Changes In Solid And Liquid Materials

Melting ice cream is a physical change because it involves a change in the state of matter from solid to liquid without any change in the chemical composition of the ice cream. The molecules in the ice cream are rearranged as heat is applied, causing the solid ice cream to melt into a liquid form. However, the chemical properties and composition of the ice cream remain the same throughout the process.

Souring milk is a chemical change. When milk sours, it undergoes a process called fermentation, where bacteria break down the lactose (sugar) in milk into lactic acid. This chemical reaction causes the milk to change in taste, smell, and texture. The transformation of lactose into lactic acid is irreversible, making it a chemical change rather than a physical change.

Baking a cake involves a chemical change because the ingredients undergo a reaction to form a new substance. When the cake batter is mixed, the chemical reactions occur between the flour, eggs, sugar, and leavening agents. As the cake bakes, these reactions continue, causing the batter to rise and solidify into a cake. The heat from the oven also plays a crucial role in these chemical reactions, causing the proteins to denature and the starches to gelatinize. Overall, baking a cake is a chemical change because it involves the formation of new substances through chemical reactions.

Rusting of keys is a chemical change. Rusting occurs when iron reacts with oxygen in the presence of water or moisture, forming iron oxide (rust). This process involves a chemical reaction where the iron atoms are oxidized, leading to the formation of a new substance (iron oxide) with different properties than the original iron. As a result, the keys undergo a chemical transformation, making it a chemical change rather than a physical change.

A change in state of matter occurs is the correct answer because physical change refers to a transformation in the physical properties of a substance without any alteration in its chemical composition. The other options, involving a chemical reaction and a new substance being formed, describe chemical changes where the composition of the substance is altered. However, a change in state of matter, such as solid to liquid or gas, is a physical change as it does not involve the formation of a new substance.