Physical Changes

Station Activities 1: Physical Change

Sublimation and Deposition

Lab Demo. Sublimation of Iodine

Iodine Test Procedure:

1. Describe the appearance of the iodine crystals.

2. What happens when a few pieces of iodine crystals are heated?

Summary: What is sublimation?Video: http://www.youtube.com/watch?v=4fAOI6BeMZY

Activity 1. Evaporation of Liquids

Describe what happens to the particles during this process. Explain why this happens.

Evaporation happens when atoms or

molecules escape from the liquid and turn into a vapor.

Alcohol Water Oil

NATURE OF THE SUBSTANCE

Some liquids have __________ forces that hold particles together making it harder for particles to escape. An example of a liquid like this is _____.

What factors can affect the rate of evaporation?

The energy you can measure with a thermometer is an average of all the molecules in the system.

There are always a few molecules with a lot of energy and some with barely any energy at all.

What factors can affect the rate of evaporation?

The molecules with a lot of energy are able to build up enough power to become a gas. Once they reach that energy level, they can leave the liquid and thus, it has evaporated.

Force of air pressing

down

Image source: http://www.vtaide.com/png/evaporation.htm

Factors affecting the rate of evaporation

Applying Understanding: Real-life situations

Why do you spread out your wet clothes to dry?

Why do you feel cool when you stand next to a fan?

Why do you feel cold when you immediately step out of the shower?

Why is it important to “ rehydrate? Relate “sweating” to evaporation and our body’s ability to regulate temperature.

Activity 2. Boiling Point of Liquids

Lab Demo:

Aim: How does a coolant help prevent overheating?

CoolantWater

Observations:

Explanation:

BOILING POINT

Evaporation takes place at surface

of liquids

Boiling takes place beneath

surface of liquid

Animated GIF "Boiling" - Courtesy of General Chemistry Help - Purdue University

Image source: http://www.elmhurst.edu/~chm/vchembook/163boilingpt.html

This animation shows how water molecules are able to break the forces of attraction .

This is what is happening inside the gas bubble as it is rising to the surface to break and release the water gas molecules.

In a liquid, molecules are packed closely together. As a liquid is heated, the temperature is increased. As the temperature increases, the kinetic energy increases which causes increasing motion. Eventually molecules break free of liquid and become a gas. At the temperature of the boiling point, the liquid turns into a gas.

Activity 2. Boiling Point of LiquidsHow does a coolant help prevent overheating?

Car Engine

The coolant doesn’t boil as easily as water because the particles have a stronger bonding force, so the coolant can absorb the energy from the engine and cool the engine down without evaporating away

http://www.youtube.com/watch?v=9q5gEZGoBnk Vacuum Chamber

How does pressure affect the boiling point?

Physics in the Kitchen

How does a

pressure cooker work?

Physics in the Kitchen

•It is the high temperature that cooks the food.

• Pressure affects boiling. Increased pressure raises the BP.

• Pressure cookers increase the pressure inside and prevent boiling.

• The increased temperature cooks food faster.

Never boil water in a MicrowaveSource: http://www.stevespangler.com/teaching-moments/warning-water-can-explode-in-the-microwave/

A man decided to have a cup of instant coffee, so he heated a cup of water in the microwave. When the timer went off, he removed the cup from the microwave and noticed that the water had not boiled.

Just then, the water literally “blew up” in his face. His whole face was blistered with first and second degree burns, which left some permanent scaring and damage to his left eye.

While at the hospital, the doctor attending him stated that his is a fairly common occurrence. Water (alone) should never be heated in a microwave oven.

Never boil water in a MicrowaveSource: http://www.stevespangler.com/teaching-moments/warning-water-can-explode-in-the-microwave/

Why? This phenomenon is known as superheating. It can occur anytime water is heated - especially if the cup or bowl is new. What happens is that the water heats faster than the vapor bubbles can form. If the cup is very new, then it is unlikely to have small surface scratches in it that provide a place for the bubbles to form (called nucleation sites).

Without bubbles, the water cannot release the heat that has built up, the liquid does not boil, and it continues to heat up past its boiling point. If the water is bumped or jarred, it's enough of a shock to cause the bubbles to rapidly form and the result is an exploding liquid that is scalding hot. One solution is to place a wooden stir stick or something non-metallic in the water to help spread the energy as it is heating in the microwave.

FREEZING POINT

Change in phase from liquid to solid

Why?When energy is taken

out from a liquid, particles slow down

until forces of attraction cause them to fuse and

form a solid.

Image source: http://www.factfrenzy.com/what-is-the-freezing-point-of-water/

Particles of a liquid

Particles slow down

Particles fuse

Substance Freezing Point (degrees Celsius)

Water 0

Diet Soda Slightly lower than 0

Regular Soda Lower than 0 (b/n -15 to -20)

Coolant -37

Alcohol (Isopropyl) - 88

Mineral Oil -30

KEY IDEAS:-Different substances have different freezing points because of the strength of the bonding forces between the particles

-The lower the freezing point, the harder it is to get particles to stick together and form a solid.

-In other words, MORE energy needs to be TAKEN away from the particles to get them to stick

Why is salt added to a snow-covered driveway?

Activity 2. Freezing Point of LiquidsWhy add salt to a snow-covered driveway?

Hypothesis: ____________________

Design a simple test. Set-up your experiment.

Salt dissolves in water pretty fast. When you sprinkle salt on ice, the salt lowers the freezing point of the water, keeping it from re-freezing as easily and helping to melt the rest of the ice. This is called lowering the freezing point.

The freezing point of water becomes lower as more particles are added until the point where the salt stops dissolving. On a driveway or street, sodium chloride can melt ice at about -9°C.

The salt disrupts the water particles ability to stick together to form a solid, thus requiring MORE energy to be TAKEN OUT of the liquid water before it will become a solid.

http://www.stevespanglerscience.com/experiment/how-does-salt-melt-ice