Common science facts: classes 6 to 8

Matter, separation methods, nutrition and deficiency diseases, acids and bases, light, sound, the solar system and the air we breathe.

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States of matter and water

Idea	Fact
Solid	Fixed shape and fixed volume
Liquid	No fixed shape, fixed volume
Gas	No fixed shape and no fixed volume
Melting point of ice	0 °C
Boiling point of water	100 °C (at sea level)
Normal human body temperature	about 37 °C
Water cycle	Evaporation, condensation, precipitation, collection

Method	Used to separate
Handpicking	Large pieces, such as stones from rice
Winnowing	Light husk from heavier grain, using wind
Sieving	Particles of different sizes, such as flour from bran
Sedimentation and decantation	Heavy insoluble particles that settle in water
Filtration	Insoluble solids from a liquid
Evaporation	A dissolved solid, such as salt from sea water
Distillation	Liquids with different boiling points
Magnetic separation	Iron or steel pieces from other materials

Nutrient	Job in the body	Deficiency problem
Carbohydrates	Give energy	Tiredness, weakness
Proteins	Growth and repair	Poor growth (for example kwashiorkor)
Fats	Store energy, keep the body warm	Lack of energy, poor absorption of some vitamins
Vitamin A	Healthy eyes and skin	Night blindness
Vitamin B1	Healthy nerves and energy release	Beriberi
Vitamin C	Healthy gums and wound healing	Scurvy
Vitamin D	Helps absorb calcium	Rickets in children
Calcium	Strong bones and teeth	Weak bones and teeth
Iron	Makes haemoglobin in blood	Anaemia
Iodine	Helps the thyroid gland	Goitre
Roughage (fibre)	Helps digestion	Constipation
Water	Carries nutrients, controls temperature	Dehydration

Part	Job
Root	Holds the plant and absorbs water and minerals
Stem	Supports the plant and carries water and food
Leaf	Makes food by photosynthesis; lets out water vapour (transpiration)
Flower	Makes seeds; has the reproductive parts
Fruit	Protects and spreads seeds

Photosynthesis: carbon dioxide + water, in sunlight and chlorophyll, gives glucose + oxygen.

Idea	Fact
Acids	Sour taste; turn blue litmus red. Examples: lemon juice (citric acid), vinegar (acetic acid)
Bases	Bitter taste, soapy feel; turn red litmus blue. Examples: baking soda, soap
Neutral substances	Do not change the colour of litmus. Example: pure water
Neutralisation	Acid + base gives salt + water

A physical change can usually be reversed and makes no new substance. A chemical change makes a new substance and is usually not reversible.

Idea	Fact
Light travels	In straight lines, at about 3 × 10⁸ m/s (300,000 km/s) in air
Law of reflection	The angle of incidence equals the angle of reflection
Rainbow colours	Violet, indigo, blue, green, yellow, orange, red (VIBGYOR)
Sound	Needs a medium; cannot travel through a vacuum
Speed of sound in air	about 340 m/s at room temperature
Audible range of human hearing	about 20 Hz to 20,000 Hz
Pitch	Depends on frequency; higher frequency means a higher pitch
Magnets	Like poles repel, unlike poles attract

Idea	Fact
Planets in order from the Sun	Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune (8 planets)
Largest planet	Jupiter
Smallest planet	Mercury
Earth rotates on its axis	Once in about 24 hours (day and night)
Earth goes around the Sun	Once in about 365¼ days (a year)
Moon goes around the Earth	about once in 27 days; the phases repeat every 29½ days
Gravity on the Moon	about one sixth of the gravity on Earth

Group	Main features	Example
Mammals	Have hair or fur, feed milk to their young	Cow, human, bat
Birds	Feathers, wings, lay eggs	Crow, peacock
Reptiles	Dry scaly skin, lay eggs	Snake, lizard, crocodile
Amphibians	Live in water and on land, moist skin	Frog, toad
Fish	Live in water, breathe with gills, have fins	Rohu, shark