Hint: In chemistry we generally discuss two different types of changes based on the nature of change which is occurring. These are physical changes and chemical changes. Physical changes are also termed a s reversible changes and chemical changes are termed as irreversible changes. Let us discuss them and answer the question.
Complete step-by-step answer:Let us firstly understand what exactly is a physical change and chemical change. A physical change is a change which changes the shape, size, color, texture of a given substance but does not alter its chemical properties. For example, melting ice. These are also termed as reversible changes because these can be reversed back or changed back onto their original substance. For example: when ice melts it converts to water, this change is physical change. This is even reversible as the water can convert back to ice on freezing. A chemical change is defined as the change which when occurs changes the chemical composition of a substance. It results in the formation of entirely new substances. For example: digestion of food. These are generally irreversible changes as they involve formation of a completely new substance which cannot be reversed back to its original form. For example: the food which is digested cannot be obtained back. In the question we are given a process ‘’ curdling of milk’’ and we need to tell whether this change is chemical change or physical change. We know that Curdling of milk refers to the conversion of milk to the curd. Now let us see how exactly this conversion occurs. For preparation of curd, we boil the milk and add a spoon of little curd in it which serves as an inoculum. We know that curd contains lactic acid. Milk contains casein protein and this protein reacts with the lactic acid. Lactic acid breaks the chemical structures/bonds of the milk and forms new bonds thus giving rise to a semi-solid form of milk called curd. We saw in the entire process that the reaction between lactic acid and milk involved breaking of bonds and forming of new bonds. Therefore, there is a chemical change associated with it and hence this change is a chemical change. Hence, curdling milk is a chemical change.
Note:We know that chemical changes are irreversible. Using this fact also we can answer the above question. Curd once formed from the milk cannot be converted back into the milk and hence this is an irreversible process. Therefore, it is a chemical change. So, whenever you get confused with the change just see whether the original substance can be obtained back or not.