i) Why is cooking difficult at high altitude? ii) Why is cooking faster in pressure cooker?

1. 

• At high altitude density of air is low which causes reduction in atmospheric pressure. 
• As pressure is less, boiling point of water lowers. 
• Water, at high altitude, starts boiling below 100°C. 
• As food is cooked mostly through the water boiling, cooking of food becomes difficult. 

2. 

• Pressure cooker operates by expelling air within the cooker and trapping steam produced from the liquid. (mostly water) boiling inside. 
• Due to high internal pressure, boiling point of liquid increases and liquid boils at temperature higher than its boiling point. 
• The increased boiling point allows more absorption of heat by liquid and steam formed is superheated.
• As a result, food gets cooked quickly.