounts of acid or alkali. It cannot neutralise unlimited quantities, and sharp colour change is the role of an indicator.

Question 35General Scientific Knowledge and Aptitude

Which reagent is used to test for the presence of reducing sugars in a food sample?

  1. Biuret solution
  2. Ethanol emulsion test
  3. Iodine solution
  4. Benedict's solution Correct answer.

Explanation

Benedict's solution contains copper(II) ions that are reduced to a brick-red copper(I) precipitate by reducing sugars on heating. The other reagents test for protein, starch and lipid respectively.

Question 36General Scientific Knowledge and Aptitude

Which structure of a nephron is primarily responsible for the reabsorption of glucose from the filtrate?

  1. Proximal convoluted tubule Correct answer.
  2. Bowman's capsule
  3. Collecting duct
  4. Descending limb of the loop of Henle

Explanation

The proximal convoluted tubule has a brush border and abundant mitochondria, enabling active co-transport that reclaims nearly all glucose from the filtrate. The loop of Henle mainly adjusts water and ion balance.

Question 37General Scientific Knowledge and Aptitude

In a plant, which tissue is chiefly responsible for the transport of sucrose from leaves to storage organs?

  1. Xylem
  2. Phloem Correct answer.
  3. Epidermis
  4. Meristem

Explanation

Phloem sieve tubes carry sucrose and other assimilates from sources such as leaves to sinks such as roots and fruits. Xylem transports water and dissolved mineral ions upward.

Question 38General Scientific Knowledge and Aptitude

A bacterial population grows exponentially with a doubling time of 2 hours. Starting from 1 000 cells, how many cells are present after 8 hours?

  1. 8 000
  2. 4 000
  3. 256 000
  4. 16 000 Correct answer.

Explanation

Eight hours contains four doubling periods, so the population is multiplied by 2⁴ = 16, giving 1 000 × 16 = 16 000 cells. Exponential growth multiplies rather than adds.

Question 39General Scientific Knowledge and Aptitude

Which of the following is an example of a derived SI unit expressed in base units?

  1. The second
  2. The mole
  3. The kilogram
  4. The newton, equal to kg m s⁻² Correct answer.

Explanation

Derived units are combinations of base units, and the newton is defined as kg m s⁻². The kilogram, second and mole are themselves SI base units.

Question 40General Scientific Knowledge and Aptitude

A solution is prepared by dissolving 4.0 g of sodium hydroxide (molar mass 40 g mol⁻¹) to make 500 cm³ of solution. What is its concentration?

  1. 0.20 mol dm⁻³ Correct answer.
  2. 2.0 mol dm⁻³
  3. 0.10 mol dm⁻³
  4. 0.40 mol dm⁻³

Explanation

The amount is 4.0 ÷ 40 = 0.10 mol, and the volume is 0.500 dm³. Dividing gives 0.10 ÷ 0.500 = 0.20 mol dm⁻³.

Question 41General Scientific Knowledge and Aptitude

Which type of isomerism arises when two compounds have the same molecular formula but different arrangements of atoms around a carbon-carbon double bond?

  1. Chain isomerism
  2. Geometric (cis-trans) isomerism Correct answer.
  3. Functional group isomerism
  4. Optical isomerism

Explanation

Restricted rotation about a double bond allows groups to lie on the same side (cis) or opposite sides (trans), which is geometric isomerism. Optical isomerism involves chirality, while chain and functional group isomerism involve different skeletons or functional groups.

Question 42General Scientific Knowledge and Aptitude

In a photoelectric experiment, increasing the intensity of incident light while keeping its frequency constant causes which change?

  1. The maximum kinetic energy of emitted electrons increases
  2. More photoelectrons are emitted per second Correct answer.
  3. The threshold frequency of the metal increases
  4. The stopping potential increases

Explanation

Intensity determines the number of photons arriving per second, so more electrons are released per second. The maximum kinetic energy depends only on the frequency of the light and the work function of the metal.

Question 43General Scientific Knowledge and Aptitude

Which of the following best describes the role of a control in a scientific experiment?

  1. It guarantees that the hypothesis is correct
  2. It ensures that all variables change at once
  3. It eliminates the need for repeated measurements
  4. It provides a baseline against which the effect of the independent variable is compared Correct answer.

Explanation

A control is treated identically to the experimental group except that the independent variable is absent or held constant, giving a baseline for comparison. It does not remove the need for replicates or guarantee any outcome.

Question 44General Scientific Knowledge and Aptitude

A radioactive isotope has a half-life of 6 days. What fraction of the original sample remains after 18 days?

  1. 1/8 Correct answer.
  2. 1/18
  3. 1/6
  4. 1/3

Explanation

Eighteen days is three half-lives, so the remaining fraction is (1/2)³ = 1/8. Each half-life halves the quantity present.

Question 45General Scientific Knowledge and Aptitude

Which of the following is the correct order of stages in the scientific problem-solving cycle used in experimental research?

  1. Conclusion, hypothesis, observation, experiment, analysis
  2. Hypothesis, observation, conclusion, experiment, analysis
  3. Experiment, observation, hypothesis, analysis, conclusion
  4. Observation, hypothesis, experiment, analysis, conclusion Correct answer.

Explanation

Scientific inquiry typically begins with observation and a question, leads to a testable hypothesis, proceeds to controlled experimentation, then analysis of data and finally a conclusion. Each stage depends on the results of the preceding one.

Question 46General Scientific Knowledge and Aptitude

A quantity y is related to x by the equation y = ax². A graph of y against x² gives a straight line. What does the gradient of that line represent?

  1. The constant 2a
  2. The square root of a
  3. The reciprocal of a
  4. The constant a Correct answer.

Explanation

Plotting y against x² linearises the relationship, giving a straight line of gradient a and zero intercept. The constant a is the coefficient of x² in the original equation.

Question 47General Scientific Knowledge and Aptitude

Which of the following best explains why a large sample size improves the reliability of an experimental conclusion?

  1. It eliminates the need for a control group
  2. It guarantees that the results are statistically significant
  3. It reduces the effect of random variation on the calculated mean Correct answer.
  4. It removes all systematic errors from the measurement

Explanation

A larger sample averages out random fluctuations, so the estimated mean is closer to the true value. Systematic errors and the need for controls are unaffected by sample size.

Question 48General Scientific Knowledge and Aptitude

A reaction between two solutions produces a precipitate. Which technique is most suitable for separating the precipitate from the liquid?

  1. Chromatography
  2. Sublimation
  3. Simple distillation
  4. Filtration Correct answer.

Explanation

Filtration separates an insoluble solid from a liquid by passing the mixture through filter paper. Distillation separates dissolved substances by boiling point, chromatography separates soluble components, and sublimation applies to solids that vaporise directly.

Question 49General Scientific Knowledge and Aptitude

An object is placed 30 cm from a converging lens of focal length 10 cm. Where is the image formed?

  1. 30 cm on the opposite side of the lens
  2. 15 cm on the opposite side of the lens Correct answer.
  3. 10 cm on the opposite side of the lens
  4. 20 cm on the same side as the object

Explanation

Using 1/f = 1/v − 1/u with u = −30 cm and f = +10 cm gives 1/v = 1/10 − 1/30 = 2/30, so v = +15 cm. The positive sign confirms a real image on the opposite side.

Question 50General Scientific Knowledge and Aptitude

Which of the following statements about enzymes is correct?

  1. They alter the equilibrium position of the reaction
  2. They are consumed permanently during the reaction
  3. They increase the activation energy so that only some molecules react
  4. They lower the activation energy of the reaction they catalyse Correct answer.

Explanation

Enzymes provide an alternative reaction pathway with a lower activation energy, increasing the proportion of molecules with sufficient energy to react. They are not consumed and do not change the equilibrium position.