A puddle vanishing on a hot day involves only a physical change in matter.
In this scenario, as the temperature rises on a hot day, the liquid water in the puddle absorbs heat energy from the surroundings and undergoes a phase change from the liquid state (water) to the gaseous state (water vapor) through the process of evaporation. This change is purely physical because the chemical composition of water remains the same; it's just changing from one phase (liquid) to another (gas).
The other options involve chemical changes:
When a log turns black and shrinks as it burns, it is undergoing a combustion reaction, which is a chemical change. The log is being chemically transformed into ash, water vapor, and various gases.
The formation of a yellow crystal when two clear liquids are mixed suggests the possibility of a chemical reaction occurring, leading to the formation of a new compound with a different chemical composition and properties.
A lawn chair rusting when left outside involves the chemical reaction of iron with oxygen in the presence of moisture, resulting in the formation of iron oxide (rust). This is also a chemical change.
So, the puddle vanishing on a hot day is the only example that involves a purely physical change in matter.