Can you please help on 17 and 18 please

17. Yes, it represents a chemical reaction. This is because new substances were formed from the original substance.

18. (a) Formation of new substances

(b) Evolution or absorption of large quantity of heat

(c)Change in mass of substances

Explanation:

17. In the model above, molecules of a compound of carbon and hydrogen (molecular formula is C2H6) combines with molecular oxygen (O2) to produce two new substances; a compound composed of two oxygen atoms and one carbon atom (CO2) and another compound composed of two atoms of hydrogen and one atom of oxygen (H2O). Since the new substances formed are completely different from the original substances, it a chemical reaction. Also, a large volume of heat would have been given off.

18. To determine if a chemical reaction has occurred, the following observations can be looked out for.

a. Formation of new substances different entirely from the original substances

b. Evolution or absorption of large quantity of heat during the change. Check whether the vessel containing the substances gets hotter or colder.

c. Change in mass of substances. Since new substances are formed in a chemical reaction, there is usually a significant change in mass of the products and reactants.

the best answer is d because the strong cohesion is caused by the ability of water molecules to undergo hydrogen bonding.

a is incorrect because when water solidifies, it becomes   ice, which expands and occupies a bigger volume for the same mass of water in its liquid state.

b is incorrect because water has high surface tension. besides bonding to itself it hydrogen bonds to other molecules at the top decreasing possibility for breaking of attractive forces.

c is incorrect because the water molecule has oxygen which is highly electronegative and so pulls the electron pair towards itself creating a dipole moment, making the molecule polar.

e is incorrect because water has strong adhesion, ability to stick to other molecules such as glass because of this molecules having stronger hydrogen bonding than water.