Exploration Class 9 CBSE Important Questions with Answers

Answer: (c) mole.

Amount of substance is a fundamental quantity measured in the mole ($\text{mol}$mol). The kilogram measures mass, the candela luminous intensity, and the kelvin temperature.

Answer: (d) Speed.

Speed is derived because $\text{speed}=\dfrac{\text{distance}}{\text{time}}$speed=distance/time, a combination of the fundamental quantities length and time. Length, time and mass are themselves fundamental.

Answer: (c) $2500\ \text{m}$2500 m.

Since $1\ \text{km}=1000\ \text{m}$1 km=1000 m, we have $2.5\ \text{km}=2.5\times1000\ \text{m}=2500\ \text{m}$2.5 km=2.5×1000 m=2500 m.

Answer: (b). A law describes what always happens, often as an exact relation that can be tested by measurement; (b) is the law of conservation of mass. Options (a), (c) and (d) explain why things happen (the particle theory of matter, the cell theory, Dalton's atomic theory), which is what a theory does.

Answer: (a) Both A and R are true and R is the correct explanation of A. A physical quantity $=\text{numerical value}\times\text{unit}$=numerical value× unit, so a bare number like $5$5 is meaningless without stating whether it is $5\ \text{m}$5 m, $5\ \text{kg}$5 kg, etc.

The main steps are:

1. Observation of a phenomenon.
2. Framing a question.
3. Proposing a hypothesis (a testable guess).
4. Performing a controlled experiment.
5. Analysing the data collected.
6. Drawing a conclusion (accepting or rejecting the hypothesis).

A scientific model is a simplified representation of a real object, system or process, used to understand, explain or predict how it behaves, especially when the real thing is too small, too large or too complex to study directly.

Examples (any two): Bohr's model of the atom (chemistry and physics); a globe as a model of the Earth (earth science); a ball-and-stick model of a molecule; a model of the human heart or of a cell (biology).

Limitation: a model leaves out details, so it is never exactly like the real thing and may fail in some situations; it must be revised when new evidence disagrees with it.

The SI (International System of Units) is preferred because:

1. It is internationally accepted, so scientists everywhere use the same units and can share results without confusion.
2. It is a decimal (metric) system — conversions use powers of $10$10 (e.g. $1\ \text{m}=100\ \text{cm}$1 m=100 cm), making calculations easy.
3. It is coherent and complete — a small set of base units generates all derived units without arbitrary conversion factors.

Older systems (like the foot-pound-second system) used awkward factors such as $12$12 inches to a foot, which made comparison difficult.

First convert to the same unit. $80\ \text{cm}=0.8\ \text{m}$80 cm=0.8 m.

Area in $\text{m}^2$m²:
$A=1.2\ \text{m}\times0.8\ \text{m}=0.96\ \text{m}^2$A=1.2 m×0.8 m=0.96 m²

Area in $\text{cm}^2$cm²: Since $1\ \text{m}=100\ \text{cm}$1 m=100 cm, $1\ \text{m}^2=10000\ \text{cm}^2$1 m²=10000 cm².
$A=0.96\times10000=9600\ \text{cm}^2$A=0.96×10000=9600 cm²

If the two sides had been multiplied as $1.2\times80$1.2×80 without converting, the answer $96$96 would be in mixed $\text{m}\cdot\text{cm}$m· cm and physically meaningless. Consistent units are essential for a correct result.

A good scientific idea makes predictions that can be tested. If the predictions come true, our confidence in the idea grows; if they fail, the idea has to be changed or replaced. This is how science corrects itself.

Thomson's model pictured the atom as a sphere of spread-out positive charge with electrons embedded in it. It predicted that fast alpha particles fired at a thin gold foil would pass straight through with only tiny deflections.

In Rutherford's experiment most did pass straight through, but a few were deflected through large angles and about $1$1 in $12000$12000 bounced back. The prediction failed, so the model was revised: Rutherford proposed that the positive charge and almost all the mass are concentrated in a tiny nucleus.

Hypothesis: Plants grow taller when exposed to more sunlight.

Method:

1. Take several identical potted plants of the same species, age and size, planted in the same soil.
2. Place them in locations receiving different amounts of sunlight — for example $2$2, $4$4 and $8$8 hours per day — while keeping water, soil and pot size the same.
3. Keep one plant in a fixed, ordinary condition to act as the control for comparison.
4. Measure each plant's height with a metre scale every day for two weeks, recording data in a table.

Variables:

• Independent variable (the one changed): hours of sunlight.
• Dependent variable (the one measured): height of the plant.
• Controlled variables (kept constant): water, soil, temperature, species.

Analysis and conclusion: Plot height against time for each group. If the plants with more sunlight grow measurably taller while all else is equal, the data support the hypothesis; otherwise it is rejected. Repeating with several plants per group reduces the effect of chance and makes the conclusion reliable.

The seven SI base quantities, their units and symbols are:

1. Length — metre — $\text{m}$m
2. Mass — kilogram — $\text{kg}$kg
3. Time — second — $\text{s}$s
4. Electric current — ampere — $\text{A}$A
5. Temperature — kelvin — $\text{K}$K
6. Amount of substance — mole — $\text{mol}$mol
7. Luminous intensity — candela — $\text{cd}$cd

Building the unit of density:

Density is defined as mass per unit volume:
$\text{density}=\frac{\text{mass}}{\text{volume}}$density=mass/volume

Mass has the base unit $\text{kg}$kg. Volume is a derived quantity, $\text{length}\times\text{breadth}\times\text{height}$length× breadth× height, with unit $\text{m}\times\text{m}\times\text{m}=\text{m}^3$m× m× m=m³.

Therefore the SI unit of density is:
$\frac{\text{kg}}{\text{m}^3}=\text{kg/m}^3$kg/m³=kg/m³

This shows how a physically meaningful derived unit is assembled entirely from base units, which is the strength of the coherent SI system.

(i) An estimate is a reasonable approximate value worked out from simple assumptions, without exact measurement. It quickly shows the size of a quantity, helps plan experiments and spot answers that are clearly wrong.

(ii) $1200\times1.5=1800$1200×1.5=1800 litres per day.

(iii) $\dfrac{1800}{1000}=1.8\ \text{m}^3$1800/1000=1.8 m³.

(iv) $\dfrac{5000}{1800}\approx2.8$5000/1800≈2.8, so a full tank lasts a little under $3$3 school days.