Grade 8 Science: Comprehensive Course Review

Comprehensive practice flashcards in Question and Answer format covering all Grade 8 Science units, including microbiology, animal and plant diversity, matter, sound, magnetism, electricity, chemical reactions, organ systems, life cycles, food preservation, astronomy, and natural disasters.

How are microorganisms defined in biology?

Unicellular (single-celled) or multicellular organisms that cannot be observed clearly by the naked eye and can only be observed clearly through microscopes.

Which scientist first observed microorganisms using an invented simple microscope, and in what year?

The Dutch scientist Antonie van Leeuwenhoek in $1674$.

What gases and by-products are formed when yeast acts in a warm sugar solution?

Carbon dioxide ($\text{CO}_2$) gas and ethyl alcohol.

How is microbial activity classified based on whether it acts on food rich in sugars, proteins, or fats?

Microbial activity on sugars leads to fermentation; on proteins, it leads to putrefaction; and on fats, it leads to rancidity.

What are three bacterial diseases and three viral diseases that infect humans?

Bacterial diseases: tuberculosis, leprosy, and typhoid fever. Viral diseases: common cold, dengue, and AIDS.

Identify the invertebrate phylum characterized by radial symmetry, a body form existing as polyps or medusae, and tentacles bearing cnidocytes.

Cnidaria (e.g., Hydra, sea anemone, jellyfish).

Which invertebrate group consists of vermiform, segmented worms that exhibit bilateral symmetry?

Annelida (e.g., earthworm, leech, Nereis).

What are the common anatomical features of organisms belonging to phylum Mollusca?

Bilaterally symmetrical, soft-bodied animals possessing a muscular foot and a skin moistened with mucus, with some species bearing shells.

What morphological characteristics define phylum Arthropoda?

Bilateral symmetry, an external skeleton (exoskeleton), an externally segmented body, and jointed appendages.

What are four flightless birds belonging to the group Aves?

Rhea, Penguin, Kiwi, Ostrich, Cassowary, and Emu.

What four diagnostic features characterize the class Mammalia?

Possession of mammary glands, skin with hair, sweat glands, and sebaceous glands, external ears with ear lobes, and lungs for breathing.

What is leaf arrangement, and what primary biological advantage does it provide?

The pattern of leaf attachment to a stem, which facilitates efficient photosynthesis by exposing all leaves to sunlight.

Give examples of plants that reproduce using runners (stolons) versus suckers.

Runners (stolons): 'Gotukola', 'Undupiyaliya', and 'Ambul ambiliya'. Suckers: Banana, 'Kalanduru', and Paddy.

How do prop roots differ from stilt roots in their origin and function?

Prop roots are adventitious roots that arise from branches and penetrate the soil to support branches (e.g., Banyan tree); stilt roots arise from the stem and grow into the ground to support the stem (e.g., 'Vetakeyya', 'Maha kadol').

What is the discontinuous or particulate nature of matter?

The principle that all matter exists as an assembly of very small particles with empty spaces between them.

What are the standard melting point and boiling point of pure water under standard atmospheric pressure?

The melting point is $0^\circ\text{C}$ and the boiling point is $100^\circ\text{C}$.

How are elements distinguished from compounds?

Elements are pure substances with definite properties that cannot be divided further by physical or chemical methods, whereas compounds are homogeneous pure substances formed when two or more elements combine chemically in a fixed ratio.

What is frequency of vibration, and what is its SI unit?

The number of vibrations produced by an object per unit time; measured in Hertz ($\text{Hz}$).

What is the normal limit of hearing (audible frequency range) for the human ear?

$20\,\text{Hz}$ to $20\,000\,\text{Hz}$.

What determines the difference between a musical tone and noise?

Musical tones are rhythmical, pleasant sounds produced by regular, methodical vibrations; noises are unpleasant sounds produced by non-formal, irregular vibrations.

What is the direction of magnetic field lines outside a bar magnet?

From the North pole ($\text{N}$) to the South pole ($\text{S}$).

Which magnetic materials are used to construct permanent magnets versus temporary magnets?

Permanent magnets are constructed from steel or ferrite; temporary magnets (electromagnets) are constructed using soft iron.

What factors can cause a permanent magnet to lose its magnetism?

Ageing, heating to high temperatures, exposure to strong magnetic fields, and strong mechanical vibrations or hammering.

How must an ammeter and a voltmeter be connected within a circuit to measure their respective quantities?

An ammeter must be connected in series to measure electric current ($I$); a voltmeter must be connected in parallel to measure potential difference ($V$).

What is electrical resistance, and what is its standard measuring unit?

The obstacle or opposition caused by a conductor to the flow of electric current through it; measured in Ohm ($\Omega$).

State the Law of Conservation of Mass and name the scientist who proved it.

During chemical reactions, total mass does not change (mass is conserved); demonstrated by the French scientist Antoine Lavoisier ($1743 - 1794$).

What are the three essential components represented in the fire triangle?

Fuel (a combustible substance), oxygen (a supporter of combustion), and heat (reaching the ignition temperature).

What environmental factors are essential for the rusting of iron, and how does galvanization prevent it?

Oxygen and water (or water vapour) are essential; galvanizing coats iron with zinc metal, which protects the iron from contacting air and moisture even if scratched.

What is a chemical neutralisation reaction?

A chemical reaction between an acid and a base wherein their acidic and basic properties disappear, producing neutral substances: a salt and water.

Why is human faeces excluded from the definition of excretory products?

Faeces is undigested waste matter passed out of the digestive tract and is not produced by chemical or metabolic reactions within body cells.

What are the four primary organs that constitute the human urinary system?

Kidneys, ureters (urinary tracts), urinary bladder, and urethra.

What are the normal components and approximate proportions of healthy human urine?

Water (about $96\%$), urea (about $2\%$), salts (about $2\%$), and trace amounts of uric acid.

What specific functions are carried out by the cerebrum, cerebellum, and medulla oblongata?

Cerebrum: controls higher brain functions (thought, intelligence, senses, voluntary actions); Cerebellum: regulates and coordinates body balance; Medulla oblongata: controls autonomic involuntary functions (heartbeat, breathing).

How many pairs of cranial nerves and spinal nerves comprise the human peripheral nervous system?

$12$ pairs of cranial nerves and $31$ pairs of spinal nerves.

What are the two principal structural layers of human skin, and where is the pigment melanin synthesized?

The outer epidermis and inner dermis; melanin is produced in the epidermal layers to shield the body from ultraviolet rays.

What three factors determine the magnetic field strength of an electromagnet?

The type of core inside the coil, the magnitude of the electric current flowing through the coil, and the number of turns in the coil.

State the word equation for the process of photosynthesis.

$$\text{Carbon dioxide} + \text{Water} \xrightarrow[\text{Chlorophyll}]{\text{Light energy}} \text{Glucose} + \text{Oxygen}$$

Define osmosis as it applies to plant root absorption.

The diffusion of water molecules through a semipermeable membrane from an area of higher water concentration to an area of lower water concentration.

How does transpiration differ from guttation in plants?

Transpiration is the evaporation of pure water vapour mainly through stomata during both day and night; guttation is the exudation of liquid droplets containing water and salts through hydathodes, occurring mostly at night when humidity is high.

What is the difference between complete and incomplete metamorphosis in insects?

Complete metamorphosis has four morphologically distinct stages: egg, larva, pupa, and adult (e.g., butterfly, mosquito); incomplete metamorphosis develops without marked intermediate differences through egg, nymph, and adult (e.g., cockroach).

Which developmental stage of the fruit fly causes destruction of crops like mango and banana?

The larval stage (larvae tunnel into and consume the fruit flesh, causing it to rot).

What temperature and duration conditions are applied in the pasteurization, ultra-pasteurization, and sterilization of milk?

Pasteurization: $72^\circ\text{C}$ for about $15\,\text{seconds}$; Ultra-pasteurization: $138^\circ\text{C}$ for about $1 - 2\,\text{seconds}$; Sterilization: $120^\circ\text{C}$ for about $15 - 20\,\text{minutes}$.

What numerical range of European Union E numbers is designated for synthetic food preservatives?

$\text{E 200}$ to $\text{E 299}$.

What causes the seasonal changes experienced on Earth?

The revolution of the Earth around the Sun while tilted at an angle of $23.5^\circ$ to the perpendicular axis of its orbital plane.

What astronomical alignments produce solar eclipses versus lunar eclipses?

A solar eclipse occurs on a new moon day when the Moon passes directly between the Sun and Earth; a lunar eclipse occurs on a full moon day when the Earth passes directly between the Sun and Moon.

What is a geostationary satellite, and who introduced the concept of a global communication satellite network in 1945?

A satellite that orbits Earth at an altitude and speed matching Earth's rotation so that it appears stationary from the surface; introduced by Sir Arthur C. Clarke.

Which constellation features Sirius, the brightest star in the night sky?

Canis Major.

Identify the zodiac constellation that contains the bright star Regulus.

Leo.

What rainfall threshold over 24 hours serves as a critical pre-sign of an impending landslide?

Receiving more than $100\,\text{mm}$ of continuous rainfall within $24\,\text{hours}$.

What are the three classifications of lightning based on the path of electrical discharge?

Earth-lightning (cloud to earth), cloud-lightning (within a cloud or between two clouds), and air-lightning (from a cloud into surrounding air).