VAPOUR PRESSURE
BY: KYLA, ALICE, AMRIT, SIERRA LINDSAY, TAYLOR
LESSON INTRODUCTION
• AN OPEN CONTAINER OF WATER IS BESIDE A CLOSED CONTAINER OF WATER, BOTH CONTAINERS HAVE
THE SAME VOLUME OF LIQUID
• THINK ABOUT:
- EVAPORATION IN EACH CONTAINER
- WILL THE CLOSED CONTAINER EVENTUALLY EVAPORATE COMPLETELY
- WILL THE TEMPERATURE AFFECT EVAPORATION
• IN THIS LESSON YOU WILL FOCUS ON ONE PHASE CHANGE, EVAPORATION, AND THE VARIABLES THAT
AFFECT THE PRESSURE MADE BY THIS PROCESS
WHAT IS VAPOUR PRESSURE
• DEFINITION: VAPOUR PRESSURE IS THE PRESSURE OF VAPOUR IN CONTACT WITH ITS LIQUID OR SOLID FORM.
OR
THE PRESSURE CREATED BY THE VAPOUR AT EQUILIBRIUM
• WHEN A SOLID OR A LIQUID EVAPORATES TO A GAS IN A CLOSED CONTAINER, THE MOLECULES CANNOT ESCAPE.
• SOME OF THE GAS MOLECULES WILL EVENTUALLY STRIKE THE CONDENSED PHASE AND CONDENSE BACK INTO IT.
• WHEN THE RATE OF CONDENSATION OF THE GAS BECOMES EQUAL TO THE RATE OF EVAPORATION OF THE LIQUID
OR SOLID, THE AMOUNT OF GAS, LIQUID AND/OR SOLID NO LONGER CHANGES.
• THE GAS IN THE CONTAINER IS IN EQUILIBRIUM WITH THE LIQUID OR SOLID.
AN EXAMPLE TO CONSIDER:
• WHAT WOULD HAPPEN IF YOU WERE TO LEAVE A CONTAINER OF LIQUID OPEN?
IT WOULD COMPLETELY EVAPORATE BECAUSE THE RATE OF EVAPORATION OF THE LIQUID WILL BE
GREATER THAN THE RATE OF CONDENSATION, AND SO OVER TIME ALL THE LIQUID WILL CONVERT TO THE
GASEOUS STATE
• WHAT WOULD HAPPEN IF YOU CLOSED THE CONTAINER?
IT WOULD NOT EVAPORATE COMPLETELY, BECAUSE THE VAPOR CANNOT ESCAPE INTO OPEN AIR SO IT
COLLECTS INSIDE THE CONTAINER AND EXERTS PRESSURE ON THE REMAINING LIQUID.
FOR EVERY MOLECULE THAT EVAPORATES, ANOTHER MUST CONDENSE BECAUSE THERE IS NOT
ENOUGH SPACE FOR BOTH
VAPOUR PRESSURE & INTERMOLECULAR FORCES
• THE RATE OF EVAPORATION IS DETERMINED BY THE STRENGTH OF INTERMOLECULAR FORCES
BETWEEN PARTICLES OF A SAMPLE
• DIFFERENT SUBSTANCES HAVE DIFFERENT INTERMOLECULAR FORCE STRENGTH MEANING THE
RATE OF EVAPORATION OF EACH SUBSTANCE WILL BE DIVERSE
• IF THE INTERMOLECULAR FORCE STRENGTH IS WEAKER, THE AMOUNT OF VAPOUR MADE,
INCREASES
• IF IT IS WEAKER IT IS EASIER FOR LIQUID PARTICLES TO LEAVE THE FORCES OF ATTRACTION IN
THE LIQUID AND BECOME GASEOUS
MEASURING PRESSURE
• MANOMETER: A DEVICE USED TO MEASURE PRESSURE
• MANOMETERS HAVE A GLASS CONTAINER ON ONE END, AND CAN BE OPENED/ CLOSED ON THE
OTHER.
• A LIQUID SUCH AS MERCURY IS PLACED IN THE "U" SHAPED TUBE
• THE PRESSURE OF THE ITEM IN THE BULB IS MEASURED BY THE DIFFERENCE OF HEIGHT IN THE
MERCURY, DUE TO PRESSURE FORCING THE LIQUID UP THE TUBE.
Example:
• A LIQUID, OFTEN MERCURY, IS PLACED IN A USHAPED TUBE (TUBES HAVE THE SAME
TEMPERATURE)
• PRESSURE IS MEASURED BY FINDING THE
DIFFERENCE IN HEIGHT BETWEEN THE 2 TUBES
• AS THE PRESSURE INCREASES THE MERCURY IS
FORCED HIGHER UP THE TUBE
WHICH HAS THE GREATER VAPOR PRESSURE?
WHY?
ANSWER:
• THE TUBE CONTAINING THE ALCOHOL HAS THE GREATER VAPOR PRESSURE
BECAUSE GENERALLY THE GREATER FORCES OF ATTRACTION LOWER THE
VAPOR PRESSURE.
• WATER HAS A GREATER FORCE OF ATTRACTION THAN THE ALCOHOL, AND
THUS HAS A LOWER VAPOR PRESSURE
VAPOR PRESSURE AND BOILING POINT
• THERE ARE TWO TYPES OF VAPORIZATION.
• EVAPORATION: THE CHANGING OF LIQUID TO GAS ON THE SURFACE OF THE LIQUID
• BOILING: VAPORIZATION TAKES PLACE THROUGHOUT THE LIQUID, REMAINING THE SAME
TEMPERATURE. THE VAPOR BUBBLES WITHIN THE LIQUID THEN RISE TO THE SURFACE.
• THE BOILING POINT OF A SUBSTANCE IS THE TEMPERATURE AT WHICH THE VAPOR PRESSURE IS
EQUAL TO THE EXTERNAL OR ATMOSPHERIC PRESSURE OF A SUBSTANCE.
ATMOSPHERIC PRESSURE AND BOILING POINT
• A CHANGE IN THE ATMOSPHERIC PRESSURE WILL CHANGE THE BOILING POINT, AS BOILING
OCCURS WHEN ATMOSPHERIC PRESSURE AND VAPOR PRESSURE ARE EQUAL.
• AS ATMOSPHERIC PRESSURE DECREASES, THE BOILING POINT WILL, TOO.
• SINCE LESS VAPOR PRESSURE IS NEEDED TO MATCH A LOWER ATMOSPHERIC PRESSURE, LESS HEAT
IS NEEDED.
• LOWER AIR PRESSURE= LOWER BOILING POINT
• NORMAL BOILING POINT: THE TEMPERATURE WHEN VAPOR PRESSURE EQUALS STANDARD
PRESSURE.
• PRESSURE COOKERS ARE OFTEN USED TO SPEED UP COOKING
ANOTHER EXAMPLE!!!
• IF YOU WERE THEN TO HARD BOIL AN EGG AT
A HIGH ALTITUDE WHERE THE AIR PRESSURE IS
LOWER, YOU WOULD HAVE A LOWER
BOILING POINT SO IT WOULD TAKE YOUR
EGG MUCH LONGER TO COOK BECAUSE THE
WATER WOULD BE BOILING AT A LOWER
TEMPERATURE
• IT WOULD BE THE OPPOSITE IF YOU WERE TO
BOIL AN EGG AT A LOW ALTITUDE WHERE THE
AIR PRESSURE IS HIGHER
PRACTICE QUESTIONS
1. WHAT IS VAPOUR PRESSURE?
2. IF THE AMOUNT OF VAPOUR MADE INCREASE, THE INTERMOLECULAR FORCE MUST BE...?
3. WHAT MEASURES PRESSURE?
4. WHAT ARE THE TWO TYPES OF VAPOURIZATION?
ANSWERS!!!!!
1. IT IS THE PRESSURE OF VAPOUR IN CONTACT WITH IT'S LIQUID OR SOLID FORM
2. WEAK
3. MANOMETER
4. EVAPORATION AND BOILING
POSSIBLE TEST QUESTIONS
• 1. IF A LIQUID IS LEFT OPEN, WILL THE LIQUID COMPLETELY EVAPORATE? IF SO HOW?
• 2. IS THE RATE OF EVAPORATION THE SAME FOR ALL LIQUIDS? IF NOT, WHAT ARE THE
DIFFERENT FACTORS?
• 3. WHEN DOES THE BOILING OF VAPOR PRESSURE OCCUR?
ANSWERS
• 1. IF A LIQUID IS LEFT OPEN, THE LIQUID WILL EVAPORATE COMPLETELY. THIS HAPPENS BECAUSE
THE RATE OF EVAPORATION WILL BE FASTER THEN THE RATE OF CONDENSATION, SO OVERALL
THE LIQUID WILL CONVERT TO GAS.
• 2. THE RATE OF EVAPORATION FOR LIQUIDS IS DIFFERENT DEPENDING ON THE STRENGTH OF THE
INTERMOLECULAR FORCE. IF THE FORCE IS WEAKER THEN THE AMOUNT OF VAPOUR THAT
EVAPORATES, IT WILL INCREASE, BECAUSE ITS EASIER FOR THE PARTICLES OF A LIQUID TO ESCAPE
THE ATTRACTION. FOR A STONGER ATTRACTION THE EVAPORATION WOULD BE SLOWER AS IT IS
HARDER FOR THE PARTICLES TO ESCAPE THE ATTRACTION.
• 3. BOILING OCCURS WHEN THE ATMOSPHERIC PRESSURE AND THE VAPOR PRESSURE OF A
LIQUID ARE EQUAL.
THANKS FOR ACTUALLY WATCHING & LISTENING
(HOPEFULLY)
XOXO