What does it mean to say that pigweed is resistant to herbicide?

It means that the variety of plant has become resistant or the herbicide ineffective against the plant. Leaving weeds around after spraying usually can result in a strain of them surviving and multiplying, sharing the same resistance until you start again with a different herbicide

Silicon is cut into tiny pieces for computer chipslead is melted to form pelletsliquid water freezes to form ice

The answer to this will be from your lab notes/procedure. it would be best to ask another student who participated in the lab as we do not know what was done, what enzyme or if the temperature is celsius or fahrenheit. if i were to guess though i'd say the rate stopped due to enzyme denturing at high temperature