Exam practice

A scientific study follows planned steps:

1. Observation and question: something in nature is observed and a question is asked about it.
2. Hypothesis: a possible answer that can be tested is suggested.
3. Experiment: the hypothesis is tested by an experiment, in a laboratory or outdoors, and data are collected by measuring (taking several readings and their average).
4. Result and conclusion: the data are analyzed and a conclusion is drawn.
5. Testing the hypothesis: the conclusion is compared with the hypothesis.
6. Report: if the hypothesis is proved, a report is written and shared; if not, the hypothesis is changed and the steps are repeated.

Example: to find whether lemon juice is acidic, we guess that it is (hypothesis), test it with pH paper (experiment), find pH about 2 (data), conclude that it is acidic, and report it.

The three main fields are biology (the study of living things), physics (matter, energy, force and motion) and chemistry (substances and how they change). Interdisciplinary fields include geology, astronomy, agriculture and environmental science.

The fields depend on each other, and many problems need several of them:

• A doctor (biology) uses X-ray machines and scans built on physics, and medicines made by chemistry.
• Agriculture uses biology (breeding better crops), chemistry (fertilizers, soil tests) and physics (irrigation pumps, weather).
• Geology uses physics to study earthquakes and chemistry to study rocks and minerals.
• Biochemistry is the chemistry of living things; biophysics studies how nerves carry electric signals.

So the fields are connected; that is why interdisciplinary fields grow out of them.

An experiment tests a hypothesis with evidence instead of opinion. It gives data that can be measured, checked and repeated by others, and it shows which guesses are right and which are wrong.
Examples: an experiment with an indicator shows that lemon juice is acidic and soap is basic; testing a vaccine on animals and then volunteers showed that it was safe and worked before it was given to everyone; timing a falling stone several times gives a reliable value for its fall time. Without experiments, a hypothesis stays a guess.

1. Do experiments only in the presence of a teacher.
2. Wear a lab coat, safety glasses and gloves.
3. Tell the teacher at once about any accident.
4. Learn how apparatus works before using it.
5. Use only the amount of chemical needed.
6. Handle chemicals safely: never taste them, and read labels.
7. Keep doors and windows open.
8. Ask the teacher when a concentrated acid is needed; dilute it by adding acid to water.
9. Heat alcohol and other things that catch fire in a water bath, never over a flame.
10. Clean up and put apparatus back after the experiment, and wash your hands with soap and water.

Scientific notation is a way of writing a very big or very small number as a coefficient from 1 up to (but less than) 10, multiplied by a power of 10. For example, 150,000,000 km = 1.5 × 10⁸ km and 0.0000001 m = 1 × 10⁻⁷ m.

1. The base is always 10.
2. The coefficient is at least 1 and less than 10: one non-zero digit before the decimal point (4.5 is right; 45 and 0.45 are not).
3. The power is a whole number: the number of places the point moved.
4. For a big number the point moves left and the power is positive: 600,000 = 6 × 10⁵.
5. For a number smaller than 1 the point moves right and the power is negative: 0.00006 = 6 × 10⁻⁵.
6. Keep the unit: 1.03 × 10⁶ m.

Metric prefixes make a unit bigger or smaller by powers of ten, so very big and very small quantities can be written with short, easy numbers and fewer zeros, and mistakes are less likely. Because they all go in steps of 10, changing from one to another is easy.
Examples: 1 kilogram = 10³ g, so the gas in a cylinder is written 14.2 kg, not 14,200 g; a tablet contains 500 mg (milligrams, 10⁻³ g) of medicine, not 0.5 g; Nepal’s hydropower potential is 83,000 MW (megawatts, 10⁶ W), not 83,000,000,000 W; a hair is 17–181 µm (micrometers, 10⁻⁶ m) wide.

The least count of an instrument is the smallest amount it can measure: the value of one smallest division on its scale. It is found by dividing the difference between two numbered marks by the number of divisions between them. A ruler’s least count is 1 mm (1 cm ÷ 10), a protractor’s is 1° (10° ÷ 10), a digital stopwatch’s is 0.01 s and a wristwatch’s is 1 s. The smaller the least count, the more exact the measurement, so the instrument must suit what is being measured.

• i.1,230,000 = 1.23 × 10⁶ (point moved 6 places left)
• ii.0.00042 = 4.2 × 10⁻⁴ (point moved 4 places right)
• iii.0.00000001 = 1 × 10⁻⁸ (point moved 8 places right)
• iv.300,000,000 = 3 × 10⁸ (point moved 8 places left; this is roughly the speed of light in m/s)