grade 7 Density

Density

Even at the same volume, different mass means different density. Density is mass divided by volume and stays constant for a given material no matter the amount. Objects denser than water sink and less dense ones float—that is why ice floats. Move mass and volume and the density, plus float or sink, shows up.

Same Size, Different Mass — Why?

Mass10 g

Volume8 cm³

💡 What is Density?

①Same volume, different mass → different densities

②If density < water it floats; if greater it sinks

③Wood floats because wood's density < water's

④Iron nails sink because iron's density > water's

⑤Ice floats because ice (0.92) < water (1.0)

Density Differs by Material

🔍 Density is a Material Property

①Same material → same density regardless of amount

②1 g or 1 kg of gold both have density 19.3 g/cm³

③Density distinguishes materials

④Higher temperature → volume expands → density decreases slightly

Density Formula

ρ = mV

Density (g/cm³) = mass (g) ÷ volume (cm³)

Solving for Mass

m = ρ × V

Mass = density × volume

Solving for Volume

V = mρ

Volume = mass ÷ density

📐 Tips for Density Calculations

①Mass: measured with an electronic balance (unit: g)

②Volume: measured with a graduated cylinder (unit: cm³ = mL)

③Irregular objects: water displacement method

④Unit conversion: 1 cm³ = 1 mL

Exam Key Points

Core Summary

ρ = mV

Density is intrinsic — independent of the amount of material

🎯 Exam Key Points

①Density = mass/volume (unit: g/cm³)

②Same material → constant density (amount-independent)

③To float in water: density < 1.0 g/cm³

④Ice floats because ice (0.92) < water (1.0)

⑤Irregular volume → water displacement method

Examples and Unit Test

Example 1

What is the density of an object with a mass of 60 g and a volume of 20 cm³?

Density is found by dividing mass by volume (density = mass ÷ volume).

60 g ÷ 20 cm³ = 3 g/cm³.

▸ 3 g/cm³

Density = mass ÷ volume, with units of g/cm³.

Example 2

If the object above, with a density of 3 g/cm³, is put in water (density 1.0 g/cm³), does it float or sink, and why?

An object sinks if its density is greater than water and floats if it is smaller.

The object density 3 g/cm³ is greater than the water density 1.0 g/cm³, so it sinks.

▸ It sinks (object density > water density)

A density greater than water sinks; smaller floats.

Unit-test style

Which statement about density is correct?

①Density is found by mass × volume

②For the same material, density is the same even if the amount differs

③An object denser than water floats

④The unit of density is g

⑤Ice is denser than water, so it sinks

▸ ② For the same material, density is the same even if the amount differs

Density is a characteristic property of a material, so it is constant for the same material regardless of amount.

Density = mass ÷ volume (①), an object denser than water sinks (③), the unit is g/cm³ (④), and ice (0.92) is less dense than water (1.0) so it floats (⑤).

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Friction

States of Matter

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