Describe what bonds are broken and formed between which molecules when calcium carbonate (chalk) and silica (silicon dioxide, or sand) react to form calcium silicate (an ingredient of some types of glass) and carbon dioxide (a common gas in the atmosphere).
A chemical reaction is when a number of chemicals are mixed together and break down into their constituent elements to then form new bonds and new chemicals. The chemical reaction sometimes needs to be encouraged with stirring, heat, electricity, catalysts and so on.
The two chemicals that are inputs to the reaction here we are told are calcium carbonate `("CaCO"_3) ` and silica, or silicon dioxide `("SiO"_2) ` . When we analyse the constituent parts of these two chemicals we can work out (also by the names in words of the chemicals) that the first contains calcium `("Ca") `, carbon `("C") ` and three parts of oxygen `("O") ` (the subscript 3 means that there are 3 oxygen molecules in the chemical). The second contains silicon `("Si") ` and two parts of oxygen `("O") `(when two oxygen molecules are joined together in a chemical this is called 'dioxide' - di (2) oxygens (oxide) ). Altogether on the lefthand side of the equation then we have 1 calcium molecule, 1 carbon molecule, 1 silicon molecule and 5 oxygen molecules.
In a chemical reaction, if the full equation is written down, all the components on the left side must also all be on the right side, each and every one. We want to know where all the molecules have gone after the two chemicals on the lefthand side have reacted and split into their components. So where does the 1 calcium molecule go? It contributes to making calcium silicate `("CaSiO_3") ` , along with the 1 silicon molecule and 3 of the 5 oxygen molecules we had on the lefthand side. What remains is the 1 carbon molecule and 2 oxygen molecules. These have combined to form the other chemical on the righthand side, namely carbon dioxide (`"CO"_2) ` (which is carbon linked with two oxygen molecules, which remember is called dioxide - di (2) oxygens (oxide)).
This is like a math calculation, except we have chemical symbols to balance instead of numbers. The two sides of the equation have to balance, so no molecule can go missing/escape in the process of the chemical reaction.