What is Density? (Student responses from pre-test go here)

Density Scientific definition: The mass per unit volume of a substance. In plain English: The amount of something packed in a volume What two units make up density? Review: How do we measure mass and volume? In grams and liters! Scientific definition: The mass per unit volume of a substance. What two units make up density?

What is the equation for Density? Density = Mass Volume This means that if we divide the mass of any object by its volume, we get its density. The density of two different things can never be exactly the same! How about some examples: (I will pass around two different objects with the same volume and two with the same mass)

How do we find an object’s Density? Simple! We know the equation for density, so all we need to do is find the mass of the object (by weighing it) Then we need to find its volume (by measuring its height, width, and length or by measuring the amount of water it displaces, as we have already done) Then we just divide the mass by the volume!

What are the densities of some common items? A penny: 9.64 grams/mL A cube of ice: .917 grams/mL A piece of coal: 1.50 grams/mL Pure water: 1.00 grams/mL!

Wait, how can the density of water be exactly 1?!? This is how volume was originally defined! – Scientists decided that 1.00 mL should be exactly equal to 1.00 grams of water! That means: Density = Mass Volume = 1.00 grams 1.00 mL =1.00 g/mL

What do we use Density for? Several things! We will mainly be looking at whether something sinks or floats in a liquid. In general, if something has a high density, it sinks! If it has a low density, it floats Before we go any further, lets find out how we can get a high density and a low density!

How to get a high Density These will be I-clicker questions! Students can vote A for up or B for down Here is our equation: Density = Mass Volume To make Density ↑ We need to: Make Mass go? Higher Mass = Higher Density Make Volume go? Lower Volume = Higher Density If your not sure, try it with numbers and see what happens! ↑ ↓

How to get a low Density ↓ ↑ Here is our equation: These will be I-clicker questions! Here is our equation: Density = Mass Volume To make Density ↓ We need to: Make Mass go? Lower Mass = Lower Density Make Volume go? Higher Volume = Lower Density Again, if your not sure, try it with numbers and see what happens! ↓ ↑

Demonstration: Floating Egg?!? Demo time! Let’s see if we can make this egg float! Remember, if the egg has a high density, it will sink, and if it has a low density, it will float. We also know that the density of water is 1, so if the density of the egg is greater than 1 it will sink in pure water, or, if the density is less than 1, it will float in pure water!

Floating Egg?!? We have two solutions: one is red and one is green! I will ask the students to write down their predictions…they will be added to where the red text is . We have two solutions: one is red and one is green! What do you think will happen when we put the egg in the red solution? What about the green solution? Let’s find out! (We will try the red solution first – the egg will sink (students will be given time to write down their observations)) Next is the green solution – the egg will float (again, the students will be given time to write down their observations)) They will then discuss their ideas and explanations with their neighbors.

Floating Egg?!? What happened? Students will start a series of think-pair- shares here. The egg sank in the red solution and floated in the green! What do these observations tell us? (I will type students’ answers here)

What if I told you the density of the egg is: 1.5 grams/mL Floating Egg?!? Why did it happen? After several answers are given, we will discuss the strengths and weaknesses of each one (after T-P-S) What if I told you the density of the egg is: 1.5 grams/mL What can you tell me now? (Students’ answers will go here again)

Ok, the I’ll tell you what I did : ) Floating Egg?!? Explanation! Ok, the I’ll tell you what I did : ) The red solution is just water with red food coloring! The green solution is water and green food coloring with salt mixed in! Why did the egg float?!?!? We know that the egg was less dense than the total solution (since it floated), but why?

I-Clicker! Why did the egg float? Please pick the best answer! After the voting is over, I will ask students to narrow down the answers A: the mass of the total solution went down B: the volume of the total solution went up C: both the mass and the volume went up, but the higher volume was more important D: both the mass and the volume went up, but the higher mass was more important E: the mass of the total solution went up I Will also make sure they understand that the “total solution” is the water and the salt – not the egg and salt water.

It looks like both solutions have about the same volume! Floating Egg?!? Explanation! It looks like both solutions have about the same volume! The green solution weighs more though! (I will weigh both containers for the students at this point so they can actually see the weight difference) Because the mass was higher, and the volume almost the same, the density of the salt water was much higher! This allowed the egg to float, because it now had a lower density than the solution!

Exit Question & Homework! You must complete the exit question before you leave! (I will hand out the exit question now – students will have the remaining time to finish. If there is no time, I will assign it as homework as well) You will also need to pick up your homework (a density worksheet) from my desk! It is due tomorrow!