9th Class Chemistry Notes 2026 — Chapter 12
Class 9 Chemistry Notes – Chapter 12
Empirical Data Collection and Analysis
Textbook Exercise Solutions
1. Tick (✓) the Correct Answer
(i) Which of the following pairs of quantities may be measured in the same unit?
Answer: (c) Heat and work (both are forms of energy measured in joules).
(ii) In which unit do we usually measure the energy present in food?
Answer: (c) Calorie
(iii) What prefix is used for 10⁻¹²?
Answer: (b) Pico
(iv) The SI unit of pressure is expressed in:
Answer: (d) Pascal (Pa), equivalent to newton per square metre (N/m²).
(v) Which symbol is used for kilogram in SI units?
Answer: (d) kg
(vi) What does a mole represent?
Answer: (a) Number (a mole represents Avogadro’s number of particles).
(vii) Which unit of volume should usually be used in Chemistry?
Answer: (c) Cubic centimetre (cm³).
(viii) Express 0.000840 in scientific notation.
Answer: (c) 8.40 × 10⁻⁴
(ix) In SI units, the prefix nano means:
Answer: (a) 10⁻⁹
(x) 65°C is equivalent to:
Answer: (b) 338 K
Calculation:
65 + 273.15 = 338.15 K ≈ 338 K
2. Questions for Short Answers
(i) What is consistency of results?
Answer: Consistency of results refers to the degree of agreement, or precision, among several repeated measurements of the same quantity under identical experimental conditions.
(ii) Why are SI units user-friendly?
Answer: SI units are user-friendly because they are based on a decimal system using powers of 10. This makes conversions between units straightforward by multiplying or dividing by 10, 100, 1,000, or other powers of 10.
(iii) Define systematic error and random error.
Answer:
Systematic Error: A consistent error that causes measurements to deviate from the true value in a particular direction. It may result from faulty equipment, incorrect calibration, or an incorrect experimental technique.
Random Error: An unpredictable variation in measurements caused by uncontrollable factors, such as environmental fluctuations or limitations in reading instruments.
(iv) What is the reason behind a random error?
Answer: Random errors occur due to slight, unpredictable variations in environmental conditions, such as temperature and pressure fluctuations or air currents, and limitations in human perception when reading instruments.
(v) Does systematic error affect accuracy?
Answer: Yes. Systematic error reduces accuracy because it consistently shifts measured values away from the true value in one direction.
(vi) Which other systems of measurement are used apart from SI units?
Answer: Other systems include the MKS (metre-kilogram-second) system, the CGS (centimetre-gram-second) system, and the FPS (foot-pound-second) system.
(vii) Define metre.
Answer: A metre is the SI base unit of length. It is defined as the distance travelled by light in a vacuum during a time interval of 1/299,792,458 of a second.
(viii) Mention two benefits scientists get by using SI units.
Answer:
Universal Standard: SI units enable scientists worldwide to communicate and compare experimental results using a common system of measurement.
Simplified Calculations: The decimal structure of SI units makes conversions and calculations easier and reduces the likelihood of conversion errors.
3. Constructed Response Questions
(i) Compare the units in the SI system with those in the MKS system.
Answer: The MKS system uses the metre for length, kilogram for mass, and second for time. The SI system incorporates these units and extends the system with seven base units, including the kelvin, ampere, mole, and candela, to provide a comprehensive international system of measurement.
(ii) What are five basic SI units which are used in Chemistry?
Answer:
Kilogram (kg) — Mass
Metre (m) — Length
Second (s) — Time
Kelvin (K) — Temperature
Mole (mol) — Amount of substance
(iii) Explain three units derived from the basic SI units.
Answer:
1. Volume (m³): Derived by multiplying three dimensions of length.
Volume = length × width × height
Unit = m × m × m = m³
2. Density (kg/m³): Derived by dividing mass by volume.
Density = Mass ÷ Volume
Unit = kg/m³
3. Concentration (mol/m³): Derived by dividing the amount of substance in moles by volume.
Concentration = Amount of substance ÷ Volume
Unit = mol/m³
In laboratory chemistry, mol/dm³ is also commonly used.
(iv) Explain why we prefer to use smaller units of mass and volume in Chemistry.
Answer: Chemistry involves laboratory experiments that often require small quantities of substances. Measuring these quantities in kilograms or cubic metres can produce inconveniently small decimal values. Smaller units, such as grams (g), milligrams (mg), cubic centimetres (cm³), and millilitres (mL), make measurements and calculations more convenient.
(v) What difficulties can we expect to encounter if we use different units of measurement in daily life?
Answer: Using different or non-standardised units can cause confusion in trade, miscommunication among scientists, and calculation errors during unit conversions. It may also lead to serious mistakes in engineering, healthcare, and other fields where accurate measurements are essential.
4. Descriptive Questions
(i) What are our indigenous units of measurement of mass, volume, and length?
Answer:
Mass: Tola, masha, ratti, seer, and maund (mund).
Volume: Pyalee and traditional measures used for liquids and grains.
Length: Gaz, haath, and kadam.
Note: Traditional units can vary by region and context. Bigha is generally a unit of land area rather than length.
(ii) Elaborate on the difference between precision and accuracy.
Answer:
Accuracy: Accuracy refers to how close a measured value is to the true or accepted value. It is affected by systematic errors.
Precision: Precision refers to how close repeated measurements of the same quantity are to one another. It reflects consistency and reproducibility and is affected by random errors.
(iii) How can you avoid systematic errors in your measurements?
Answer:
Calibrating Instruments: Regularly calibrate balances, thermometers, and volumetric equipment against known standards.
Using Control Standards: Test known control samples alongside experimental samples to identify measurement problems.
Correcting Experimental Techniques: Avoid parallax errors, read instruments correctly, and maintain appropriate experimental conditions.
Checking Zero Errors: Ensure that instruments are properly zeroed before taking measurements.
(iv) How do measurements in SI units ensure safety and reliability?
Answer: Standardised SI measurements reduce ambiguity in pharmaceutical dosing, chemical manufacturing, structural engineering, and scientific communication. They help prevent dangerous errors caused by misunderstandings or incorrect conversions between different units.
(v) Can a student be both inaccurate and imprecise in his measurements?
Answer: Yes. If a student uses poor experimental techniques, uncalibrated equipment, or works under fluctuating conditions, repeated measurements may differ considerably from one another, making them imprecise. If these measurements are also far from the true value, they are inaccurate.
5. Investigative Question
(i) Elaborate on the importance of using SI units in space exploration.
Answer: Space exploration involves collaboration among space agencies, contractors, and research facilities worldwide. Using standardised SI units helps ensure that measurements, calculations, and engineering data are understood consistently by all teams.
A famous example is the loss of the Mars Climate Orbiter in 1999. One engineering team supplied thruster-related data in Imperial units, while another team used SI units in its navigation calculations. The mismatch contributed to an incorrect assessment of the spacecraft’s trajectory, causing it to enter the Martian atmosphere at an unsafe altitude and be lost.
This incident demonstrates that consistent use of units, careful conversion, and verification of calculations are essential for mission safety, accurate trajectory planning, and successful space exploration.
Notes by Sir Dilawar
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