To determine the density of a substance for Class 7, you need to find its mass and its volume and then use a simple formula.
Understanding Density
Density is a fundamental property of matter that tells us how much "stuff" (mass) is packed into a certain amount of space (volume). Think of it like this: a feather and a rock of the same size don't weigh the same because the rock is much denser – it has more mass in the same volume.
The Density Formula
The key to finding density lies in this simple mathematical relationship:
DENSITY = MASS / VOLUME
This means if you know the mass of a substance and its volume, you can calculate its density by dividing the mass by the volume.
What is Mass?
- Mass is the amount of matter in an object.
- It is usually measured in grams (g) or kilograms (kg).
- You can measure the mass of a substance using a balance or a weighing scale.
What is Volume?
- Volume is the amount of space a substance occupies.
- It is usually measured in cubic centimeters (cm³) or milliliters (mL) for liquids. Remember that 1 cm³ is equal to 1 mL. For larger volumes, cubic meters (m³) or liters (L) are used.
Measuring Volume:
- For liquids: You can easily measure the volume of a liquid using measuring cylinders, beakers, or flasks with volume markings.
- For regularly shaped solids (like a cube or a rectangular block): You can measure its length, width, and height and multiply them together: Volume = length × width × height.
- For irregularly shaped solids (like a rock): You can use the water displacement method.
- Pour a known volume of water into a measuring cylinder. Note the initial volume (V₁).
- Carefully place the solid object into the water.
- The water level will rise. Note the final volume (V₂).
- The volume of the object is the difference between the final and initial volumes: Volume = V₂ - V₁.
Steps to Determine Density
Here's a simple process:
- Measure the Mass: Use a balance to find the mass of the substance (in grams or kilograms).
- Measure the Volume:
- If it's a liquid, use a measuring cylinder.
- If it's a regular solid, measure its dimensions and calculate the volume (length × width × height).
- If it's an irregular solid, use the water displacement method to find its volume.
- Calculate Density: Divide the mass you measured by the volume you measured using the formula: DENSITY = MASS / VOLUME.
- Include Units: Make sure to state the units for density, which are typically g/cm³ or kg/m³.
Example:
Suppose you have a small stone:
- You measure its mass and find it is 50 grams.
- You use the water displacement method to find its volume.
- Initial water volume = 20 mL
- Final water volume with stone = 40 mL
- Stone's volume = 40 mL - 20 mL = 20 mL (or 20 cm³)
- Now, calculate the density:
- Density = Mass / Volume
- Density = 50 g / 20 cm³
- Density = 2.5 g/cm³
So, the density of the stone is 2.5 g/cm³.
Units of Density
Density units combine a unit of mass and a unit of volume. Common units include:
| Mass Unit | Volume Unit | Density Unit |
|---|---|---|
| grams (g) | cubic centimeter (cm³) | g/cm³ |
| grams (g) | milliliter (mL) | g/mL |
| kilograms (kg) | cubic meter (m³) | kg/m³ |
Understanding density helps us compare different substances and explain why some things float (less dense than water) and others sink (more dense than water).
