The IISER Aptitude Test (IAT) 2026 question paper had 60 multiple-choice questions across Biology, Chemistry, Mathematics and Physics. Each subject had 15 questions, and students had three hours to attempt the paper. This article explains selected questions with their answers and calculation steps, followed by observations about the four sections. The complete question paper PDF and available solutions are linked below.

The paper included genetics, chemical reactions, coordinate geometry, electric forces and several other topics. Some questions could be answered by applying one concept. Others required students to follow a sequence of changes, read a diagram or connect a formula to the conditions given in the question.

IISER IAT 2026 paper pattern

Each subject therefore carried 60 marks. Equal marks did not mean that every question needed equal working time. For example, identifying the outcome of a genetic cross involved a short set of possible combinations, while a solution-concentration question required tracking both the amount of solute and the changing volume.

Section-wise paper analysis

Biology

The Biology questions covered photosynthesis, plant vascular systems, interactions between organisms, biotechnology, hormones and inheritance. Several used diagrams or graphs, including enzyme activity plots, a pedigree and lac operon arrangements. These questions asked students to interpret the information shown, not just recognise a topic name. The pBR322 and blood-group questions below illustrate how a familiar concept can be tested through a specific situation.

Chemistry

Chemistry included coordination compounds, redox reactions, molecular orbitals, organic reactions, chromatography and solution concentration. The dichromate question tested electron transfer and balancing in an acidic medium. The NaOH question tested dilution, removal of a portion of the solution and mixing. Other questions used reaction drawings or experimental information, so the conditions given with the question were part of the problem.

Mathematics

The Mathematics section included three-dimensional geometry, functions, sequences, statistics and remainders. Some questions could be simplified before doing a long calculation. The angle between two lines came from their direction vectors, while the progression question required using a recurrence relation to find the common ratio. Conditions such as positive terms or real values could remove an otherwise possible algebraic answer.

Physics

Physics included mechanics, optics, electricity, magnetism, radiation and thermodynamics. Questions used situations such as an elastic collision, a charged pendulum, a particle moving in an electric field and light striking a surface. Several asked for a ratio or how a quantity changed. Identifying which quantities stayed fixed was important before substituting into a formula.

Some of the Questions of the Paper

The following questions are shortened versions of questions in the linked paper. Answers are given by their meaning or value, with the working needed to understand them.

Biology: inserting DNA into pBR322 (Question 4)

Question: A foreign DNA fragment is inserted into the tetracycline-resistance gene of pBR322. After the recombinant plasmid enters E. coli, which antibiotic can the cells grow in?

Answer: They can grow in ampicillin, but not in tetracycline.

The insertion disrupts the tetracycline-resistance gene. This is called insertional inactivation. The ampicillin-resistance gene remains intact, so it still allows the transformed cells to survive in ampicillin. Cells carrying the original, non-recombinant plasmid retain both resistance genes. The location of the inserted DNA is what separates the two outcomes; the question does not say that both genes are damaged.

Biology: blood groups of children (Question 12)

Question: Which parental blood-group combination gives children whose blood group is different from that of either parent?

Answer: An AB father and an O mother, under the usual ABO inheritance model.

The father's genotype is I^AI^B, while the mother's is ii. The father can pass on I^A or I^B, and the mother passes on i. The possible children are therefore I^Ai, with group A, and I^Bi, with group B. Neither outcome is AB or O. Each has a probability of one-half for an individual child; this does not mean that every family must have equal numbers of A and B children.

Chemistry: reduction of dichromate (Question 17)

Question: How many protons and electrons are needed to reduce one dichromate ion to chromium(III) ions in an acidic solution?

Answer: 14 protons and 6 electrons.

The balanced half-reaction is:

Cr₂O₇^2- + 14H⁺ + 6e^- → 2Cr³⁺ + 7H₂O

Each chromium atom changes from oxidation state +6 to +3 and gains three electrons. Two chromium atoms therefore need six electrons. Seven water molecules balance the oxygen atoms, and 14 hydrogen ions balance the hydrogen atoms. As a final check, the total charge on each side is +6.

Chemistry: dilution and mixing of NaOH (Question 26)

Question: A 100 mL sample of 1.0 M NaOH is diluted to 1.0 L. Half is discarded, then 100 mL of 0.5 M NaOH is added to the remaining solution. Find the final concentration.

Answer: Approximately 0.17 M.

1. The starting solution contains 1.0 × 0.100 = 0.100 mol of NaOH.
2. Dilution changes the volume to 1.0 L but leaves 0.100 mol of NaOH.
3. Discarding half leaves 0.050 mol in 0.500 L.
4. The added solution supplies 0.5 × 0.100 = 0.050 mol.
5. The final solution has 0.100 mol in 0.600 L, giving 0.100/0.600 = 0.1667 M.

Discarding half of a well-mixed solution halves its volume and its solute amount together. It does not halve its concentration. Also, the two concentrations cannot simply be averaged because the volumes being mixed are different.

Mathematics: angle between two lines (Question 34)

Question: Find the acute angle between the line through (1, 1, 1) and (3, 1, 3), and the line through (0, 2, -1) and (2, 0, 3).

Answer: 30 degrees.

Subtracting the coordinates gives direction vectors (2, 0, 2) and (2, -2, 4). These can be simplified to u = (1, 0, 1) and v = (1, -1, 2). Their dot product is 3, and their lengths are √2 and √6.

Therefore, cos θ = |u · v|/(|u||v|) = 3/(√2 × √6) = √3/2. This gives θ = 30 degrees. The points locate the lines, but their direction vectors determine the angle.

Mathematics: sum of a geometric progression (Question 42)

Question: A geometric progression has positive integer terms, starts with 3 and satisfies a_n+2 - 2a_n = a_n+1. Find the sum of its first five terms.

Answer: 93.

If the common ratio is r, then a_n+1 = ra_n and a_n+2 = r²a_n. Dividing the given relation by the non-zero term a_n gives r² - 2 = r. Thus, (r - 2)(r + 1) = 0.

The ratio is either 2 or -1. A ratio of -1 would produce negative terms, which the question excludes. With r = 2, the first five terms are 3, 6, 12, 24 and 48. Their sum is 93.

Physics: travel time in an electric field (Question 49)

Question: A charged particle starts from rest in a uniform electric field. It takes time t₁ to travel distance d when its mass is m₁, and time t₂ when its mass is m₂. Find t₁/t₂.

Answer: √(m₁/m₂).

For the same charge and field, the force magnitude is |q|E. Acceleration is therefore |q|E/m. Starting from rest, d = at²/2, so t = √(2dm/|q|E). The distance, charge and field cancel when the two times are divided. Time is proportional to the square root of mass, not mass itself. The heavier particle accelerates less and takes longer.

Physics: force exerted by light (Question 55)

Question: Light of power P strikes a flat surface normally. The surface absorbs 10%, transmits 10% and reflects the remaining light. What force does it experience?

Answer: 1.7P/c, where c is the speed of light.

The absorbed part transfers momentum at a rate 0.1P/c. The reflected part is 80% of the beam. Reflection reverses its direction, giving twice the momentum transfer: 2 × 0.8P/c = 1.6P/c. The transmitted part continues through without contributing to the force in this model. Adding the contributions gives 0.1P/c + 1.6P/c = 1.7P/c.

What these questions show about the paper

The examples show why small details in a question matter. In Biology, the inserted DNA affects one resistance gene, not both. In Chemistry, removing solution is different from adding water. In Mathematics, the words "positive integer terms" rule out one root. In Physics, reflection transfers more momentum than absorption.

Several answers can also be checked without repeating all the working. The final NaOH concentration must lie between the concentrations being mixed. A heavier particle should take longer to cover the same distance under the same electric force. These checks help identify an inverted ratio or an incorrect volume before choosing an answer.

Frequently asked questions

How many questions were in IISER IAT 2026?

There were 60 questions, divided equally among Biology, Chemistry, Mathematics and Physics.

What were the maximum marks and duration?

The paper carried 240 marks and lasted 180 minutes. A correct answer earned four marks, while an incorrect answer lost one mark.

Does this article contain the complete paper?

No. It explains selected questions from all four subjects. The complete-paper page linked above provides access to the question paper and a solution PDF.

Why is the NaOH answer 0.17 M rather than 0.10 M?

The remaining 0.10 M solution receives more NaOH when the 0.50 M solution is added. The final amount is 0.100 mol in 0.600 L, giving approximately 0.17 M.