No, alkynes are not freely rotatable. The carbon-carbon triple bond in alkynes is made up of one sigma bond and two pi bonds, which restricts the rotation around the bond. This rigidity gives alkynes unique chemical and physical properties compared to alkenes and alkanes. The lack of free rotation also affects the geometry and reactivity of alkynes in various chemical reactions. **

Yes, the violin peg is rotatable. The pegs are used to tune the strings of the violin by tightening or loosening them. By turning the pegs, the tension on the strings can be adjusted to achieve the desired pitch. It is important to rotate the pegs carefully and in the correct direction to avoid damaging the instrument. **

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Peptide bonds are not freely rotatable because of the partial double bond character that arises from resonance in the peptide bond. This restricts the rotation around the bond and locks the peptide backbone into a planar structure. The resonance forms a rigid, flat structure that allows for the formation of the alpha helix and beta sheet secondary structures in proteins. This rigidity is essential for maintaining the specific three-dimensional shape and function of proteins. **

The screw may no longer be rotatable due to corrosion or rusting of the screw threads, which can cause them to become stuck or seized. Additionally, the screw may have been over-tightened, causing the threads to become deformed and preventing further rotation. Another reason could be that the screw has reached the end of its thread and can no longer be turned. **